Provided by: lvm2_2.02.133-1ubuntu10_amd64 bug

NAME

       lvmthin — LVM thin provisioning

DESCRIPTION

       Blocks  in  a  standard  logical  volume  are  allocated when the LV is
       created, but blocks in a thin provisioned logical volume are  allocated
       as they are written.  Because of this, a thin provisioned LV is given a
       virtual size, and can then be much  larger  than  physically  available
       storage.   The amount of physical storage provided for thin provisioned
       LVs can be increased later as the need arises.

       Blocks in a standard LV are allocated (during creation)  from  the  VG,
       but blocks in a thin LV are allocated (during use) from a special "thin
       pool LV".  The thin pool LV contains blocks of  physical  storage,  and
       blocks in thin LVs just reference blocks in the thin pool LV.

       A  thin  pool  LV must be created before thin LVs can be created within
       it.  A thin pool LV is created by combining two standard LVs:  a  large
       data LV that will hold blocks for thin LVs, and a metadata LV that will
       hold metadata.  The metadata tracks which data blocks  belong  to  each
       thin LV.

       Snapshots of thin LVs are efficient because the data blocks common to a
       thin LV and any of its snapshots are shared.  Snapshots may be taken of
       thin  LVs  or  of  other  thin  snapshots.   Blocks common to recursive
       snapshots are also shared in the thin pool.  There is no  limit  to  or
       degradation from sequences of snapshots.

       As thin LVs or snapshot LVs are written to, they consume data blocks in
       the thin pool.  As free data blocks in the  pool  decrease,  more  free
       blocks  may  need  to  be supplied.  This is done by extending the thin
       pool data LV with additional physical space from the VG.  Removing thin
       LVs  or  snapshots  from the thin pool can also free blocks in the thin
       pool.  However, removing LVs is not always an effective way of  freeing
       space  in  a  thin  pool because the amount is limited to the number of
       blocks not shared with other LVs in the pool.

       Incremental block allocation from thin pools  can  cause  thin  LVs  to
       become  fragmented.   Standard  LVs  generally  avoid  this  problem by
       allocating all the blocks at once during creation.

Thin Terms

       ThinDataLV
              thin data LV
              large LV created in a VG
              used by thin pool to store ThinLV blocks

       ThinMetaLV
              thin metadata LV
              small LV created in a VG
              used by thin pool to track data block usage

       ThinPoolLV
              thin pool LV
              combination of ThinDataLV and ThinMetaLV
              contains ThinLVs and SnapLVs

       ThinLV
              thin LV
              created from ThinPoolLV
              appears blank after creation

       SnapLV
              snapshot LV
              created from ThinPoolLV
              appears as a snapshot of another LV after creation

Thin Usage

       The primary method for using lvm thin provisioning:

   1. create ThinDataLV
       Create an LV that will hold thin pool data.

       lvcreate -n ThinDataLV -L LargeSize VG

       Example
       # lvcreate -n pool0 -L 10G vg

   2. create ThinMetaLV
       Create an LV that will hold thin pool metadata.

       lvcreate -n ThinMetaLV -L SmallSize VG

       Example
       # lvcreate -n pool0meta -L 1G vg

       # lvs
         LV        VG Attr       LSize
         pool0     vg -wi-a----- 10.00g
         pool0meta vg -wi-a----- 1.00g

   3. create ThinPoolLV
       Combine the data and metadata LVs into a thin pool LV.
       ThinDataLV is renamed to hidden ThinPoolLV_tdata.
       ThinMetaLV is renamed to hidden ThinPoolLV_tmeta.
       The new ThinPoolLV takes the previous name of ThinDataLV.

       lvconvert --type thin-pool --poolmetadata VG/ThinMetaLV VG/ThinDataLV

       Example
       # lvconvert --type thin-pool --poolmetadata vg/pool0meta vg/pool0

       # lvs vg/pool0
         LV    VG Attr       LSize  Pool Origin Data% Meta%
         pool0 vg twi-a-tz-- 10.00g      0.00   0.00

       # lvs -a
         LV            VG Attr       LSize
         pool0         vg twi-a-tz-- 10.00g
         [pool0_tdata] vg Twi-ao---- 10.00g
         [pool0_tmeta] vg ewi-ao---- 1.00g

   4. create ThinLV
       Create a new thin LV from the thin pool LV.
       The thin LV is created with a virtual size.
       Multiple new thin LVs may be created in the thin pool.
       Thin LV names must be unique in the VG.
       The '--type thin' option is inferred from the virtual size option.
       The --thinpool argument specifies which thin pool will
       contain the ThinLV.

       lvcreate -n ThinLV -V VirtualSize --thinpool VG/ThinPoolLV

       Example
       Create a thin LV in a thin pool:
       # lvcreate -n thin1 -V 1T --thinpool vg/pool0

       Create another thin LV in the same thin pool:
       # lvcreate -n thin2 -V 1T --thinpool vg/pool0

       # lvs vg/thin1 vg/thin2
         LV    VG Attr       LSize Pool  Origin Data%
         thin1 vg Vwi-a-tz-- 1.00t pool0        0.00
         thin2 vg Vwi-a-tz-- 1.00t pool0        0.00

   5. create SnapLV
       Create snapshots of an existing ThinLV or SnapLV.
       Do not specify -L, --size when creating a thin snapshot.
       A size argument will cause an old COW snapshot to be created.

       lvcreate -n SnapLV -s VG/ThinLV
       lvcreate -n SnapLV -s VG/PrevSnapLV

       Example
       Create first snapshot of an existing ThinLV:
       # lvcreate -n thin1s1 -s vg/thin1

       Create second snapshot of the same ThinLV:
       # lvcreate -n thin1s2 -s vg/thin1

       Create a snapshot of the first snapshot:
       # lvcreate -n thin1s1s1 -s vg/thin1s1

       # lvs vg/thin1s1 vg/thin1s2 vg/thin1s1s1
         LV        VG Attr       LSize Pool  Origin
         thin1s1   vg Vwi---tz-k 1.00t pool0 thin1
         thin1s2   vg Vwi---tz-k 1.00t pool0 thin1
         thin1s1s1 vg Vwi---tz-k 1.00t pool0 thin1s1

   6. activate SnapLV
       Thin snapshots are created with the persistent "activation skip"  flag,
       indicated  by  the  "k" attribute.  Use -K with lvchange or vgchange to
       activate thin snapshots with the "k" attribute.

       lvchange -ay -K VG/SnapLV

       Example
       # lvchange -ay -K vg/thin1s1

       # lvs vg/thin1s1
         LV      VG Attr       LSize Pool  Origin
         thin1s1 vg Vwi-a-tz-k 1.00t pool0 thin1

Thin Topics

       Alternate syntax for specifying type thin-pool
       Automatic pool metadata LV
       Specify devices for data and metadata LVs
       Tolerate device failures using raid
       Spare metadata LV
       Metadata check and repair
       Activation of thin snapshots
       Removing thin pool LVs, thin LVs and snapshots
       Manually manage free data space of thin pool LV
       Manually manage free metadata space of a thin pool LV
       Using fstrim to increase free space in a thin pool LV
       Automatically extend thin pool LV
       Data space exhaustion
       Metadata space exhaustion
       Automatic extend settings
       Zeroing
       Discard
       Chunk size
       Size of pool metadata LV
       Create a thin snapshot of an external, read only LV
       Convert a standard LV to a thin LV with an external origin
       Single step thin pool LV creation
       Single step thin pool LV and thin LV creation
       Merge thin snapshots
       XFS on snapshots

   Alternate syntax for specifying type thin-pool

       The fully specified syntax for creating a thin pool LV shown above is:

       lvconvert --type thin-pool --poolmetadata VG/ThinMetaLV VG/ThinDataLV

       An existing LV is converted to a thin pool  by  changing  its  type  to
       thin-pool.  An alternate syntax may be used for the same operation:

       lvconvert --thinpool VG/ThinDataLV --poolmetadata VG/ThinMetaLV

       The  thin-pool type is inferred by lvm; the --thinpool option is not an
       alias for --type thin-pool.  The use of the --thinpool option  here  is
       different  from  the  use of the --thinpool option when creating a thin
       LV, where it specifies the pool in which the thin LV is created.

   Automatic pool metadata LV

       A thin data LV can be converted to a thin pool LV without specifying  a
       thin  pool  metadata  LV.  LVM automatically creates a metadata LV from
       the same VG.

       lvcreate -n ThinDataLV -L LargeSize VG
       lvconvert --type thin-pool VG/ThinDataLV

       Example
       # lvcreate -n pool0 -L 10G vg
       # lvconvert --type thin-pool vg/pool0

       # lvs -a
         pool0           vg          twi-a-tz--  10.00g
         [pool0_tdata]   vg          Twi-ao----  10.00g
         [pool0_tmeta]   vg          ewi-ao----  16.00m

   Specify devices for data and metadata LVs

       The data and metadata LVs in a thin pool are best created  on  separate
       physical devices.  To do that, specify the device name(s) at the end of
       the lvcreate line.  It can be especially helpful to  use  fast  devices
       for the metadata LV.

       lvcreate -n ThinDataLV -L LargeSize VG LargePV
       lvcreate -n ThinMetaLV -L SmallSize VG SmallPV
       lvconvert --type thin-pool --poolmetadata VG/ThinMetaLV VG/ThinDataLV

       Example
       # lvcreate -n pool0 -L 10G vg /dev/sdA
       # lvcreate -n pool0meta -L 1G vg /dev/sdB
       # lvconvert --type thin-pool --poolmetadata vg/pool0meta vg/pool0

       lvm.conf(5) thin_pool_metadata_require_separate_pvs
       controls the default PV usage for thin pool creation.

   Tolerate device failures using raid

       To  tolerate  device  failures,  use raid for the pool data LV and pool
       metadata LV.  This is especially recommended for pool metadata LVs.

       lvcreate --type raid1 -m 1 -n ThinMetaLV -L SmallSize VG PVA PVB
       lvcreate --type raid1 -m 1 -n ThinDataLV -L LargeSize VG PVC PVD
       lvconvert --type thin-pool --poolmetadata VG/ThinMetaLV VG/ThinDataLV

       Example
       # lvcreate --type raid1 -m 1 -n pool0 -L 10G vg /dev/sdA /dev/sdB
       # lvcreate --type raid1 -m 1 -n pool0meta -L 1G vg /dev/sdC /dev/sdD
       # lvconvert --type thin-pool --poolmetadata vg/pool0meta vg/pool0

   Spare metadata LV

       The first time a thin pool LV is  created,  lvm  will  create  a  spare
       metadata LV in the VG.  This behavior can be controlled with the option
       --poolmetadataspare y|n.  (Future thin pool creations will also attempt
       to create the pmspare LV if none exists.)

       To  create the pmspare ("pool metadata spare") LV, lvm first creates an
       LV with a default name, e.g. lvol0, and then  converts  this  LV  to  a
       hidden LV with the _pmspare suffix, e.g. lvol0_pmspare.

       One pmspare LV is kept in a VG to be used for any thin pool.

       The  pmspare  LV  cannot  be  created  explicitly,  but  may be removed
       explicitly.

       Example
       # lvcreate -n pool0 -L 10G vg
       # lvcreate -n pool0meta -L 1G vg
       # lvconvert --type thin-pool --poolmetadata vg/pool0meta vg/pool0

       # lvs -a
         [lvol0_pmspare] vg          ewi-------
         pool0           vg          twi---tz--
         [pool0_tdata]   vg          Twi-------

         [pool0_tmeta]   vg          ewi-------
       The "Metadata check and  repair"  section  describes  the  use  of  the
       pmspare LV.

   Metadata check and repair

       If  thin  pool metadata is damaged, it may be repairable.  Checking and
       repairing thin pool metadata is analagous to running  fsck  on  a  file
       system.

       When  a  thin  pool LV is activated, lvm runs the thin_check command to
       check the correctness of the metadata on the pool metadata LV.

       lvm.conf(5) thin_check_executable
       can be set to an empty string ("")  to  disable  the  thin_check  step.
       This is not recommended.

       lvm.conf(5) thin_check_options
       controls the command options used for the thin_check command.

       If  the  thin_check command finds a problem with the metadata, the thin
       pool LV is not activated, and  the  thin  pool  metadata  needs  to  be
       repaired.

       Simple  repair commands are not always successful.  Advanced repair may
       require editing thin pool metadata and lvm metadata.  Newer versions of
       the  kernel and lvm tools may be more successful at repair.  Report the
       details of damaged thin metadata to get the best advice on recovery.

       Command to repair a thin pool:
       lvconvert --repair VG/ThinPoolLV

       Repair performs the following steps:

       1. Creates a new, repaired copy of the metadata.
       lvconvert runs the thin_repair command to read  damaged  metadata  from
       the  existing  pool  metadata LV, and writes a new repaired copy to the
       VG's pmspare LV.

       2. Replaces the thin pool metadata LV.
       If step 1 is successful, the thin pool metadata LV is replaced with the
       pmspare  LV  containing the corrected metadata.  The previous thin pool
       metadata LV, containing the damaged metadata, becomes visible with  the
       new name ThinPoolLV_tmetaN (where N is 0,1,...).

       If  the  repair  works,  the  thin  pool  LV  and  its  thin LVs can be
       activated, and the LV containing the damaged thin pool metadata can  be
       removed.   It  may  be  useful  to move the new metadata LV (previously
       pmspare) to a better PV.

       If the repair does not work, the thin pool LV  and  its  thin  LVs  are
       lost.

       If  metadata  is  manually restored with thin_repair directly, the pool
       metadata LV can be manually swapped  with  another  LV  containing  new
       metadata:

       lvconvert --thinpool VG/ThinPoolLV --poolmetadata VG/NewThinMetaLV

   Activation of thin snapshots

       When  a  thin  snapshot  LV  is  created,  it  is  by default given the
       "activation skip" flag.  This flag is indicated by  the  "k"  attribute
       displayed by lvs:

       # lvs vg/thin1s1
         LV         VG  Attr       LSize Pool  Origin
         thin1s1    vg  Vwi---tz-k 1.00t pool0 thin1

       This  flag causes the snapshot LV to be skipped, i.e. not activated, by
       normal activation commands.  The skipping behavior does  not  apply  to
       deactivation commands.

       A  snapshot LV with the "k" attribute can be activated using the -K (or
       --ignoreactivationskip) option in addition  to  the  standard  -ay  (or
       --activate y) option.

       Command to activate a thin snapshot LV:
       lvchange -ay -K VG/SnapLV

       The  persistent  "activation  skip"  flag  can  be  turned  off  during
       lvcreate, or later with lvchange using the -kn (or  --setactivationskip
       n)  option.  It can be turned on again with -ky (or --setactivationskip
       y).

       When the "activation skip" flag is removed, normal activation  commands
       will activate the LV, and the -K activation option is not needed.

       Command to create snapshot LV without the activation skip flag:
       lvcreate -kn -n SnapLV -s VG/ThinLV

       Command to remove the activation skip flag from a snapshot LV:
       lvchange -kn VG/SnapLV

       lvm.conf(5) auto_set_activation_skip
       controls the default activation skip setting used by lvcreate.

   Removing thin pool LVs, thin LVs and snapshots

       Removing a thin LV and its related snapshots returns the blocks it used
       to the thin pool LV.  These blocks will be reused for  other  thin  LVs
       and snapshots.

       Removing  a  thin  pool LV removes both the data LV and metadata LV and
       returns the space to the VG.

       lvremove of thin pool LVs, thin LVs and snapshots  cannot  be  reversed
       with vgcfgrestore.

       vgcfgbackup does not back up thin pool metadata.

   Manually manage free data space of thin pool LV

       The  available  free  space in a thin pool LV can be displayed with the
       lvs command.  Free space can be added by extending the thin pool LV.

       Command to extend thin pool data space:
       lvextend -L Size VG/ThinPoolLV

       Example
       1. A thin pool LV is using 26.96% of its data blocks.
       # lvs
         LV    VG           Attr       LSize   Pool  Origin Data%
         pool0 vg           twi-a-tz--  10.00g               26.96

       2. Double the amount of physical space in the thin pool LV.
       # lvextend -L+10G vg/pool0

       3. The percentage of used data blocks is half the previous value.
       # lvs
         LV    VG           Attr       LSize   Pool  Origin Data%
         pool0 vg           twi-a-tz--  20.00g               13.48

       Other methods of increasing free data space in a thin pool  LV  include
       removing  a  thin LV and its related snapsots, or running fstrim on the
       file system using a thin LV.

   Manually manage free metadata space of a thin pool LV

       The available metadata space in a thin pool LV can  be  displayed  with
       the lvs -o+metadata_percent command.

       Command to extend thin pool metadata space:
       lvextend --poolmetadatasize Size VG/ThinPoolLV

       Example
       1. A thin pool LV is using 12.40% of its metadata blocks.
       # lvs -oname,size,data_percent,metadata_percent vg/pool0
         LV    LSize   Data%  Meta%
         pool0  20.00g  13.48  12.40

       2.  Display  a  thin  pool  LV with its component thin data LV and thin
       metadata LV.
       # lvs -a -oname,attr,size vg
         LV              Attr       LSize
         pool0           twi-a-tz--  20.00g
         [pool0_tdata]   Twi-ao----  20.00g
         [pool0_tmeta]   ewi-ao----  12.00m

       3. Double the amount of physical space in the thin metadata LV.
       # lvextend --poolmetadatasize +12M vg/pool0

       4. The percentage of used metadata blocks is half the previous value.
       # lvs -a -oname,size,data_percent,metadata_percent vg
         LV              LSize   Data%  Meta%
         pool0            20.00g  13.48   6.20
         [pool0_tdata]    20.00g
         [pool0_tmeta]    24.00m

   Using fstrim to increase free space in a thin pool LV

       Removing files in a file system on top of a thin LV does not  generally
       add  free  space  back  to  the thin pool.  Manually running the fstrim
       command can return space back to the thin pool that had  been  used  by
       removed files.  fstrim uses discards and will not work if the thin pool
       LV has discards mode set to ignore.

       Example
       A thin pool has 10G of physical data space, and a thin LV has a virtual
       size  of  100G.   Writing a 1G file to the file system reduces the free
       space in the thin pool by 10% and increases the virtual  usage  of  the
       file system by 1%.  Removing the 1G file restores the virtual 1% to the
       file system, but does not restore the physical 10% to  the  thin  pool.
       The fstrim command restores the physical space to the thin pool.

       # lvs -a -oname,attr,size,pool_lv,origin,data_percent,metadata_percent vg
       LV              Attr       LSize   Pool  Origin Data%  Meta%
       pool0           twi-a-tz--  10.00g               47.01  21.03
       thin1           Vwi-aotz-- 100.00g pool0          2.70

       # df -h /mnt/X
       Filesystem            Size  Used Avail Use% Mounted on
       /dev/mapper/vg-thin1   99G  1.1G   93G   2% /mnt/X

       # dd if=/dev/zero of=/mnt/X/1Gfile bs=4096 count=262144; sync

       # lvs
       pool0           vg   twi-a-tz--  10.00g               57.01  25.26
       thin1           vg   Vwi-aotz-- 100.00g pool0          3.70

       # df -h /mnt/X
       /dev/mapper/vg-thin1   99G  2.1G   92G   3% /mnt/X

       # rm /mnt/X/1Gfile

       # lvs
       pool0           vg   twi-a-tz--  10.00g               57.01  25.26
       thin1           vg   Vwi-aotz-- 100.00g pool0          3.70

       # df -h /mnt/X
       /dev/mapper/vg-thin1   99G  1.1G   93G   2% /mnt/X

       # fstrim -v /mnt/X

       # lvs
       pool0           vg   twi-a-tz--  10.00g               47.01  21.03
       thin1           vg   Vwi-aotz-- 100.00g pool0          2.70

       The  "Discard"  section covers an option for automatically freeing data
       space in a thin pool.

   Automatically extend thin pool LV

       The lvm daemon dmeventd (lvm2-monitor) monitors the data usage of  thin
       pool  LVs and extends them when the usage reaches a certain level.  The
       necessary free space must exist in the VG  to  extend  thin  pool  LVs.
       Monitoring and extension of thin pool LVs are controlled independently.

       monitoring

       When  a thin pool LV is activated, dmeventd will begin monitoring it by
       default.

       Command to start or stop dmeventd monitoring a thin pool LV:
       lvchange --monitor {y|n} VG/ThinPoolLV

       The current dmeventd monitoring  status  of  a  thin  pool  LV  can  be
       displayed with the command lvs -o+seg_monitor.

       autoextend

       dmeventd  should  be configured to extend thin pool LVs before all data
       space is used.  Warnings are emitted through syslog when the use  of  a
       thin  pool reaches 80%, 85%, 90% and 95%.  (See the section "Data space
       exhaustion" for the effects of not extending  a  thin  pool  LV.)   The
       point  at  which  dmeventd  extends  thin  pool LVs, and the amount are
       controlled with two configuration settings:

       lvm.conf(5) thin_pool_autoextend_threshold
       is a percentage full value that defines when the thin pool LV should be
       extended.   Setting  this  to  100  disables  automatic extention.  The
       minimum value is 50.

       lvm.conf(5) thin_pool_autoextend_percent
       defines how much extra data space should be added to the thin  pool  LV
       from the VG, in percent of its current size.

       disabling

       There  are  multiple  ways  that  extension  of  thin  pools  could  be
       prevented:

       · If the dmeventd daemon is not running,  no  monitoring  or  automatic
         extension will occur.

       · Even  when  dmeventd  is running, all monitoring can be disabled with
         the lvm.conf monitoring setting.

       · To activate or  create  a  thin  pool  LV  without  interacting  with
         dmeventd,  the  --ignoremonitoring  option  can  be  used.  With this
         option, the command will not ask dmeventd to monitor  the  thin  pool
         LV.

       · Setting  thin_pool_autoextend_threshould  to  100  disables automatic
         extension of thin pool LVs, even  if  they  are  being  monitored  by
         dmeventd.

       Example
       If        thin_pool_autoextend_threshold        is        70        and
       thin_pool_autoextend_percent is 20, whenever a pool exceeds 70%  usage,
       it  will  be  extended  by another 20%.  For a 1G pool, using 700M will
       trigger a resize to 1.2G. When the usage exceeds 840M, the pool will be
       extended to 1.44G, and so on.

   Data space exhaustion

       When  properly  managed, thin pool data space should be extended before
       it is all used (see the section "Automatically extend thin  pool  LV").
       If  thin pool data space is already exhausted, it can still be extended
       (see the section "Manually manage free data space of thin pool LV".)

       The  behavior  of  a  full  thin  pool   is   configurable   with   the
       --errorwhenfull  y|n option to lvcreate or lvchange.  The errorwhenfull
       setting applies only to writes; reading thin LVs can continue even when
       data space is exhausted.

       Command to change the handling of a full thin pool:
       lvchange --errorwhenfull {y|n} VG/ThinPoolLV

       lvm.conf(5) error_when_full
       controls the default error when full behavior.

       The  current  setting  of  a  thin  pool  LV  can be displayed with the
       command: lvs -o+lv_when_full.

       The errorwhenfull setting does not effect the monitoring and autoextend
       settings,  and  the  monitoring/autoextend  settings  do not effect the
       errorwhenfull setting.  It is only when monitoring/autoextend  are  not
       effective that the thin pool becomes full and the errorwhenfull setting
       is applied.

       errorwhenfull n

       This is the default.  Writes to thin LVs are accepted and queued,  with
       the  expectation that pool data space will be extended soon.  Once data
       space is extended, the queued writes will be processed,  and  the  thin
       pool will return to normal operation.

       While waiting to be extended, the thin pool will queue writes for up to
       60 seconds (the default).  If data space has not  been  extended  after
       this  time,  the queued writes will return an error to the caller, e.g.
       the file system.  This can result in file system  corruption  for  non-
       journaled file systems that may require fsck.  When a thin pool returns
       errors for writes to a thin LV, any file system is  subject  to  losing
       unsynced user data.

       The  60 second timeout can be changed or disabled with the dm-thin-pool
       kernel module option no_space_timeout.  This option sets the number  of
       seconds  that  thin  pools will queue writes.  If set to 0, writes will
       not time out.  Disabling timeouts can result in the system running  out
       of  resources,  memory  exhaustion,  hung  tasks,  and deadlocks.  (The
       timeout applies to all thin pools on the system.)

       errorwhenfull y

       Writes to thin LVs immediately return  an  error,  and  no  writes  are
       queued.   In  the  case of a file system, this can result in corruption
       that may require fsck (the specific consequences depend on the thin  LV
       user.)

       data percent

       When  data space is exhausted, the lvs command displays 100 under Data%
       for the thin pool LV:

       # lvs vg/pool0
         LV     VG           Attr       LSize   Pool  Origin Data%
         pool0  vg           twi-a-tz-- 512.00m              100.00

       causes

       A thin pool may run out of data space for any of the following reasons:

       · Automatic extension of the thin pool is disabled, and the  thin  pool
         is  not  manually  extended.   (Disabling  automatic extension is not
         recommended.)

       · The dmeventd daemon is not running and the thin pool is not  manually
         extended.  (Disabling dmeventd is not recommended.)

       · Automatic  extension  of  the thin pool is too slow given the rate of
         writes to thin LVs in the pool.  (This can be addressed by tuning the
         thin_pool_autoextend_threshold and thin_pool_autoextend_percent.  See
         "Automatic extend settings".)

       · The VG does not have enough free blocks to extend the thin pool.

   Metadata space exhaustion

       If thin pool metadata space is  exhausted  (or  a  thin  pool  metadata
       operation  fails),  errors  will  be returned for IO operations on thin
       LVs.

       When metadata space is exhausted, the lvs command  displays  100  under
       Meta% for the thin pool LV:

       # lvs -o lv_name,size,data_percent,metadata_percent vg/pool0
         LV    LSize Data%  Meta%
         pool0              100.00

       The same reasons for thin pool data space exhaustion apply to thin pool
       metadata space.

       Metadata space exhaustion can lead to inconsistent thin  pool  metadata
       and  inconsistent  file  systems,  so  the  response  requires  offline
       checking and repair.

       1. Deactivate the thin pool LV, or reboot the system  if  this  is  not
       possible.

       2. Repair thin pool with lvconvert --repair.
          See "Metadata check and repair".

       3. Extend pool metadata space with lvextend --poolmetadatasize.
          See "Manually manage free metadata space of a thin pool LV".

       4. Check and repair file system with fsck.

   Automatic extend settings

       Thin  pool LVs can be extended according to preset values.  The presets
       determine if the LV should be extended based on how full it is, and  if
       so by how much.  When dmeventd monitors thin pool LVs, it uses lvextend
       with these presets.  (See "Automatically extend thin pool LV".)

       Command to extend a thin pool data LV using presets:
       lvextend --use-policies VG/ThinPoolLV

       The command uses these settings:

       lvm.conf(5) thin_pool_autoextend_threshold
       autoextend the LV when its usage exceeds this percent.

       lvm.conf(5) thin_pool_autoextend_percent
       autoextend the LV by this much additional space.

       To see the default values of these settings, run:

       lvmconfig --type default --withcomment
              activation/thin_pool_autoextend_threshold

       lvmconfig --type default --withcomment
              activation/thin_pool_autoextend_percent

       To change these values globally, edit lvm.conf(5).

       To change these values on a per-VG or per-LV basis, attach a  "profile"
       to  the  VG or LV.  A profile is a collection of config settings, saved
       in a local text file  (using  the  lvm.conf  format).   lvm  looks  for
       profiles  in  the  profile_dir directory, e.g. /etc/lvm/profile/.  Once
       attached to a VG or LV, lvm  will  process  the  VG  or  LV  using  the
       settings  from the attached profile.  A profile is named and referenced
       by its file name.

       To use a profile to customize the lvextend settings for an LV:

       · Create a file containing settings, saved  in  profile_dir.   For  the
         profile_dir location, run:
         lvmconfig config/profile_dir

       · Attach the profile to an LV, using the command:
         lvchange --metadataprofile ProfileName VG/ThinPoolLV

       · Extend the LV using the profile settings:
         lvextend --use-policies VG/ThinPoolLV

       Example
       # lvmconfig config/profile_dir
       profile_dir="/etc/lvm/profile"

       # cat /etc/lvm/profile/pool0extend.profile
       activation {
               thin_pool_autoextend_threshold=50
               thin_pool_autoextend_percent=10
       }

       # lvchange --metadataprofile pool0extend vg/pool0

       # lvextend --use-policies vg/pool0

       Notes

       · A  profile  is  attached  to  a  VG  or  LV  by  name, where the name
         references a local file in  profile_dir.   If  the  VG  is  moved  to
         another machine, the file with the profile also needs to be moved.

       · Only certain settings can be used in a VG or LV profile, see:
         lvmconfig --type profilable-metadata.

       · An LV without a profile of its own will inherit the VG profile.

       · Remove a profile from an LV using the command:
         lvchange --detachprofile VG/ThinPoolLV.

       · Commands  can  also have profiles applied to them.  The settings that
         can be applied to a command are different than the settings that  can
         be  applied  to a VG or LV.  See lvmconfig --type profilable-command.
         To apply a profile to a command, write a  profile,  save  it  in  the
         profile   directory,   and   run   the   command  using  the  option:
         --commandprofile ProfileName.

   Zeroing

       When a thin pool provisions a new data block for a  thin  LV,  the  new
       block  is  first overwritten with zeros.  The zeroing mode is indicated
       by the "z" attribute displayed by lvs.  The option -Z (or  --zero)  can
       be added to commands to specify the zeroing mode.

       Command to set the zeroing mode when creating a thin pool LV:
       lvconvert --type thin-pool -Z{y|n}
              --poolmetadata VG/ThinMetaLV VG/ThinDataLV

       Command to change the zeroing mode of an existing thin pool LV:
       lvchange -Z{y|n} VG/ThinPoolLV

       If  zeroing  mode  is  changed  from "n" to "y", previously provisioned
       blocks are not zeroed.

       Provisioning of large zeroed chunks impacts performance.

       lvm.conf(5) thin_pool_zero
       controls the default zeroing mode used when creating a thin pool.

   Discard

       The discard behavior of a thin pool LV determines how discard  requests
       are handled.  Enabling discard under a file system may adversely affect
       the  file  system  performance  (see  the  section  on  fstrim  for  an
       alternative.)  Possible discard behaviors:

       ignore: Ignore any discards that are received.

       nopassdown:  Process any discards in the thin pool itself and allow the
       no longer needed extends to be overwritten by new data.

       passdown: Process discards in the thin pool (as with  nopassdown),  and
       pass  the discards down the the underlying device.  This is the default
       mode.

       Command to display the current discard mode of a thin pool LV:
       lvs -o+discards VG/ThinPoolLV

       Command to set the discard mode when creating a thin pool LV:
       lvconvert --discards {ignore|nopassdown|passdown}
              --type thin-pool --poolmetadata VG/ThinMetaLV VG/ThinDataLV

       Command to change the discard mode of an existing thin pool LV:
       lvchange --discards {ignore|nopassdown|passdown} VG/ThinPoolLV

       Example
       # lvs -o name,discards vg/pool0
       pool0 passdown

       # lvchange --discards ignore vg/pool0

       lvm.conf(5) thin_pool_discards
       controls the default discards mode used when creating a thin pool.

   Chunk size

       The size of data blocks managed by a thin pool can  be  specified  with
       the  --chunksize  option when the thin pool LV is created.  The default
       unit is KiB. The value must be a multiple of 64KiB  between  64KiB  and
       1GiB.

       When a thin pool is used primarily for the thin provisioning feature, a
       larger value is optimal.  To optimize for  many  snapshots,  a  smaller
       value reduces copying time and consumes less space.

       Command to display the thin pool LV chunk size:
       lvs -o+chunksize VG/ThinPoolLV

       Example
       # lvs -o name,chunksize
         pool0 64.00k

       lvm.conf(5) thin_pool_chunk_size
       controls the default chunk size used when creating a thin pool.

       The default value is shown by:
       lvmconfig --type default allocation/thin_pool_chunk_size

   Size of pool metadata LV

       The  amount  of  thin  metadata  depends  on how many blocks are shared
       between thin LVs (i.e. through  snapshots).   A  thin  pool  with  many
       snapshots  may  need a larger metadata LV.  Thin pool metadata LV sizes
       can be from 2MiB to 16GiB.

       When using lvcreate to create what will become a thin metadata LV,  the
       size is specified with the -L|--size option.

       When  an LVM command automatically creates a thin metadata LV, the size
       is specified with the --poolmetadatasize option.  When this  option  is
       not  given, LVM automatically chooses a size based on the data size and
       chunk size.

       It can be hard to predict the amount of metadata  space  that  will  be
       needed,  so it is recommended to start with a size of 1GiB which should
       be enough for all practical purposes.  A  thin  pool  metadata  LV  can
       later be manually or automatically extended if needed.

   Create a thin snapshot of an external, read only LV

       Thin  snapshots  are  typically  taken  of other thin LVs or other thin
       snapshot LVs within the same thin pool.  It is also  possible  to  take
       thin  snapshots of external, read only LVs.  Writes to the snapshot are
       stored in the thin pool, and the external LV is used to read  unwritten
       parts of the thin snapshot.

       lvcreate -n SnapLV -s VG/ExternalOriginLV --thinpool VG/ThinPoolLV

       Example
       # lvchange -an vg/lve
       # lvchange --permission r vg/lve
       # lvcreate -n snaplve -s vg/lve --thinpool vg/pool0

       # lvs vg/lve vg/snaplve
         LV      VG  Attr       LSize  Pool  Origin Data%
         lve     vg  ori------- 10.00g
         snaplve vg  Vwi-a-tz-- 10.00g pool0 lve      0.00

   Convert a standard LV to a thin LV with an external origin

       A new thin LV can be created and given the name of an existing standard
       LV.  At the same time, the existing LV is  converted  to  a  read  only
       external  LV  with  a  new name.  Unwritten portions of the thin LV are
       read from the external LV.  The new name given to the existing  LV  can
       be specified with --originname, otherwise the existing LV will be given
       a default name, e.g. lvol#.

       Convert ExampleLV into a read  only  external  LV  with  the  new  name
       NewExternalOriginLV,  and  create  a  new  thin  LV  that  is given the
       previous name of ExampleLV.

       lvconvert --type thin --thinpool VG/ThinPoolLV
              --originname NewExternalOriginLV --thin VG/ExampleLV

       Example
       # lvcreate -n lv_example -L 10G vg

       # lvs
         lv_example      vg          -wi-a-----  10.00g

       # lvconvert --type thin --thinpool vg/pool0
                 --originname lv_external --thin vg/lv_example

       # lvs
         LV              VG          Attr       LSize   Pool  Origin
         lv_example      vg          Vwi-a-tz--  10.00g pool0 lv_external
         lv_external     vg          ori-------  10.00g

   Single step thin pool LV creation

       A thin pool LV can be created with a single  lvcreate  command,  rather
       than  using lvconvert on existing LVs.  This one command creates a thin
       data LV, a thin metadata LV, and combines the two into a thin pool LV.

       lvcreate --type thin-pool -L LargeSize -n ThinPoolLV VG

       Example
       # lvcreate --type thin-pool -L8M -n pool0 vg

       # lvs vg/pool0
         LV    VG  Attr       LSize Pool Origin Data%
         pool0 vg  twi-a-tz-- 8.00m               0.00

       # lvs -a
         pool0           vg          twi-a-tz--   8.00m
         [pool0_tdata]   vg          Twi-ao----   8.00m
         [pool0_tmeta]   vg          ewi-ao----   8.00m

   Single step thin pool LV and thin LV creation

       A thin pool LV and a thin LV can be  created  with  a  single  lvcreate
       command.   This one command creates a thin data LV, a thin metadata LV,
       combines the two into a thin pool LV, and creates a thin LV in the  new
       pool.
       -L LargeSize specifies the physical size of the thin pool LV.
       -V VirtualSize specifies the virtual size of the thin LV.

       lvcreate -V VirtualSize -L LargeSize
              -n ThinLV --thinpool VG/ThinPoolLV

       Equivalent to:
       lvcreate --type thin-pool -L LargeSize VG/ThinPoolLV
       lvcreate -n ThinLV -V VirtualSize --thinpool VG/ThinPoolLV

       Example
       # lvcreate -L8M -V2G -n thin1 --thinpool vg/pool0

       # lvs -a
         pool0           vg          twi-a-tz--   8.00m
         [pool0_tdata]   vg          Twi-ao----   8.00m
         [pool0_tmeta]   vg          ewi-ao----   8.00m
         thin1           vg          Vwi-a-tz--   2.00g pool0

   Merge thin snapshots

       A  thin  snapshot  can  be  merged  into  its  origin thin LV using the
       lvconvert --merge command.  The result of a snapshot merge is that  the
       origin  thin  LV takes the content of the snapshot LV, and the snapshot
       LV is removed.  Any content that was unique to the origin  thin  LV  is
       lost after the merge.

       Because  a  merge changes the content of an LV, it cannot be done while
       the LVs are open, e.g. mounted.  If a merge is initiated while the  LVs
       are  open,  the effect of the merge is delayed until the origin thin LV
       is next activated.

       lvconvert --merge VG/SnapLV

       Example
       # lvs vg
         LV      VG Attr       LSize   Pool  Origin
         pool0   vg twi-a-tz--  10.00g
         thin1   vg Vwi-a-tz-- 100.00g pool0
         thin1s1 vg Vwi-a-tz-k 100.00g pool0 thin1

       # lvconvert --merge vg/thin1s1

       # lvs vg
         LV      VG Attr       LSize   Pool  Origin
         pool0   vg twi-a-tz--  10.00g
         thin1   vg Vwi-a-tz-- 100.00g pool0

       Example
       Delayed merging of open LVs.

       # lvs vg
         LV      VG Attr       LSize   Pool  Origin
         pool0   vg twi-a-tz--  10.00g
         thin1   vg Vwi-aotz-- 100.00g pool0
         thin1s1 vg Vwi-aotz-k 100.00g pool0 thin1

       # df
       /dev/mapper/vg-thin1            100G   33M  100G   1% /mnt/X
       /dev/mapper/vg-thin1s1          100G   33M  100G   1% /mnt/Xs

       # ls /mnt/X
       file1 file2 file3
       # ls /mnt/Xs
       file3 file4 file5

       # lvconvert --merge vg/thin1s1
       Logical volume vg/thin1s1 contains a filesystem in use.
       Delaying merge since snapshot is open.
       Merging of thin snapshot thin1s1 will occur on next activation.

       # umount /mnt/X
       # umount /mnt/Xs

       # lvs -a vg
         LV              VG   Attr       LSize   Pool  Origin
         pool0           vg   twi-a-tz--  10.00g
         [pool0_tdata]   vg   Twi-ao----  10.00g
         [pool0_tmeta]   vg   ewi-ao----   1.00g
         thin1           vg   Owi-a-tz-- 100.00g pool0
         [thin1s1]       vg   Swi-a-tz-k 100.00g pool0 thin1

       # lvchange -an vg/thin1
       # lvchange -ay vg/thin1

       # mount /dev/vg/thin1 /mnt/X

       # ls /mnt/X
       file3 file4 file5

   XFS on snapshots

       Mounting an XFS file system on a new snapshot LV requires attention  to
       the  file system's log state and uuid.  On the snapshot LV, the xfs log
       will contain a dummy transaction, and the xfs uuid will match the  uuid
       from the file system on the origin LV.

       If  the snapshot LV is writable, mounting will recover the log to clear
       the dummy transaction, but will require skipping the uuid check:

       mount /dev/VG/SnapLV /mnt -o nouuid

       Or, the uuid can be changed on disk before mounting:

       xfs_admin -U generate /dev/VG/SnapLV
       mount /dev/VG/SnapLV /mnt

       If the snapshot LV is readonly, the log recovery and uuid check need to
       be skipped while mounting readonly:

       mount /dev/VG/SnapLV /mnt -o ro,nouuid,norecovery

SEE ALSO

       lvm(8),    lvm.conf(5),    lvmconfig(8),   lvcreate(8),   lvconvert(8),
       lvchange(8),   lvextend(8),    lvremove(8),    lvs(8),    thin_dump(8),
       thin_repair(8) thin_restore(8)