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NAME

       lvmraid — LVM RAID

DESCRIPTION

       lvm(8)  RAID  is a way to create a Logical Volume (LV) that uses multiple physical devices
       to improve performance or tolerate device failures.  In  LVM,  the  physical  devices  are
       Physical Volumes (PVs) in a single Volume Group (VG).

       How  LV  data blocks are placed onto PVs is determined by the RAID level.  RAID levels are
       commonly referred to as 'raid'  followed  by  a  number,  e.g.   raid1,  raid5  or  raid6.
       Selecting  a  RAID  level  involves  making tradeoffs among: physical device requirements,
       fault tolerance, and performance.  A description of the RAID levels can be found at
       www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf

       LVM RAID uses both Device Mapper (DM) and Multiple Device  (MD)  drivers  from  the  Linux
       kernel.  DM is used to create and manage visible LVM devices, and MD is used to place data
       on physical devices.

       LVM creates hidden LVs (dm devices) layered between the visible LV and  physical  devices.
       LVs  in  the  middle layers are called sub LVs.  For LVM raid, a sub LV pair to store data
       and metadata (raid superblock and write intent bitmap) is created per raid image/leg  (see
       lvs command examples below).

Create a RAID LV

       To  create  a  RAID LV, use lvcreate and specify an LV type.  The LV type corresponds to a
       RAID level.  The basic RAID levels that can be  used  are:  raid0,  raid1,  raid4,  raid5,
       raid6, raid10.

       lvcreate --type RaidLevel [OPTIONS] --name Name --size Size VG [PVs]

       To display the LV type of an existing LV, run:

       lvs -o name,segtype LV

       (The LV type is also referred to as "segment type" or "segtype".)

       LVs can be created with the following types:

   raid0

       Also  called  striping,  raid0  spreads LV data across multiple devices in units of stripe
       size.  This is used to increase performance.  LV data will be lost if any of  the  devices
       fail.

       lvcreate --type raid0 [--stripes Number --stripesize Size] VG [PVs]

       --stripes specifies the number of devices to spread the LV across.

       --stripesize  specifies  the size of each stripe in kilobytes.  This is the amount of data
              that is written to one device before moving to the next.

       PVs specifies the devices to use.  If not specified, lvm will choose Number  devices,  one
       for each stripe based on the number of PVs available or supplied.

   raid1

       Also  called  mirroring,  raid1  uses  multiple devices to duplicate LV data.  The LV data
       remains available if all but one of the devices fail.  The minimum number of devices (i.e.
       sub LV pairs) required is 2.

       lvcreate --type raid1 [--mirrors Number] VG [PVs]

       --mirrors specifies the number of mirror images in addition to the original LV image, e.g.
              --mirrors 1 means there are two images of the data, the  original  and  one  mirror
              image.

       PVs  specifies  the devices to use.  If not specified, lvm will choose Number devices, one
       for each image.

   raid4

       raid4 is a form of striping that uses an extra, first device dedicated to  storing  parity
       blocks.   The  LV  data  remains  available  if  one  device fails.  The parity is used to
       recalculate data that is lost from  a  single  device.   The  minimum  number  of  devices
       required is 3.

       lvcreate --type raid4 [--stripes Number --stripesize Size] VG [PVs]

       --stripes  specifies  the number of devices to use for LV data.  This does not include the
              extra device lvm adds for storing parity blocks.  A raid4 LV  with  Number  stripes
              requires Number+1 devices.  Number must be 2 or more.

       --stripesize  specifies  the size of each stripe in kilobytes.  This is the amount of data
              that is written to one device before moving to the next.

       PVs specifies the devices to use.  If not specified, lvm  will  choose  Number+1  separate
       devices.

       raid4  is  called  non-rotating  parity because the parity blocks are always stored on the
       same device.

   raid5

       raid5 is a form of striping that uses an extra device for storing parity blocks.  LV  data
       and  parity  blocks  are  stored  on  each  device,  typically  in  a rotating pattern for
       performance reasons.  The LV data remains available if one device fails.   The  parity  is
       used to recalculate data that is lost from a single device.  The minimum number of devices
       required is 3 (unless converting from 2 legged raid1  to  reshape  to  more  stripes;  see
       reshaping).

       lvcreate --type raid5 [--stripes Number --stripesize Size] VG [PVs]

       --stripes  specifies  the number of devices to use for LV data.  This does not include the
              extra device lvm adds for storing parity blocks.  A raid5 LV  with  Number  stripes
              requires Number+1 devices.  Number must be 2 or more.

       --stripesize  specifies  the size of each stripe in kilobytes.  This is the amount of data
              that is written to one device before moving to the next.

       PVs specifies the devices to use.  If not specified, lvm  will  choose  Number+1  separate
       devices.

       raid5  is called rotating parity because the parity blocks are placed on different devices
       in a round-robin sequence.  There are variations of raid5 with  different  algorithms  for
       placing  the  parity blocks.  The default variant is raid5_ls (raid5 left symmetric, which
       is a rotating parity 0 with data restart.)  See RAID5 variants below.

   raid6

       raid6 is a form of striping like raid5, but uses two extra devices for parity blocks.   LV
       data  and  parity  blocks  are  stored on each device, typically in a rotating pattern for
       perfomramce reasons.  The LV data remains available if up to two devices fail.  The parity
       is  used  to recalculate data that is lost from one or two devices.  The minimum number of
       devices required is 5.

       lvcreate --type raid6 [--stripes Number --stripesize Size] VG [PVs]

       --stripes specifies the number of devices to use for LV data.  This does not  include  the
              extra  two  devices  lvm  adds  for  storing parity blocks.  A raid6 LV with Number
              stripes requires Number+2 devices.  Number must be 3 or more.

       --stripesize specifies the size of each stripe in kilobytes.  This is the amount  of  data
              that is written to one device before moving to the next.

       PVs  specifies  the  devices  to use.  If not specified, lvm will choose Number+2 separate
       devices.

       Like raid5, there are variations of raid6 with different algorithms for placing the parity
       blocks.  The default variant is raid6_zr (raid6 zero restart, aka left symmetric, which is
       a rotating parity 0 with data restart.)  See RAID6 variants below.

   raid10

       raid10 is a combination of raid1 and raid0, striping data  across  mirrored  devices.   LV
       data  remains  available  if  one or more devices remains in each mirror set.  The minimum
       number of devices required is 4.

       lvcreate --type raid10
              [--mirrors NumberMirrors]
              [--stripes NumberStripes --stripesize Size]
              VG [PVs]

       --mirrors specifies the number of mirror images within each  stripe.   e.g.   --mirrors  1
              means there are two images of the data, the original and one mirror image.

       --stripes specifies the total number of devices to use in all raid1 images (not the number
              of raid1 devices to spread the  LV  across,  even  though  that  is  the  effective
              result).     The    number    of   devices   in   each   raid1   mirror   will   be
              NumberStripes/(NumberMirrors+1), e.g. mirrors 1 and  stripes  4  will  stripe  data
              across two raid1 mirrors, where each mirror is devices.

       --stripesize  specifies  the size of each stripe in kilobytes.  This is the amount of data
              that is written to one device before moving to the next.

       PVs specifies the devices to use.   If  not  specified,  lvm  will  choose  the  necessary
       devices.   Devices  are  used  to  create mirrors in the order listed, e.g. for mirrors 1,
       stripes 2, listing PV1 PV2 PV3 PV4 results in mirrors PV1/PV2 and PV3/PV4.

       RAID10 is not mirroring on top of stripes, which would be RAID01, which is  less  tolerant
       of device failures.

Synchronization

       Synchronization  is  the  process  that makes all the devices in a RAID LV consistent with
       each other.

       In a RAID1 LV, all mirror images should have the same data.  When a new  mirror  image  is
       added,  or  a  mirror  image  is  missing data, then images need to be synchronized.  Data
       blocks are copied from an existing image to a new or outdated image to make them match.

       In a RAID 4/5/6 LV, parity blocks and  data  blocks  should  match  based  on  the  parity
       calculation.   When the devices in a RAID LV change, the data and parity blocks can become
       inconsistent and need to be synchronized.  Correct blocks are read, parity is  calculated,
       and recalculated blocks are written.

       The  RAID implementation keeps track of which parts of a RAID LV are synchronized.  When a
       RAID LV is first created and activated the first synchronization is called initialization.
       A  pointer  stored  in  the  raid  metadata keeps track of the initialization process thus
       allowing it to be restarted after a deactivation of the RaidLV or a crash.  Any writes  to
       the  RaidLV  dirties the respective region of the write intent bitmap which allow for fast
       recovery of the regions after a crash.  Without this, the  entire  LV  would  need  to  be
       synchronized every time it was activated.

       Automatic  synchronization  happens when a RAID LV is activated, but it is usually partial
       because the bitmaps reduce the areas that are checked.  A full sync becomes necessary when
       devices in the RAID LV are replaced.

       The  synchronization  status  of  a  RAID  LV  is reported by the following command, where
       "Cpy%Sync" = "100%" means sync is complete:

       lvs -a -o name,sync_percent

   Scrubbing
       Scrubbing is a full scan of the RAID LV requested by a user.  Scrubbing can find  problems
       that are missed by partial synchronization.

       Scrubbing  assumes  that  RAID  metadata and bitmaps may be inaccurate, so it verifies all
       RAID metadata, LV data, and parity blocks.  Scrubbing can find inconsistencies  caused  by
       hardware  errors  or  degradation.  These kinds of problems may be undetected by automatic
       synchronization which excludes areas outside of the RAID write-intent bitmap.

       The command to scrub a RAID LV can operate in two different modes:

       lvchange --syncaction check|repair LV

       check Check mode is read-only and only detects inconsistent areas in the RAID LV, it  does
              not correct them.

       repair  Repair  mode  checks  and  writes corrected blocks to synchronize any inconsistent
              areas.

       Scrubbing can consume a lot of bandwidth and slow down application I/O on the RAID LV.  To
       control the I/O rate used for scrubbing, use:

       --maxrecoveryrate Size[k|UNIT]
              Sets  the  maximum recovery rate for a RAID LV.  Size is specified as an amount per
              second for each device in the array.  If no suffix is given, then KiB/sec/device is
              used.  Setting the recovery rate to 0 means it will be unbounded.

       --minrecoveryrate Size[k|UNIT]
              Sets  the  minimum recovery rate for a RAID LV.  Size is specified as an amount per
              second for each device in the array.  If no suffix is given, then KiB/sec/device is
              used.  Setting the recovery rate to 0 means it will be unbounded.

       To  display  the current scrubbing in progress on an LV, including the syncaction mode and
       percent complete, run:

       lvs -a -o name,raid_sync_action,sync_percent

       After scrubbing is complete, to display the number of inconsistent blocks found, run:

       lvs -o name,raid_mismatch_count

       Also, if mismatches were found, the lvs attr field will display the letter "m"  (mismatch)
       in the 9th position, e.g.

       # lvs -o name,vgname,segtype,attr vg/lv
         LV VG   Type  Attr

         lv vg   raid1 Rwi-a-r-m-
   Scrubbing Limitations
       The  check  mode can only report the number of inconsistent blocks, it cannot report which
       blocks are inconsistent.  This makes it impossible to know which device has errors, or  if
       the errors affect file system data, metadata or nothing at all.

       The  repair  mode can make the RAID LV data consistent, but it does not know which data is
       correct.  The result may be consistent but incorrect data.  When two different  blocks  of
       data  must  be  made  consistent,  it chooses the block from the device that would be used
       during RAID intialization.  However, if the PV holding corrupt  data  is  known,  lvchange
       --rebuild can be used in place of scrubbing to reconstruct the data on the bad device.

       Future developments might include:

       Allowing a user to choose the correct version of data during repair.

       Using  a  majority  of  devices to determine the correct version of data to use in a 3-way
       RAID1 or RAID6 LV.

       Using a checksumming device to pin-point when and where an error occurs, allowing it to be
       rewritten.

SubLVs

       An  LV  is  often  a combination of other hidden LVs called SubLVs.  The SubLVs either use
       physical devices, or are built from other SubLVs themselves.  SubLVs hold LV data  blocks,
       RAID  parity blocks, and RAID metadata.  SubLVs are generally hidden, so the lvs -a option
       is required to display them:

       lvs -a -o name,segtype,devices

       SubLV names begin with the visible LV name, and have an automatic  suffix  indicating  its
       role:

       ·  SubLVs  holding  LV  data or parity blocks have the suffix _rimage_#.  These SubLVs are
          sometimes referred to as DataLVs.

       ·  SubLVs holding  RAID  metadata  have  the  suffix  _rmeta_#.   RAID  metadata  includes
          superblock information, RAID type, bitmap, and device health information.  These SubLVs
          are sometimes referred to as MetaLVs.

       SubLVs are an internal implementation detail of LVM.  The way they are  used,  constructed
       and named may change.

       The  following  examples  show  the SubLV arrangement for each of the basic RAID LV types,
       using the fewest number of devices allowed for each.

   Examples
       raid0
       Each rimage SubLV holds a portion of LV data.  No parity is used.   No  RAID  metadata  is
       used.

       # lvcreate --type raid0 --stripes 2 --name lvr0 ...

       # lvs -a -o name,segtype,devices
         lvr0            raid0  lvr0_rimage_0(0),lvr0_rimage_1(0)
         [lvr0_rimage_0] linear /dev/sda(...)
         [lvr0_rimage_1] linear /dev/sdb(...)

       raid1
       Each  rimage SubLV holds a complete copy of LV data.  No parity is used.  Each rmeta SubLV
       holds RAID metadata.

       # lvcreate --type raid1 --mirrors 1 --name lvr1 ...

       # lvs -a -o name,segtype,devices
         lvr1            raid1  lvr1_rimage_0(0),lvr1_rimage_1(0)
         [lvr1_rimage_0] linear /dev/sda(...)
         [lvr1_rimage_1] linear /dev/sdb(...)
         [lvr1_rmeta_0]  linear /dev/sda(...)
         [lvr1_rmeta_1]  linear /dev/sdb(...)

       raid4
       At least three rimage SubLVs each hold a portion of LV data and  one  rimage  SubLV  holds
       parity.  Each rmeta SubLV holds RAID metadata.

       # lvcreate --type raid4 --stripes 2 --name lvr4 ...

       # lvs -a -o name,segtype,devices
         lvr4            raid4  lvr4_rimage_0(0),\
                                lvr4_rimage_1(0),\
                                lvr4_rimage_2(0)
         [lvr4_rimage_0] linear /dev/sda(...)
         [lvr4_rimage_1] linear /dev/sdb(...)
         [lvr4_rimage_2] linear /dev/sdc(...)
         [lvr4_rmeta_0]  linear /dev/sda(...)
         [lvr4_rmeta_1]  linear /dev/sdb(...)
         [lvr4_rmeta_2]  linear /dev/sdc(...)

       raid5
       At  least  three  rimage  SubLVs  each typcially hold a portion of LV data and parity (see
       section on raid5) Each rmeta SubLV holds RAID metadata.

       # lvcreate --type raid5 --stripes 2 --name lvr5 ...

       # lvs -a -o name,segtype,devices
         lvr5            raid5  lvr5_rimage_0(0),\
                                lvr5_rimage_1(0),\
                                lvr5_rimage_2(0)
         [lvr5_rimage_0] linear /dev/sda(...)
         [lvr5_rimage_1] linear /dev/sdb(...)
         [lvr5_rimage_2] linear /dev/sdc(...)
         [lvr5_rmeta_0]  linear /dev/sda(...)
         [lvr5_rmeta_1]  linear /dev/sdb(...)
         [lvr5_rmeta_2]  linear /dev/sdc(...)

       raid6
       At least five rimage SubLVs each typically hold a portion of LV  data  and  parity.   (see
       section on raid6) Each rmeta SubLV holds RAID metadata.

       # lvcreate --type raid6 --stripes 3 --name lvr6

       # lvs -a -o name,segtype,devices
         lvr6            raid6  lvr6_rimage_0(0),\
                                lvr6_rimage_1(0),\
                                lvr6_rimage_2(0),\
                                lvr6_rimage_3(0),\
                                lvr6_rimage_4(0),\
                                lvr6_rimage_5(0)
         [lvr6_rimage_0] linear /dev/sda(...)
         [lvr6_rimage_1] linear /dev/sdb(...)
         [lvr6_rimage_2] linear /dev/sdc(...)
         [lvr6_rimage_3] linear /dev/sdd(...)
         [lvr6_rimage_4] linear /dev/sde(...)
         [lvr6_rimage_5] linear /dev/sdf(...)
         [lvr6_rmeta_0]  linear /dev/sda(...)
         [lvr6_rmeta_1]  linear /dev/sdb(...)
         [lvr6_rmeta_2]  linear /dev/sdc(...)
         [lvr6_rmeta_3]  linear /dev/sdd(...)
         [lvr6_rmeta_4]  linear /dev/sde(...)
         [lvr6_rmeta_5]  linear /dev/sdf(...)

       raid10
       At  least  four  rimage  SubLVs  each hold a portion of LV data.  No parity is used.  Each
       rmeta SubLV holds RAID metadata.

       # lvcreate --type raid10 --stripes 2 --mirrors 1 --name lvr10

       # lvs -a -o name,segtype,devices
         lvr10            raid10 lvr10_rimage_0(0),\
                                 lvr10_rimage_1(0),\
                                 lvr10_rimage_2(0),\
                                 lvr10_rimage_3(0)
         [lvr10_rimage_0] linear /dev/sda(...)
         [lvr10_rimage_1] linear /dev/sdb(...)
         [lvr10_rimage_2] linear /dev/sdc(...)
         [lvr10_rimage_3] linear /dev/sdd(...)
         [lvr10_rmeta_0]  linear /dev/sda(...)
         [lvr10_rmeta_1]  linear /dev/sdb(...)
         [lvr10_rmeta_2]  linear /dev/sdc(...)
         [lvr10_rmeta_3]  linear /dev/sdd(...)

Device Failure

       Physical devices in a RAID LV can fail or be lost for multiple reasons.  A device could be
       disconnected,  permanently  failed,  or temporarily disconnected.  The purpose of RAID LVs
       (levels 1 and higher) is to continue operating in a degraded mode, without losing LV data,
       even  after  a  device  fails.  The number of devices that can fail without the loss of LV
       data depends on the RAID level:

       ·  RAID0 (striped) LVs cannot tolerate losing any devices.  LV data will be  lost  if  any
          devices fail.

       ·  RAID1 LVs can tolerate losing all but one device without LV data loss.

       ·  RAID4 and RAID5 LVs can tolerate losing one device without LV data loss.

       ·  RAID6 LVs can tolerate losing two devices without LV data loss.

       ·  RAID10  is variable, and depends on which devices are lost.  It stripes across multiple
          mirror groups with raid1 layout thus it can tolerate losing all but one device in  each
          of these groups without LV data loss.

       If a RAID LV is missing devices, or has other device-related problems, lvs reports this in
       the health_status (and attr) fields:

       lvs -o name,lv_health_status

       partial
       Devices are missing from the LV.  This is also indicated by the letter  "p"  (partial)  in
       the 9th position of the lvs attr field.

       refresh needed
       A  device  was  temporarily missing but has returned.  The LV needs to be refreshed to use
       the device again (which will usually  require  partial  synchronization).   This  is  also
       indicated  by  the  letter "r" (refresh needed) in the 9th position of the lvs attr field.
       See Refreshing an LV.  This could also indicate a problem with the device, in  which  case
       it should be be replaced, see Replacing Devices.

       mismatches exist
       See Scrubbing.

       Most commands will also print a warning if a device is missing, e.g.
       WARNING: Device for PV uItL3Z-wBME-DQy0-... not found or rejected ...

       This  warning  will  go away if the device returns or is removed from the VG (see vgreduce
       --removemissing).

   Activating an LV with missing devices
       A RAID LV that is missing devices may be activated or not, depending  on  the  "activation
       mode" used in lvchange:

       lvchange -ay --activationmode complete|degraded|partial LV

       complete
       The LV is only activated if all devices are present.

       degraded
       The  LV  is  activated  with  missing devices if the RAID level can tolerate the number of
       missing devices without LV data loss.

       partial
       The LV is always activated, even if portions of the LV data are  missing  because  of  the
       missing  device(s).   This  should  only  be  used  to  perform extreme recovery or repair
       operations.

       lvm.conf(5) activation/activation_mode
       controls the activation mode when not specified by the command.

       The default value is printed by:
       lvmconfig --type default activation/activation_mode

   Replacing Devices
       Devices in a RAID LV can be replaced by other devices in the VG.  When  replacing  devices
       that  are no longer visible on the system, use lvconvert --repair.  When replacing devices
       that are still visible, use lvconvert --replace.   The  repair  command  will  attempt  to
       restore  the  same  number of data LVs that were previously in the LV.  The replace option
       can be repeated to replace multiple PVs.  Replacement devices  can  be  optionally  listed
       with either option.

       lvconvert --repair LV [NewPVs]

       lvconvert --replace OldPV LV [NewPV]

       lvconvert --replace OldPV1 --replace OldPV2 LV [NewPVs]

       New devices require synchronization with existing devices, see Synchronization.

   Refreshing an LV
       Refreshing  a  RAID  LV  clears  any  transient  device  failures  (device was temporarily
       disconnected) and returns the LV to its fully redundant mode.   Restoring  a  device  will
       usually  require at least partial synchronization (see Synchronization).  Failure to clear
       a transient failure results in the  RAID  LV  operating  in  degraded  mode  until  it  is
       reactivated.  Use the lvchange command to refresh an LV:

       lvchange --refresh LV

       # lvs -o name,vgname,segtype,attr,size vg
         LV VG   Type  Attr       LSize
         lv vg   raid1 Rwi-a-r-r- 100.00g

       # lvchange --refresh vg/lv

       # lvs -o name,vgname,segtype,attr,size vg
         LV VG   Type  Attr       LSize
         lv vg   raid1 Rwi-a-r--- 100.00g

   Automatic repair
       If  a  device  in  a  RAID  LV fails, device-mapper in the kernel notifies the dmeventd(8)
       monitoring process (see Monitoring).  dmeventd can be configured to automatically  respond
       using:

       lvm.conf(5) activation/raid_fault_policy

       Possible settings are:

       warn
       A  warning  is added to the system log indicating that a device has failed in the RAID LV.
       It is left to the user to repair the LV, e.g.  replace failed devices.

       allocate
       dmeventd automatically attempts to repair the LV using spare devices in the VG.  Note that
       even  a  transient  failure  is  treated as a permanent failure under this setting.  A new
       device is allocated and full synchronization is started.

       The specific command run by dmeventd to warn or repair is:
       lvconvert --repair --use-policies LV

   Corrupted Data
       Data on a device can be corrupted due to hardware errors without  the  device  ever  being
       disconnected  or  there  being any fault in the software.  This should be rare, and can be
       detected (see Scrubbing).

   Rebuild specific PVs
       If specific PVs in a RAID LV are known to have corrupt data, the data on those PVs can  be
       reconstructed with:

       lvchange --rebuild PV LV

       The rebuild option can be repeated with different PVs to replace the data on multiple PVs.

Monitoring

       When  a  RAID  LV is activated the dmeventd(8) process is started to monitor the health of
       the LV.  Various events detected in the kernel can cause a notification to  be  sent  from
       device-mapper  to  the  monitoring  process, including device failures and synchronization
       completion (e.g.  for initialization or scrubbing).

       The LVM configuration file contains options that affect how the  monitoring  process  will
       respond  to  failure  events  (e.g. raid_fault_policy).  It is possible to turn on and off
       monitoring with lvchange, but it is not recommended to turn this off  unless  you  have  a
       thorough knowledge of the consequences.

Configuration Options

       There  are  a  number of options in the LVM configuration file that affect the behavior of
       RAID LVs.  The tunable options are listed below.  A detailed description of  each  can  be
       found in the LVM configuration file itself.
               mirror_segtype_default
               raid10_segtype_default
               raid_region_size
               raid_fault_policy
               activation_mode

RAID1 Tuning

       A  RAID1 LV can be tuned so that certain devices are avoided for reading while all devices
       are still written to.

       lvchange --[raid]writemostly PV[:y|n|t] LV

       The specified device will be marked as "write mostly", which means that reading from  this
       device  will  be avoided, and other devices will be preferred for reading (unless no other
       devices are available.)  This minimizes the I/O to the specified device.

       If the PV name has no suffix, the write mostly attribute is set.  If the PV name  has  the
       suffix  :n,  the  write mostly attribute is cleared, and the suffix :t toggles the current
       setting.

       The write mostly option can be repeated on the command line to change multiple devices  at
       once.

       To report the current write mostly setting, the lvs attr field will show the letter "w" in
       the 9th position when write mostly is set:

       lvs -a -o name,attr

       When a device is marked write mostly, the maximum number of  outstanding  writes  to  that
       device can be configured.  Once the maximum is reached, further writes become synchronous.
       When synchronous, a write to the LV will not complete  until  writes  to  all  the  mirror
       images are complete.

       lvchange --[raid]writebehind Number LV

       To report the current write behind setting, run:

       lvs -o name,raid_write_behind

       When write behind is not configured, or set to 0, all LV writes are synchronous.

RAID Takeover

       RAID  takeover  is  converting  a  RAID  LV from one RAID level to another, e.g.  raid5 to
       raid6.  Changing the RAID level is usually done to  increase  or  decrease  resilience  to
       device  failures  or to restripe LVs.  This is done using lvconvert and specifying the new
       RAID level as the LV type:

       lvconvert --type RaidLevel LV [PVs]

       The most common and recommended RAID takeover conversions are:

       linear to raid1
              Linear is a single image of LV data, and converting it to raid1 adds a mirror image
              which is a direct copy of the original linear image.

       striped/raid0 to raid4/5/6
              Adding parity devices to a striped volume results in raid4/5/6.

       Unnatural conversions that are not recommended include converting between striped and non-
       striped types.  This is because file systems often optimize I/O patterns based  on  device
       striping values.  If those values change, it can decrease performance.

       Converting  to  a  higher RAID level requires allocating new SubLVs to hold RAID metadata,
       and new SubLVs to hold parity blocks for LV  data.   Converting  to  a  lower  RAID  level
       removes the SubLVs that are no longer needed.

       Conversion  often  requires  full  synchronization  of  the RAID LV (see Synchronization).
       Converting to RAID1 requires copying all LV data blocks to N new images  on  new  devices.
       Converting to a parity RAID level requires reading all LV data blocks, calculating parity,
       and writing the new parity blocks.  Synchronization can take a long time depending on  the
       throughpout  of  the  devices used and the size of the RaidLV.  It can degrade performance
       (rate controls also apply to conversion; see --minrecoveryrate and --maxrecoveryrate.)

       Warning: though it is possible to create striped LVs  with up to 128 stripes, a maximum of
       64  stripes  can be converted to raid0, 63 to raid4/5 and 62 to raid6 because of the added
       parity SubLVs.  A striped LV with a maximum of 32 stripes can be converted to raid10.

       The following takeover conversions are currently possible:

       ·  between striped and raid0.

       ·  between linear and raid1.

       ·  between mirror and raid1.

       ·  between raid1 with two images and raid4/5.

       ·  between striped/raid0 and raid4.

       ·  between striped/raid0 and raid5.

       ·  between striped/raid0 and raid6.

       ·  between raid4 and raid5.

       ·  between raid4/raid5 and raid6.

       ·  between striped/raid0 and raid10.

       ·  between striped and raid4.

   Indirect conversions
       Converting from one raid level to another may require multiple steps, converting first  to
       intermediate raid levels.

       linear to raid6

       To convert an LV from linear to raid6:
       1. convert to raid1 with two images
       2. convert to raid5 (internally raid5_ls) with two images
       3. convert to raid5 with three or more stripes (reshape)
       4. convert to raid6 (internally raid6_ls_6)
       5. convert to raid6 (internally raid6_zr, reshape)

       The commands to perform the steps above are:
       1. lvconvert --type raid1 --mirrors 1 LV
       2. lvconvert --type raid5 LV
       3. lvconvert --stripes 3 LV
       4. lvconvert --type raid6 LV
       5. lvconvert --type raid6 LV

       The  final  conversion  from  raid6_ls_6  to  raid6_zr  is  done  to  avoid  the potential
       write/recovery performance reduction in raid6_ls_6 because of the dedicated parity device.
       raid6_zr rotates data and parity blocks to avoid this.

       linear to striped

       To convert an LV from linear to striped:
       1. convert to raid1 with two images
       2. convert to raid5_n
       3. convert to raid5_n with five 128k stripes (reshape)
       4. convert raid5_n to striped

       The commands to perform the steps above are:
       1. lvconvert --type raid1 --mirrors 1 LV
       2. lvconvert --type raid5_n LV
       3. lvconvert --stripes 5 --stripesize 128k LV
       4. lvconvert --type striped LV

       The  raid5_n type in step 2 is used because it has dedicated parity SubLVs at the end, and
       can be converted to striped directly.  The stripe size is increased in step 3 to add extra
       space for the conversion process.  This step grows the LV size by a factor of five.  After
       conversion, this extra space can be reduced (or used to grow the  file  system  using  the
       LV).

       Reversing these steps will convert a striped LV to linear.

       raid6 to striped

       To convert an LV from raid6_nr to striped:
       1. convert to raid6_n_6
       2. convert to striped

       The commands to perform the steps above are:
       1. lvconvert --type raid6_n_6 LV
       2. lvconvert --type striped LV

   Examples
       Converting an LV from linear to raid1.

       # lvs -a -o name,segtype,size vg
         LV   Type   LSize
         lv   linear 300.00g

       # lvconvert --type raid1 --mirrors 1 vg/lv

       # lvs -a -o name,segtype,size vg
         LV            Type   LSize
         lv            raid1  300.00g
         [lv_rimage_0] linear 300.00g
         [lv_rimage_1] linear 300.00g
         [lv_rmeta_0]  linear   3.00m
         [lv_rmeta_1]  linear   3.00m

       Converting an LV from mirror to raid1.

       # lvs -a -o name,segtype,size vg
         LV            Type   LSize
         lv            mirror 100.00g
         [lv_mimage_0] linear 100.00g
         [lv_mimage_1] linear 100.00g
         [lv_mlog]     linear   3.00m

       # lvconvert --type raid1 vg/lv

       # lvs -a -o name,segtype,size vg
         LV            Type   LSize
         lv            raid1  100.00g
         [lv_rimage_0] linear 100.00g
         [lv_rimage_1] linear 100.00g
         [lv_rmeta_0]  linear   3.00m
         [lv_rmeta_1]  linear   3.00m

       Converting an LV from linear to raid1 (with 3 images).

       # lvconvert --type raid1 --mirrors 2 vg/lv

       Converting an LV from striped (with 4 stripes) to raid6_n_6.

       # lvcreate --stripes 4 -L64M -n lv vg

       # lvconvert --type raid6 vg/lv

       # lvs -a -o lv_name,segtype,sync_percent,data_copies
         LV            Type      Cpy%Sync #Cpy
         lv            raid6_n_6 100.00      3
         [lv_rimage_0] linear
         [lv_rimage_1] linear
         [lv_rimage_2] linear
         [lv_rimage_3] linear
         [lv_rimage_4] linear
         [lv_rimage_5] linear
         [lv_rmeta_0]  linear
         [lv_rmeta_1]  linear
         [lv_rmeta_2]  linear
         [lv_rmeta_3]  linear
         [lv_rmeta_4]  linear
         [lv_rmeta_5]  linear

       This  convert  begins  by  allocating  MetaLVs  (rmeta_#)  for each of the existing stripe
       devices.   It  then  creates  2  additional  MetaLV/DataLV  pairs  (rmeta_#/rimage_#)  for
       dedicated raid6 parity.

       If  rotating  data/parity is required, such as with raid6_nr, it must be done by reshaping
       (see below).

RAID Reshaping

       RAID reshaping is changing attributes of a RAID LV while  keeping  the  same  RAID  level.
       This includes changing RAID layout, stripe size, or number of stripes.

       When  changing  the RAID layout or stripe size, no new SubLVs (MetaLVs or DataLVs) need to
       be allocated, but DataLVs are extended by a small amount (typically 1 extent).  The  extra
       space  allows  blocks  in a stripe to be updated safely, and not be corrupted in case of a
       crash.  If a crash occurs, reshaping can just be restarted.

       (If blocks in a stripe were updated in place, a crash could leave them  partially  updated
       and  corrupted.  Instead, an existing stripe is quiesced, read, changed in layout, and the
       new stripe written to free space.  Once that is done, the new  stripe  is  unquiesced  and
       used.)

   Examples
       (Command output shown in examples may change.)

       Converting raid6_n_6 to raid6_nr with rotating data/parity.

       This  conversion  naturally  follows a previous conversion from striped/raid0 to raid6_n_6
       (shown above).  It completes the transition to a more traditional RAID6.

       # lvs -o lv_name,segtype,sync_percent,data_copies
         LV            Type      Cpy%Sync #Cpy
         lv            raid6_n_6 100.00      3
         [lv_rimage_0] linear
         [lv_rimage_1] linear
         [lv_rimage_2] linear
         [lv_rimage_3] linear
         [lv_rimage_4] linear
         [lv_rimage_5] linear
         [lv_rmeta_0]  linear
         [lv_rmeta_1]  linear
         [lv_rmeta_2]  linear
         [lv_rmeta_3]  linear
         [lv_rmeta_4]  linear
         [lv_rmeta_5]  linear

       # lvconvert --type raid6_nr vg/lv

       # lvs -a -o lv_name,segtype,sync_percent,data_copies
         LV            Type     Cpy%Sync #Cpy
         lv            raid6_nr 100.00      3
         [lv_rimage_0] linear
         [lv_rimage_0] linear
         [lv_rimage_1] linear
         [lv_rimage_1] linear
         [lv_rimage_2] linear
         [lv_rimage_2] linear
         [lv_rimage_3] linear
         [lv_rimage_3] linear
         [lv_rimage_4] linear
         [lv_rimage_5] linear
         [lv_rmeta_0]  linear
         [lv_rmeta_1]  linear
         [lv_rmeta_2]  linear
         [lv_rmeta_3]  linear
         [lv_rmeta_4]  linear
         [lv_rmeta_5]  linear

       The DataLVs are larger (additional segment in each) which provides space for  out-of-place
       reshaping.  The result is:

       # lvs -a -o lv_name,segtype,seg_pe_ranges,dataoffset
         LV            Type     PE Ranges          DOff
         lv            raid6_nr lv_rimage_0:0-32 \
                                lv_rimage_1:0-32 \
                                lv_rimage_2:0-32 \
                                lv_rimage_3:0-32
         [lv_rimage_0] linear   /dev/sda:0-31      2048
         [lv_rimage_0] linear   /dev/sda:33-33
         [lv_rimage_1] linear   /dev/sdaa:0-31     2048
         [lv_rimage_1] linear   /dev/sdaa:33-33
         [lv_rimage_2] linear   /dev/sdab:1-33     2048
         [lv_rimage_3] linear   /dev/sdac:1-33     2048
         [lv_rmeta_0]  linear   /dev/sda:32-32
         [lv_rmeta_1]  linear   /dev/sdaa:32-32
         [lv_rmeta_2]  linear   /dev/sdab:0-0
         [lv_rmeta_3]  linear   /dev/sdac:0-0

       All  segments  with  PE  ranges  '33-33'  provide  the  out-of-place  reshape  space.  The
       dataoffset column shows that the data was moved from initial offset 0 to 2048  sectors  on
       each component DataLV.

       For  performance  reasons  the  raid6_nr  RaidLV  can be restriped.  Convert it from 3-way
       striped to 5-way-striped.

       # lvconvert --stripes 5 vg/lv
         Using default stripesize 64.00 KiB.
         WARNING: Adding stripes to active logical volume vg/lv will \
         grow it from 99 to 165 extents!
         Run "lvresize -l99 vg/lv" to shrink it or use the additional \
         capacity.
         Logical volume vg/lv successfully converted.

       # lvs vg/lv
         LV   VG     Attr       LSize   Cpy%Sync
         lv   vg     rwi-a-r-s- 652.00m 52.94

       # lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV            Attr       Type     PE Ranges          DOff
         lv            rwi-a-r--- raid6_nr lv_rimage_0:0-33 \
                                           lv_rimage_1:0-33 \
                                           lv_rimage_2:0-33 ... \
                                           lv_rimage_5:0-33 \
                                           lv_rimage_6:0-33   0
         [lv_rimage_0] iwi-aor--- linear   /dev/sda:0-32      0
         [lv_rimage_0] iwi-aor--- linear   /dev/sda:34-34
         [lv_rimage_1] iwi-aor--- linear   /dev/sdaa:0-32     0
         [lv_rimage_1] iwi-aor--- linear   /dev/sdaa:34-34
         [lv_rimage_2] iwi-aor--- linear   /dev/sdab:0-32     0
         [lv_rimage_2] iwi-aor--- linear   /dev/sdab:34-34
         [lv_rimage_3] iwi-aor--- linear   /dev/sdac:1-34     0
         [lv_rimage_4] iwi-aor--- linear   /dev/sdad:1-34     0
         [lv_rimage_5] iwi-aor--- linear   /dev/sdae:1-34     0
         [lv_rimage_6] iwi-aor--- linear   /dev/sdaf:1-34     0
         [lv_rmeta_0]  ewi-aor--- linear   /dev/sda:33-33
         [lv_rmeta_1]  ewi-aor--- linear   /dev/sdaa:33-33
         [lv_rmeta_2]  ewi-aor--- linear   /dev/sdab:33-33
         [lv_rmeta_3]  ewi-aor--- linear   /dev/sdac:0-0
         [lv_rmeta_4]  ewi-aor--- linear   /dev/sdad:0-0
         [lv_rmeta_5]  ewi-aor--- linear   /dev/sdae:0-0
         [lv_rmeta_6]  ewi-aor--- linear   /dev/sdaf:0-0

       Stripes also can be removed from raid5 and 6.  Convert the 5-way striped  raid6_nr  LV  to
       4-way-striped.   The  force  option needs to be used, because removing stripes (i.e. image
       SubLVs) from a RaidLV will shrink its size.

       # lvconvert --stripes 4 vg/lv
         Using default stripesize 64.00 KiB.
         WARNING: Removing stripes from active logical volume vg/lv will \
         shrink it from 660.00 MiB to 528.00 MiB!
         THIS MAY DESTROY (PARTS OF) YOUR DATA!
         If that leaves the logical volume larger than 206 extents due \
         to stripe rounding,
         you may want to grow the content afterwards (filesystem etc.)
         WARNING: to remove freed stripes after the conversion has finished,\
         you have to run "lvconvert --stripes 4 vg/lv"
         Logical volume vg/lv successfully converted.

       # lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV            Attr       Type     PE Ranges          DOff
         lv            rwi-a-r-s- raid6_nr lv_rimage_0:0-33 \
                                           lv_rimage_1:0-33 \
                                           lv_rimage_2:0-33 ... \
                                           lv_rimage_5:0-33 \
                                           lv_rimage_6:0-33   0
         [lv_rimage_0] Iwi-aor--- linear   /dev/sda:0-32      0
         [lv_rimage_0] Iwi-aor--- linear   /dev/sda:34-34
         [lv_rimage_1] Iwi-aor--- linear   /dev/sdaa:0-32     0
         [lv_rimage_1] Iwi-aor--- linear   /dev/sdaa:34-34
         [lv_rimage_2] Iwi-aor--- linear   /dev/sdab:0-32     0
         [lv_rimage_2] Iwi-aor--- linear   /dev/sdab:34-34
         [lv_rimage_3] Iwi-aor--- linear   /dev/sdac:1-34     0
         [lv_rimage_4] Iwi-aor--- linear   /dev/sdad:1-34     0
         [lv_rimage_5] Iwi-aor--- linear   /dev/sdae:1-34     0
         [lv_rimage_6] Iwi-aor-R- linear   /dev/sdaf:1-34     0
         [lv_rmeta_0]  ewi-aor--- linear   /dev/sda:33-33
         [lv_rmeta_1]  ewi-aor--- linear   /dev/sdaa:33-33
         [lv_rmeta_2]  ewi-aor--- linear   /dev/sdab:33-33
         [lv_rmeta_3]  ewi-aor--- linear   /dev/sdac:0-0
         [lv_rmeta_4]  ewi-aor--- linear   /dev/sdad:0-0
         [lv_rmeta_5]  ewi-aor--- linear   /dev/sdae:0-0
         [lv_rmeta_6]  ewi-aor-R- linear   /dev/sdaf:0-0

       The 's' in column 9 of the attribute field shows the RaidLV is still reshaping.   The  'R'
       in  the  same  column of the attribute field shows the freed image Sub LVs which will need
       removing once the reshaping finished.

       # lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV   Attr       Type     PE Ranges          DOff
         lv   rwi-a-r-R- raid6_nr lv_rimage_0:0-33 \
                                  lv_rimage_1:0-33 \
                                  lv_rimage_2:0-33 ... \
                                  lv_rimage_5:0-33 \
                                  lv_rimage_6:0-33   8192

       Now that the reshape is finished the 'R' atribute  on  the  RaidLV  shows  images  can  be
       removed.

       # lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV   Attr       Type     PE Ranges          DOff
         lv   rwi-a-r-R- raid6_nr lv_rimage_0:0-33 \
                                  lv_rimage_1:0-33 \
                                  lv_rimage_2:0-33 ... \
                                  lv_rimage_5:0-33 \
                                  lv_rimage_6:0-33   8192

       This  is  achieved  by  repeating  the  command  ("lvconvert  --stripes  4 vg/lv" would be
       sufficient).

       # lvconvert --stripes 4 vg/lv
         Using default stripesize 64.00 KiB.
         Logical volume vg/lv successfully converted.

       # lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV            Attr       Type     PE Ranges          DOff
         lv            rwi-a-r--- raid6_nr lv_rimage_0:0-33 \
                                           lv_rimage_1:0-33 \
                                           lv_rimage_2:0-33 ... \
                                           lv_rimage_5:0-33   8192
         [lv_rimage_0] iwi-aor--- linear   /dev/sda:0-32      8192
         [lv_rimage_0] iwi-aor--- linear   /dev/sda:34-34
         [lv_rimage_1] iwi-aor--- linear   /dev/sdaa:0-32     8192
         [lv_rimage_1] iwi-aor--- linear   /dev/sdaa:34-34
         [lv_rimage_2] iwi-aor--- linear   /dev/sdab:0-32     8192
         [lv_rimage_2] iwi-aor--- linear   /dev/sdab:34-34
         [lv_rimage_3] iwi-aor--- linear   /dev/sdac:1-34     8192
         [lv_rimage_4] iwi-aor--- linear   /dev/sdad:1-34     8192
         [lv_rimage_5] iwi-aor--- linear   /dev/sdae:1-34     8192
         [lv_rmeta_0]  ewi-aor--- linear   /dev/sda:33-33
         [lv_rmeta_1]  ewi-aor--- linear   /dev/sdaa:33-33
         [lv_rmeta_2]  ewi-aor--- linear   /dev/sdab:33-33
         [lv_rmeta_3]  ewi-aor--- linear   /dev/sdac:0-0
         [lv_rmeta_4]  ewi-aor--- linear   /dev/sdad:0-0
         [lv_rmeta_5]  ewi-aor--- linear   /dev/sdae:0-0

       # lvs -a -o lv_name,attr,segtype,reshapelen vg
         LV            Attr       Type     RSize
         lv            rwi-a-r--- raid6_nr 24.00m
         [lv_rimage_0] iwi-aor--- linear    4.00m
         [lv_rimage_0] iwi-aor--- linear
         [lv_rimage_1] iwi-aor--- linear    4.00m
         [lv_rimage_1] iwi-aor--- linear
         [lv_rimage_2] iwi-aor--- linear    4.00m
         [lv_rimage_2] iwi-aor--- linear
         [lv_rimage_3] iwi-aor--- linear    4.00m
         [lv_rimage_4] iwi-aor--- linear    4.00m
         [lv_rimage_5] iwi-aor--- linear    4.00m
         [lv_rmeta_0]  ewi-aor--- linear
         [lv_rmeta_1]  ewi-aor--- linear
         [lv_rmeta_2]  ewi-aor--- linear
         [lv_rmeta_3]  ewi-aor--- linear
         [lv_rmeta_4]  ewi-aor--- linear
         [lv_rmeta_5]  ewi-aor--- linear

       Future developments might include automatic removal of the freed images.

       If the reshape space shall be removed any lvconvert command not changing the layout can be
       used:

       # lvconvert --stripes 4 vg/lv
         Using default stripesize 64.00 KiB.
         No change in RAID LV vg/lv layout, freeing reshape space.
         Logical volume vg/lv successfully converted.

       # lvs -a -o lv_name,attr,segtype,reshapelen vg
         LV            Attr       Type     RSize
         lv            rwi-a-r--- raid6_nr    0
         [lv_rimage_0] iwi-aor--- linear      0
         [lv_rimage_0] iwi-aor--- linear
         [lv_rimage_1] iwi-aor--- linear      0
         [lv_rimage_1] iwi-aor--- linear
         [lv_rimage_2] iwi-aor--- linear      0
         [lv_rimage_2] iwi-aor--- linear
         [lv_rimage_3] iwi-aor--- linear      0
         [lv_rimage_4] iwi-aor--- linear      0
         [lv_rimage_5] iwi-aor--- linear      0
         [lv_rmeta_0]  ewi-aor--- linear
         [lv_rmeta_1]  ewi-aor--- linear
         [lv_rmeta_2]  ewi-aor--- linear
         [lv_rmeta_3]  ewi-aor--- linear
         [lv_rmeta_4]  ewi-aor--- linear
         [lv_rmeta_5]  ewi-aor--- linear

       In case the RaidLV should be converted to striped:

       # lvconvert --type striped vg/lv
         Unable to convert LV vg/lv from raid6_nr to striped.
         Converting vg/lv from raid6_nr is directly possible to the \
         following layouts:
           raid6_nc
           raid6_zr
           raid6_la_6
           raid6_ls_6
           raid6_ra_6
           raid6_rs_6
           raid6_n_6

       A  direct  conversion  isn't  possible  thus the command informed about the possible ones.
       raid6_n_6 is suitable to convert to striped so convert to it  first  (this  is  a  reshape
       changing the raid6 layout from raid6_nr to raid6_n_6).

       # lvconvert --type raid6_n_6
         Using default stripesize 64.00 KiB.
         Converting raid6_nr LV vg/lv to raid6_n_6.
       Are you sure you want to convert raid6_nr LV vg/lv? [y/n]: y
         Logical volume vg/lv successfully converted.

       Wait for the reshape to finish.

       # lvconvert --type striped vg/lv
         Logical volume vg/lv successfully converted.

       # lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV   Attr       Type    PE Ranges  DOff
         lv   -wi-a----- striped /dev/sda:2-32 \
                                 /dev/sdaa:2-32 \
                                 /dev/sdab:2-32 \
                                 /dev/sdac:3-33
         lv   -wi-a----- striped /dev/sda:34-35 \
                                 /dev/sdaa:34-35 \
                                 /dev/sdab:34-35 \
                                 /dev/sdac:34-35

       From striped we can convert to raid10

       # lvconvert --type raid10 vg/lv
         Using default stripesize 64.00 KiB.
         Logical volume vg/lv successfully converted.

       # lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV   Attr       Type   PE Ranges          DOff
         lv   rwi-a-r--- raid10 lv_rimage_0:0-32 \
                                lv_rimage_4:0-32 \
                                lv_rimage_1:0-32 ... \
                                lv_rimage_3:0-32 \
                                lv_rimage_7:0-32   0

       # lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         WARNING: Cannot find matching striped segment for vg/lv_rimage_3.
         LV            Attr       Type   PE Ranges          DOff
         lv            rwi-a-r--- raid10 lv_rimage_0:0-32 \
                                         lv_rimage_4:0-32 \
                                         lv_rimage_1:0-32 ... \
                                         lv_rimage_3:0-32 \
                                         lv_rimage_7:0-32   0
         [lv_rimage_0] iwi-aor--- linear /dev/sda:2-32      0
         [lv_rimage_0] iwi-aor--- linear /dev/sda:34-35
         [lv_rimage_1] iwi-aor--- linear /dev/sdaa:2-32     0
         [lv_rimage_1] iwi-aor--- linear /dev/sdaa:34-35
         [lv_rimage_2] iwi-aor--- linear /dev/sdab:2-32     0
         [lv_rimage_2] iwi-aor--- linear /dev/sdab:34-35
         [lv_rimage_3] iwi-XXr--- linear /dev/sdac:3-35     0
         [lv_rimage_4] iwi-aor--- linear /dev/sdad:1-33     0
         [lv_rimage_5] iwi-aor--- linear /dev/sdae:1-33     0
         [lv_rimage_6] iwi-aor--- linear /dev/sdaf:1-33     0
         [lv_rimage_7] iwi-aor--- linear /dev/sdag:1-33     0
         [lv_rmeta_0]  ewi-aor--- linear /dev/sda:0-0
         [lv_rmeta_1]  ewi-aor--- linear /dev/sdaa:0-0
         [lv_rmeta_2]  ewi-aor--- linear /dev/sdab:0-0
         [lv_rmeta_3]  ewi-aor--- linear /dev/sdac:0-0
         [lv_rmeta_4]  ewi-aor--- linear /dev/sdad:0-0
         [lv_rmeta_5]  ewi-aor--- linear /dev/sdae:0-0
         [lv_rmeta_6]  ewi-aor--- linear /dev/sdaf:0-0
         [lv_rmeta_7]  ewi-aor--- linear /dev/sdag:0-0

       raid10 allows to add stripes but can't remove them.

       A  more  elaborate  example  to convert from linear to striped with interim conversions to
       raid1 then raid5 followed by restripe (4 steps).

       We start with the linear LV.

       # lvs -a -o name,size,segtype,syncpercent,datastripes,\
                   stripesize,reshapelenle,devices vg
         LV   LSize   Type   Cpy%Sync #DStr Stripe RSize Devices
         lv   128.00m linear              1     0        /dev/sda(0)

       Then convert it to a 2-way raid1.

       # lvconvert --mirrors 1 vg/lv
         Logical volume vg/lv successfully converted.

       # lvs -a -o name,size,segtype,datastripes,\
                   stripesize,reshapelenle,devices vg
         LV            LSize   Type   #DStr Stripe RSize Devices
         lv            128.00m raid1      2     0        lv_rimage_0(0),\
                                                         lv_rimage_1(0)
         [lv_rimage_0] 128.00m linear     1     0        /dev/sda(0)
         [lv_rimage_1] 128.00m linear     1     0        /dev/sdhx(1)
         [lv_rmeta_0]    4.00m linear     1     0        /dev/sda(32)
         [lv_rmeta_1]    4.00m linear     1     0        /dev/sdhx(0)

       Once the raid1 LV is fully synchronized we convert it to raid5_n (only 2-way raid1 LVs can
       be  converted to raid5).  We select raid5_n here because it has dedicated parity SubLVs at
       the end and can be converted to striped directly without any additional conversion.

       # lvconvert --type raid5_n vg/lv
         Using default stripesize 64.00 KiB.
         Logical volume vg/lv successfully converted.

       # lvs -a -o name,size,segtype,syncpercent,datastripes,\
                   stripesize,reshapelenle,devices vg
         LV            LSize   Type    #DStr Stripe RSize Devices
         lv            128.00m raid5_n     1 64.00k     0 lv_rimage_0(0),\
                                                          lv_rimage_1(0)
         [lv_rimage_0] 128.00m linear      1     0      0 /dev/sda(0)
         [lv_rimage_1] 128.00m linear      1     0      0 /dev/sdhx(1)
         [lv_rmeta_0]    4.00m linear      1     0        /dev/sda(32)
         [lv_rmeta_1]    4.00m linear      1     0        /dev/sdhx(0)

       Now we'll change the number of data stripes from 1 to 5 and request 128K  stripe  size  in
       one command.  This will grow the size of the LV by a factor of 5 (we add 4 data stripes to
       the one given).  That additonal space can be used by e.g. growing any contained filesystem
       or the LV can be reduced in size after the reshaping conversion has finished.

       # lvconvert --stripesize 128k --stripes 5 vg/lv
         Converting stripesize 64.00 KiB of raid5_n LV vg/lv to 128.00 KiB.
         WARNING: Adding stripes to active logical volume vg/lv will grow \
         it from 32 to 160 extents!
         Run "lvresize -l32 vg/lv" to shrink it or use the additional capacity.
         Logical volume vg/lv successfully converted.

       # lvs -a -o name,size,segtype,datastripes,\
                   stripesize,reshapelenle,devices
         LV            LSize   Type    #DStr Stripe  RSize Devices
         lv            640.00m raid5_n     5 128.00k     6 lv_rimage_0(0),\
                                                           lv_rimage_1(0),\
                                                           lv_rimage_2(0),\
                                                           lv_rimage_3(0),\
                                                           lv_rimage_4(0),\
                                                           lv_rimage_5(0)
         [lv_rimage_0] 132.00m linear      1      0      1 /dev/sda(33)
         [lv_rimage_0] 132.00m linear      1      0        /dev/sda(0)
         [lv_rimage_1] 132.00m linear      1      0      1 /dev/sdhx(33)
         [lv_rimage_1] 132.00m linear      1      0        /dev/sdhx(1)
         [lv_rimage_2] 132.00m linear      1      0      1 /dev/sdhw(33)
         [lv_rimage_2] 132.00m linear      1      0        /dev/sdhw(1)
         [lv_rimage_3] 132.00m linear      1      0      1 /dev/sdhv(33)
         [lv_rimage_3] 132.00m linear      1      0        /dev/sdhv(1)
         [lv_rimage_4] 132.00m linear      1      0      1 /dev/sdhu(33)
         [lv_rimage_4] 132.00m linear      1      0        /dev/sdhu(1)
         [lv_rimage_5] 132.00m linear      1      0      1 /dev/sdht(33)
         [lv_rimage_5] 132.00m linear      1      0        /dev/sdht(1)
         [lv_rmeta_0]    4.00m linear      1      0        /dev/sda(32)
         [lv_rmeta_1]    4.00m linear      1      0        /dev/sdhx(0)
         [lv_rmeta_2]    4.00m linear      1      0        /dev/sdhw(0)
         [lv_rmeta_3]    4.00m linear      1      0        /dev/sdhv(0)
         [lv_rmeta_4]    4.00m linear      1      0        /dev/sdhu(0)
         [lv_rmeta_5]    4.00m linear      1      0        /dev/sdht(0)

       Once the conversion has finished we can can convert to striped.

       # lvconvert --type striped vg/lv
         Logical volume vg/lv successfully converted.

       # lvs -a -o name,size,segtype,datastripes,\
                   stripesize,reshapelenle,devices vg
         LV   LSize   Type    #DStr Stripe  RSize Devices
         lv   640.00m striped     5 128.00k       /dev/sda(33),\
                                                  /dev/sdhx(33),\
                                                  /dev/sdhw(33),\
                                                  /dev/sdhv(33),\
                                                  /dev/sdhu(33)
         lv   640.00m striped     5 128.00k       /dev/sda(0),\
                                                  /dev/sdhx(1),\
                                                  /dev/sdhw(1),\
                                                  /dev/sdhv(1),\
                                                  /dev/sdhu(1)

       Reversing these steps will convert a given striped LV to linear.

       Mind  the  facts  that stripes are removed thus the capacity of the RaidLV will shrink and
       that changing the RaidLV layout will influence its performance.

       "lvconvert --stripes 1 vg/lv" for converting to 1 stripe will  inform  upfront  about  the
       reduced  size  to  allow  for  resizing  the content or growing the RaidLV before actually
       converting to 1 stripe.  The --force option is needed to allow stripe removing conversions
       to prevent data loss.

       Of course any interim step can be the intended last one (e.g. striped-> raid1).

RAID5 Variants

       raid5_ls
       · RAID5 left symmetric
       · Rotating parity N with data restart

       raid5_la
       · RAID5 left symmetric
       · Rotating parity N with data continuation

       raid5_rs
       · RAID5 right symmetric
       · Rotating parity 0 with data restart

       raid5_ra
       · RAID5 right asymmetric
       · Rotating parity 0 with data continuation

       raid5_n
       · RAID5 parity n
       · Dedicated parity device n used for striped/raid0 conversions
       · Used for RAID Takeover

RAID6 Variants

       raid6
       · RAID6 zero restart (aka left symmetric)
       · Rotating parity 0 with data restart
       · Same as raid6_zr

       raid6_zr
       · RAID6 zero restart (aka left symmetric)
       · Rotating parity 0 with data restart

       raid6_nr
       · RAID6 N restart (aka right symmetric)
       · Rotating parity N with data restart

       raid6_nc
       · RAID6 N continue
       · Rotating parity N with data continuation

       raid6_n_6
       · RAID6 last parity devices
       · Fixed dedicated last devices (P-Syndrome N-1 and Q-Syndrome N)
         with striped data used for striped/raid0 conversions
       · Used for RAID Takeover

       raid6_{ls,rs,la,ra}_6
       · RAID6 last parity device
       · Dedicated last parity device used for conversions from/to
         raid5_{ls,rs,la,ra}

       raid6_ls_6
       · RAID6 N continue
       · Same as raid5_ls for N-1 disks with fixed Q-Syndrome N
       · Used for RAID Takeover

       raid6_la_6
       · RAID6 N continue
       · Same as raid5_la for N-1 disks with fixed Q-Syndrome N
       · Used forRAID Takeover

       raid6_rs_6
       · RAID6 N continue
       · Same as raid5_rs for N-1 disks with fixed Q-Syndrome N
       · Used for RAID Takeover

       raid6_ra_6
       · RAID6 N continue
       · Same as raid5_ra for N-1 disks with fixed Q-Syndrome N
       · Used for RAID Takeover

History

       The  2.6.38-rc1 version of the Linux kernel introduced a device-mapper target to interface
       with the software RAID (MD) personalities.  This provided device-mapper  with  RAID  4/5/6
       capabilities  and  a  larger development community.  Later, support for RAID1, RAID10, and
       RAID1E (RAID 10 variants) were added.  Support for these new kernel RAID targets was added
       to  LVM  version  2.02.87.   The capabilities of the LVM raid1 type have surpassed the old
       mirror type.  raid1 is now recommended instead of mirror.  raid1 became  the  default  for
       mirroring in LVM version 2.02.100.