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man lvmcache: updates for mode and policy

Browse Source 
Correct some things, e.g. set mode and policy on
the cache lv, not the pool, lvm.conf field for
mode changed.

Add smq which was missing.

Make the sections on cache mode and cache policy
consistent in structure and style.
This commit is contained in:
David Teigland 2015-10-01 11:39:07 -05:00
parent 26da6a3e10
commit 5e5d48348b
1 changed files with 43 additions and 33 deletions
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76
man/lvmcache.7.in
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@ -195,6 +195,7 @@ Users who are concerned about the possibility of failures in their fast
devices that could lead to data loss might consider making their cache
pool sub-LVs redundant.
.I Example
.nf
0. Create an origin LV we wish to cache
# lvcreate \-L 10G \-n lv1 vg /dev/slow_devs
@ -228,14 +229,18 @@ from the cache pool back to the origin LV. This mode will increase
performance, but the loss of a device associated with the cache pool LV
can result in lost data.
The cache mode can be specified with the --cachemode option when a cache
pool LV is created.
With the \-\-cachemode option, the cache mode can be set when creating a
cache LV, or changed on an existing cache LV. The current cache mode of a
cache LV can be displayed with the cache_mode reporting option:
.B lvs \-o+cache_mode VG/CacheLV
.BR lvm.conf (5)
.B cache_pool_cachemode
.B allocation/cache_mode
.br
defines the default cache mode.
.I Example
.nf
0. Create an origin LV we wish to cache (yours may already exist)
# lvcreate \-L 10G \-n lv1 vg /dev/slow
@ -246,50 +251,55 @@ defines the default cache mode.
2. Create a cache metadata LV
# lvcreate \-L 8M \-n cache1meta vg /dev/fast
3. Create a cache pool LV specifying cache mode "writethrough"
# lvconvert \-\-type cache\-pool \-\-poolmetadata vg/cache1meta \\
\-\-cachemode writethrough vg/cache1
3. Create a cache pool LV
# lvconvert \-\-type cache\-pool \-\-poolmetadata vg/cache1meta vg/cache1
4. Create a cache LV by combining the cache pool LV and origin LV
# lvconvert \-\-type cache \-\-cachepool vg/cache1 vg/lv1
4. Create a cache LV by combining the cache pool LV and origin LV,
and use the writethrough cache mode.
# lvconvert \-\-type cache \-\-cachepool vg/cache1 \\
\-\-cachemode writethrough vg/lv1
.fi
The cache mode can be changed on an existing LV with the command:
.B lvconvert --cachemode writethrough|writeback VG/CacheLV
.SS Cache policy & policy settings
.SS Cache policy
\&
The cache subsystem has an additional per-LV parameter, namely the cache policy
to use, and possibly the tunable parameters of the said cache policy. In the
current implementation, two policies are available, "mq" which is the default
policy and "cleaner" which is used to force the cache to write back (flush) all
cached writes to the origin LV. Moreover, the "mq" policy has a number of
tunable parameters: the defaults are chosen to be suitable for the vast
majority of systems. However, under special circumstances, changing the tunable
settings of the cache policy can improve performance.
The cache subsystem has additional per-LV parameters: the cache policy to
use, and possibly tunable parameters for the cache policy. Three policies
are currently available: "smq" is the default policy, "mq" is an older
implementation, and "cleaner" is used to force the cache to write back
(flush) all cached writes to the origin LV.
On an existing cache LV, the policy can be set (to "mq") and the cache settings
can be changed using commands like these:
The "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.
With the \-\-cachepolicy and \-\-cachesettings options, the cache policy
and settings can be set when creating a cache LV, or changed on an
existing cache LV (both options can be used together). The current cache
policy and settings of a cache LV can be displayed with the cache_policy
and cache_settings reporting options:
.B lvs \-o+cache_policy,cache_settings VG/CacheLV
.I Example
.nf
# lvchange \-\-cachepolicy mq vg/lv1
# lvchange \-\-cachesettings \(aqmigration_threshold=2048 random_threshold=4\(aq \\
vg/lv1
Change the cache policy and settings of an existing cache LV.
# lvchange \-\-cachepolicy mq \-\-cachesettings \\
\(aqmigration_threshold=2048 random_threshold=4\(aq vg/lv1
.fi
Both commands can be combined, setting both cache policy and its settings
together. Moreover, when creating a cache LV for the first time (using
lvcreate), the \-\-cachepolicy and \-\-cachesettings parameters can be used as
well. The current policy and the policy settings can be listed using the lvs
command, using 'cache_policy' and 'cache_settings' fields:
.BR lvm.conf (5)
.B allocation/cache_policy
.br
defines the default cache policy.
.BR lvm.conf (5)
.B allocation/cache_settings
.br
defines the default cache settings.
# lvs -o +cache_policy,cache_settings
.SS Spare metadata LV