focal (7) lvmthin.7.gz

Provided by: lvm2_2.03.07-1ubuntu1_amd64 bug

NAME

       lvmthin — LVM thin provisioning

DESCRIPTION

       Blocks  in  a  standard  lvm(8) Logical Volume (LV) are allocated when the LV is created, but blocks in a
       thin provisioned LV 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 Volume Group (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 ThinPoolLV VG

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

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

       # 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 --snapshot VG/ThinLV
       lvcreate -n SnapLV --snapshot 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

       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

   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/repair 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 repair.  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 fs repair (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.

   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  extents  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 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 --type thin -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

       After the first mount with the above approach, the UUID can subsequently be changed using:

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

       Once the UUID has been changed, the mount command will no longer require the nouuid option.

       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)