тонкое предоставление LVM (LVM thin provisioning)
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 analogous 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_metaN
(where N is 0,1,...).
If the repair works, the thin pool LV and its thin LVs can be
activated. User should manually check if repaired thin pool
kernel metadata has all data for all lvm2 known LVs by individual
activation of every thin LV. When all works, user should continue
with fsck of all filesystems present on these volumes. Once the
thin pool is considered fully functional user may remove
ThinPoolLV_metaN (the LV containing the damaged thin pool
metadata) for possible space reuse. For a better performance it
may be useful to pvmove the new repaired metadata LV (written to
previous pmspare volume) to a faster PV, e.g. SSD.
If the repair operation fails, the thin pool LV and its thin LVs
are not accessible and it may be necessary to restore their
content from a backup. In such case the content of unmodified
original damaged ThinPoolLV_metaN volume can be used by your
support for more advanced recovery methods.
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
Note: Thin pool metadata is compact so even small corruptions in
them may result in significant portions of mappings to be lost.
It is recommended to use fast resilient storage for them.
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 snapshots, 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.
— Autoextending —
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
extension. 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_threshold 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 approximately 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.
Configurable setting lvm.conf(5)
allocation/thin_pool_crop_metadata gives control over cropping to
15.81GiB to stay backward compatible with older versions of lvm2.
With enabled cropping there can be observed some problems when
using volumes above this size with thin tools (i.e. thin_repair).
Cropping should be enabled only when compatibility is required.
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
