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man: update lvmcache cache setting

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Correct setting of migration_threshold and add clarify
how the user can restore default cache settings for cache policies.
Also document mq aliasing with smq for newer kernels.
This commit is contained in:
Zdenek Kabelac 2023-02-11 20:32:45 +01:00
parent 31076942f1
commit 02fdb2480e
1 changed files with 24 additions and 4 deletions
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28
man/lvmcache.7_main
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@ -501,7 +501,18 @@ Checking migration threshold (in sectors) of running cached LV:
.br
.B lvs -o+kernel_cache_settings VG/LV
.
.SS dm-cache migration threshold
.SS dm-cache cache settings
.
To set dm-cache cache setting use:
.P
--cachesettings 'option1=N option2=N ...'
.P
To unset/drop cache setting and restore its default kernel value
use special keyword 'default' as option parameter:
.P
--cachesettings 'option1=default option2=default ...'
.
.SS dm-cache migration threshold cache setting
.
Migrating data between the origin and cache LV uses bandwidth.
The user can set a throttle to prevent more than a certain amount of
@ -510,17 +521,22 @@ account of normal io traffic going to the devices.
.P
User can set migration threshold via cache policy settings as
"migration_threshold=<#sectors>" to set the maximum number
of sectors being migrated, the default being 2048 sectors (1 MiB).
of sectors being migrated, the default being 2048 sectors (1 MiB)
or 8 cache chunks whichever of those two values is larger.
.P
Command to set migration threshold to 2 MiB (4096 sectors):
.P
.B lvcreate --cachepolicy 'migration_threshold=4096' VG/LV
.B lvcreate --cachesettings 'migration_threshold=4096' VG/LV
.P
Command to display the migration threshold:
.P
.B lvs -o+kernel_cache_settings,cache_settings VG/LV
.br
.B lvs -o+chunksize VG/LV
.P
Command to restore/revert to default value:
.P
.B lvchange --cachesettings 'migration_threshold=default' VG/LV
.
.SS dm-cache cache policy
.
@ -533,6 +549,11 @@ back (flush) all cached writes to the origin LV.
The older "mq" policy has a number of tunable parameters. The defaults are
chosen to be suitable for the majority of systems, but in special
circumstances, changing the settings can improve performance.
Newer kernels however alias this policy with "smq" policy. Cache settings
used to configure "mq" policy [random_threshold, sequential_threshold,
discard_promote_adjustment, read_promote_adjustment,
write_promote_adjustment] are thus silently ignored also performance
matches "smq".
.P
With the --cachepolicy and --cachesettings options, the cache policy and
settings can be set when caching is started, or changed on an existing
@ -582,7 +603,6 @@ cache_settings {
smq {
.RS
migration_threshold=8192
random_threshold=4096
.RE
}
.RE