€•LD      Œsphinx.addnodes”Œdocument”“”)”}”(Œ	rawsource”Œ ”Œchildren”]”(Œtranslations”ŒLanguagesNode”“”)”}”(hhh]”(h Œpending_xref”“”)”}”(hhh]”Œdocutils.nodes”ŒText”“”ŒChinese (Simplified)”…””}”Œparent”hsbaŒ
attributes”}”(Œids”]”Œclasses”]”Œnames”]”Œdupnames”]”Œbackrefs”]”Œ	refdomain”Œstd”Œreftype”Œdoc”Œ	reftarget”Œ</translations/zh_CN/admin-guide/device-mapper/cache-policies”Œmodname”NŒ	classname”NŒrefexplicit”ˆuŒtagname”hhhubh)”}”(hhh]”hŒChinese (Traditional)”…””}”hh2sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ</translations/zh_TW/admin-guide/device-mapper/cache-policies”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒItalian”…””}”hhFsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ</translations/it_IT/admin-guide/device-mapper/cache-policies”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒJapanese”…””}”hhZsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ</translations/ja_JP/admin-guide/device-mapper/cache-policies”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒKorean”…””}”hhnsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ</translations/ko_KR/admin-guide/device-mapper/cache-policies”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒSpanish”…””}”hh‚sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ</translations/sp_SP/admin-guide/device-mapper/cache-policies”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubeh}”(h]”h ]”h"]”h$]”h&]”Œcurrent_language”ŒEnglish”uh1h
hhŒ	_document”hŒsource”NŒline”NubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒGuidance for writing policies”h]”hŒGuidance for writing policies”…””}”(hh¨hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hh£hžhhŸŒV/var/lib/git/docbuild/linux/Documentation/admin-guide/device-mapper/cache-policies.rst”h KubhŒ	paragraph”“”)”}”(hŒªTry to keep transactionality out of it.  The core is careful to
avoid asking about anything that is migrating.  This is a pain, but
makes it easier to write the policies.”h]”hŒªTry to keep transactionality out of it.  The core is careful to
avoid asking about anything that is migrating.  This is a pain, but
makes it easier to write the policies.”…””}”(hh¹hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¸)”}”(hŒ9Mappings are loaded into the policy at construction time.”h]”hŒ9Mappings are loaded into the policy at construction time.”…””}”(hhÇhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K	hh£hžhubh¸)”}”(hŒ‚Every bio that is mapped by the target is referred to the policy.
The policy can return a simple HIT or MISS or issue a migration.”h]”hŒ‚Every bio that is mapped by the target is referred to the policy.
The policy can return a simple HIT or MISS or issue a migration.”…””}”(hhÕhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¸)”}”(hŒŒCurrently there's no way for the policy to issue background work,
e.g. to start writing back dirty blocks that are going to be evicted
soon.”h]”hŒŽCurrently thereâ€™s no way for the policy to issue background work,
e.g. to start writing back dirty blocks that are going to be evicted
soon.”…””}”(hhãhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¸)”}”(hX†  Because we map bios, rather than requests it's easy for the policy
to get fooled by many small bios.  For this reason the core target
issues periodic ticks to the policy.  It's suggested that the policy
doesn't update states (eg, hit counts) for a block more than once
for each tick.  The core ticks by watching bios complete, and so
trying to see when the io scheduler has let the ios run.”h]”hXŒ  Because we map bios, rather than requests itâ€™s easy for the policy
to get fooled by many small bios.  For this reason the core target
issues periodic ticks to the policy.  Itâ€™s suggested that the policy
doesnâ€™t update states (eg, hit counts) for a block more than once
for each tick.  The core ticks by watching bios complete, and so
trying to see when the io scheduler has let the ios run.”…””}”(hhñhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¢)”}”(hhh]”(h§)”}”(hŒ/Overview of supplied cache replacement policies”h]”hŒ/Overview of supplied cache replacement policies”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hhÿhžhhŸh¶h Kubh¢)”}”(hhh]”(h§)”}”(hŒmultiqueue (mq)”h]”hŒmultiqueue (mq)”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hj  hžhhŸh¶h Kubh¸)”}”(hŒ0This policy is now an alias for smq (see below).”h]”hŒ0This policy is now an alias for smq (see below).”…””}”(hj!  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K hj  hžhubh¸)”}”(hŒ9The following tunables are accepted, but have no effect::”h]”hŒ8The following tunables are accepted, but have no effect:”…””}”(hj/  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K"hj  hžhubhŒliteral_block”“”)”}”(hŒ¹'sequential_threshold <#nr_sequential_ios>'
'random_threshold <#nr_random_ios>'
'read_promote_adjustment <value>'
'write_promote_adjustment <value>'
'discard_promote_adjustment <value>'”h]”hŒ¹'sequential_threshold <#nr_sequential_ios>'
'random_threshold <#nr_random_ios>'
'read_promote_adjustment <value>'
'write_promote_adjustment <value>'
'discard_promote_adjustment <value>'”…””}”hj?  sbah}”(h]”h ]”h"]”h$]”h&]”Œ	xml:space”Œpreserve”uh1j=  hŸh¶h K$hj  hžhubeh}”(h]”Œmultiqueue-mq”ah ]”h"]”Œmultiqueue (mq)”ah$]”h&]”uh1h¡hhÿhžhhŸh¶h Kubh¢)”}”(hhh]”(h§)”}”(hŒStochastic multiqueue (smq)”h]”hŒStochastic multiqueue (smq)”…””}”(hjZ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hjW  hžhhŸh¶h K+ubh¸)”}”(hŒThis policy is the default.”h]”hŒThis policy is the default.”…””}”(hjh  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K-hjW  hžhubh¸)”}”(hŒgThe stochastic multi-queue (smq) policy addresses some of the problems
with the multiqueue (mq) policy.”h]”hŒgThe stochastic multi-queue (smq) policy addresses some of the problems
with the multiqueue (mq) policy.”…””}”(hjv  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K/hjW  hžhubh¸)”}”(hŒÍThe smq policy (vs mq) offers the promise of less memory utilization,
improved performance and increased adaptability in the face of changing
workloads.  smq also does not have any cumbersome tuning knobs.”h]”hŒÍThe smq policy (vs mq) offers the promise of less memory utilization,
improved performance and increased adaptability in the face of changing
workloads.  smq also does not have any cumbersome tuning knobs.”…””}”(hj„  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K2hjW  hžhubh¸)”}”(hX1  Users may switch from "mq" to "smq" simply by appropriately reloading a
DM table that is using the cache target.  Doing so will cause all of the
mq policy's hints to be dropped.  Also, performance of the cache may
degrade slightly until smq recalculates the origin device's hotspots
that should be cached.”h]”hX=  Users may switch from â€œmqâ€ to â€œsmqâ€ simply by appropriately reloading a
DM table that is using the cache target.  Doing so will cause all of the
mq policyâ€™s hints to be dropped.  Also, performance of the cache may
degrade slightly until smq recalculates the origin deviceâ€™s hotspots
that should be cached.”…””}”(hj’  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K6hjW  hžhubh¢)”}”(hhh]”(h§)”}”(hŒMemory usage”h]”hŒMemory usage”…””}”(hj£  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hj   hžhhŸh¶h K=ubh¸)”}”(hŒQThe mq policy used a lot of memory; 88 bytes per cache block on a 64
bit machine.”h]”hŒQThe mq policy used a lot of memory; 88 bytes per cache block on a 64
bit machine.”…””}”(hj±  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K?hj   hžhubh¸)”}”(hX#  smq uses 28bit indexes to implement its data structures rather than
pointers.  It avoids storing an explicit hit count for each block.  It
has a 'hotspot' queue, rather than a pre-cache, which uses a quarter of
the entries (each hotspot block covers a larger area than a single
cache block).”h]”hX'  smq uses 28bit indexes to implement its data structures rather than
pointers.  It avoids storing an explicit hit count for each block.  It
has a â€˜hotspotâ€™ queue, rather than a pre-cache, which uses a quarter of
the entries (each hotspot block covers a larger area than a single
cache block).”…””}”(hj¿  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KBhj   hžhubh¸)”}”(hŒtAll this means smq uses ~25bytes per cache block.  Still a lot of
memory, but a substantial improvement nonetheless.”h]”hŒtAll this means smq uses ~25bytes per cache block.  Still a lot of
memory, but a substantial improvement nonetheless.”…””}”(hjÍ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KHhj   hžhubeh}”(h]”Œmemory-usage”ah ]”h"]”Œmemory usage”ah$]”h&]”uh1h¡hjW  hžhhŸh¶h K=ubh¢)”}”(hhh]”(h§)”}”(hŒLevel balancing”h]”hŒLevel balancing”…””}”(hjæ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hjã  hžhhŸh¶h KLubh¸)”}”(hX  mq placed entries in different levels of the multiqueue structures
based on their hit count (~ln(hit count)).  This meant the bottom
levels generally had the most entries, and the top ones had very
few.  Having unbalanced levels like this reduced the efficacy of the
multiqueue.”h]”hX  mq placed entries in different levels of the multiqueue structures
based on their hit count (~ln(hit count)).  This meant the bottom
levels generally had the most entries, and the top ones had very
few.  Having unbalanced levels like this reduced the efficacy of the
multiqueue.”…””}”(hjô  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KNhjã  hžhubh¸)”}”(hX?  smq does not maintain a hit count, instead it swaps hit entries with
the least recently used entry from the level above.  The overall
ordering being a side effect of this stochastic process.  With this
scheme we can decide how many entries occupy each multiqueue level,
resulting in better promotion/demotion decisions.”h]”hX?  smq does not maintain a hit count, instead it swaps hit entries with
the least recently used entry from the level above.  The overall
ordering being a side effect of this stochastic process.  With this
scheme we can decide how many entries occupy each multiqueue level,
resulting in better promotion/demotion decisions.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KThjã  hžhubh¸)”}”(hX  Adaptability:
The mq policy maintained a hit count for each cache block.  For a
different block to get promoted to the cache its hit count has to
exceed the lowest currently in the cache.  This meant it could take a
long time for the cache to adapt between varying IO patterns.”h]”hX  Adaptability:
The mq policy maintained a hit count for each cache block.  For a
different block to get promoted to the cache its hit count has to
exceed the lowest currently in the cache.  This meant it could take a
long time for the cache to adapt between varying IO patterns.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KZhjã  hžhubh¸)”}”(hXM  smq doesn't maintain hit counts, so a lot of this problem just goes
away.  In addition it tracks performance of the hotspot queue, which
is used to decide which blocks to promote.  If the hotspot queue is
performing badly then it starts moving entries more quickly between
levels.  This lets it adapt to new IO patterns very quickly.”h]”hXO  smq doesnâ€™t maintain hit counts, so a lot of this problem just goes
away.  In addition it tracks performance of the hotspot queue, which
is used to decide which blocks to promote.  If the hotspot queue is
performing badly then it starts moving entries more quickly between
levels.  This lets it adapt to new IO patterns very quickly.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K`hjã  hžhubeh}”(h]”Œlevel-balancing”ah ]”h"]”Œlevel balancing”ah$]”h&]”uh1h¡hjW  hžhhŸh¶h KLubh¢)”}”(hhh]”(h§)”}”(hŒPerformance”h]”hŒPerformance”…””}”(hj7  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hj4  hžhhŸh¶h Kgubh¸)”}”(hŒ;Testing smq shows substantially better performance than mq.”h]”hŒ;Testing smq shows substantially better performance than mq.”…””}”(hjE  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Kihj4  hžhubeh}”(h]”Œperformance”ah ]”h"]”Œperformance”ah$]”h&]”uh1h¡hjW  hžhhŸh¶h Kgubeh}”(h]”Œstochastic-multiqueue-smq”ah ]”h"]”Œstochastic multiqueue (smq)”ah$]”h&]”uh1h¡hhÿhžhhŸh¶h K+ubh¢)”}”(hhh]”(h§)”}”(hŒcleaner”h]”hŒcleaner”…””}”(hjf  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hjc  hžhhŸh¶h Klubh¸)”}”(hŒGThe cleaner writes back all dirty blocks in a cache to decommission it.”h]”hŒGThe cleaner writes back all dirty blocks in a cache to decommission it.”…””}”(hjt  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Knhjc  hžhubeh}”(h]”Œcleaner”ah ]”h"]”Œcleaner”ah$]”h&]”uh1h¡hhÿhžhhŸh¶h Klubeh}”(h]”Œ/overview-of-supplied-cache-replacement-policies”ah ]”h"]”Œ/overview of supplied cache replacement policies”ah$]”h&]”uh1h¡hh£hžhhŸh¶h Kubh¢)”}”(hhh]”(h§)”}”(hŒExamples”h]”hŒExamples”…””}”(hj•  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hj’  hžhhŸh¶h Kqubh¸)”}”(hŒThe syntax for a table is::”h]”hŒThe syntax for a table is:”…””}”(hj£  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Kshj’  hžhubj>  )”}”(hŒƒcache <metadata dev> <cache dev> <origin dev> <block size>
<#feature_args> [<feature arg>]*
<policy> <#policy_args> [<policy arg>]*”h]”hŒƒcache <metadata dev> <cache dev> <origin dev> <block size>
<#feature_args> [<feature arg>]*
<policy> <#policy_args> [<policy arg>]*”…””}”hj±  sbah}”(h]”h ]”h"]”h$]”h&]”jM  jN  uh1j=  hŸh¶h Kuhj’  hžhubh¸)”}”(hŒ;The syntax to send a message using the dmsetup command is::”h]”hŒ:The syntax to send a message using the dmsetup command is:”…””}”(hj¿  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Kyhj’  hžhubj>  )”}”(hŒpdmsetup message <mapped device> 0 sequential_threshold 1024
dmsetup message <mapped device> 0 random_threshold 8”h]”hŒpdmsetup message <mapped device> 0 sequential_threshold 1024
dmsetup message <mapped device> 0 random_threshold 8”…””}”hjÍ  sbah}”(h]”h ]”h"]”h$]”h&]”jM  jN  uh1j=  hŸh¶h K{hj’  hžhubh¸)”}”(hŒUsing dmsetup::”h]”hŒUsing dmsetup:”…””}”(hjÛ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K~hj’  hžhubj>  )”}”(hX  dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \
    /dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8"
creates a 128GB large mapped device named 'blah' with the
sequential threshold set to 1024 and the random_threshold set to 8.”h]”hX  dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \
    /dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8"
creates a 128GB large mapped device named 'blah' with the
sequential threshold set to 1024 and the random_threshold set to 8.”…””}”hjé  sbah}”(h]”h ]”h"]”h$]”h&]”jM  jN  uh1j=  hŸh¶h K€hj’  hžhubeh}”(h]”Œexamples”ah ]”h"]”Œexamples”ah$]”h&]”uh1h¡hh£hžhhŸh¶h Kqubeh}”(h]”Œguidance-for-writing-policies”ah ]”h"]”Œguidance for writing policies”ah$]”h&]”uh1h¡hhhžhhŸh¶h Kubeh}”(h]”h ]”h"]”h$]”h&]”Œsource”h¶uh1hŒcurrent_source”NŒcurrent_line”NŒsettings”Œdocutils.frontend”ŒValues”“”)”}”(h¦NŒ	generator”NŒ	datestamp”NŒsource_link”NŒ
source_url”NŒtoc_backlinks”Œentry”Œfootnote_backlinks”KŒsectnum_xform”KŒstrip_comments”NŒstrip_elements_with_classes”NŒstrip_classes”NŒreport_level”KŒ
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id_counter”Œcollections”ŒCounter”“”}”…”R”Œparse_messages”]”Œtransform_messages”]”Œtransformer”NŒinclude_log”]”Œ
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