€•”9      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”Œ1/translations/zh_CN/block/writeback_cache_control”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$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ1/translations/zh_TW/block/writeback_cache_control”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒItalian”…””}”hhFsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ1/translations/it_IT/block/writeback_cache_control”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒJapanese”…””}”hhZsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ1/translations/ja_JP/block/writeback_cache_control”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒKorean”…””}”hhnsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ1/translations/ko_KR/block/writeback_cache_control”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒSpanish”…””}”hh‚sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ1/translations/sp_SP/block/writeback_cache_control”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubeh}”(h]”h ]”h"]”h$]”h&]”Œcurrent_language”ŒEnglish”uh1h
hhŒ	_document”hŒsource”NŒline”NubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒ*Explicit volatile write back cache control”h]”hŒ*Explicit volatile write back cache control”…””}”(hh¨hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hh£hžhhŸŒK/var/lib/git/docbuild/linux/Documentation/block/writeback_cache_control.rst”h Kubh¢)”}”(hhh]”(h§)”}”(hŒIntroduction”h]”hŒIntroduction”…””}”(hhºhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hh·hžhhŸh¶h KubhŒ	paragraph”“”)”}”(hX³  Many storage devices, especially in the consumer market, come with volatile
write back caches.  That means the devices signal I/O completion to the
operating system before data actually has hit the non-volatile storage.  This
behavior obviously speeds up various workloads, but it means the operating
system needs to force data out to the non-volatile storage when it performs
a data integrity operation like fsync, sync or an unmount.”h]”hX³  Many storage devices, especially in the consumer market, come with volatile
write back caches.  That means the devices signal I/O completion to the
operating system before data actually has hit the non-volatile storage.  This
behavior obviously speeds up various workloads, but it means the operating
system needs to force data out to the non-volatile storage when it performs
a data integrity operation like fsync, sync or an unmount.”…””}”(hhÊhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h Khh·hžhubhÉ)”}”(hŒÜThe Linux block layer provides two simple mechanisms that let filesystems
control the caching behavior of the storage device.  These mechanisms are
a forced cache flush, and the Force Unit Access (FUA) flag for requests.”h]”hŒÜThe Linux block layer provides two simple mechanisms that let filesystems
control the caching behavior of the storage device.  These mechanisms are
a forced cache flush, and the Force Unit Access (FUA) flag for requests.”…””}”(hhØhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h Khh·hžhubeh}”(h]”Œintroduction”ah ]”h"]”Œintroduction”ah$]”h&]”uh1h¡hh£hžhhŸh¶h Kubh¢)”}”(hhh]”(h§)”}”(hŒExplicit cache flushes”h]”hŒExplicit cache flushes”…””}”(hhñhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hhîhžhhŸh¶h KubhÉ)”}”(hX<  The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from
the filesystem and will make sure the volatile cache of the storage device
has been flushed before the actual I/O operation is started.  This explicitly
guarantees that previously completed write requests are on non-volatile
storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be
set on an otherwise empty bio structure, which causes only an explicit cache
flush without any dependent I/O.  It is recommend to use
the blkdev_issue_flush() helper for a pure cache flush.”h]”hX<  The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from
the filesystem and will make sure the volatile cache of the storage device
has been flushed before the actual I/O operation is started.  This explicitly
guarantees that previously completed write requests are on non-volatile
storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be
set on an otherwise empty bio structure, which causes only an explicit cache
flush without any dependent I/O.  It is recommend to use
the blkdev_issue_flush() helper for a pure cache flush.”…””}”(hhÿhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h Khhîhžhubeh}”(h]”Œexplicit-cache-flushes”ah ]”h"]”Œexplicit cache flushes”ah$]”h&]”uh1h¡hh£hžhhŸh¶h Kubh¢)”}”(hhh]”(h§)”}”(hŒForced Unit Access”h]”hŒForced Unit Access”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hj  hžhhŸh¶h K"ubhÉ)”}”(hŒÛThe REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the
filesystem and will make sure that I/O completion for this request is only
signaled after the data has been committed to non-volatile storage.”h]”hŒÛThe REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the
filesystem and will make sure that I/O completion for this request is only
signaled after the data has been committed to non-volatile storage.”…””}”(hj&  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h K$hj  hžhubeh}”(h]”Œforced-unit-access”ah ]”h"]”Œforced unit access”ah$]”h&]”uh1h¡hh£hžhhŸh¶h K"ubh¢)”}”(hhh]”(h§)”}”(hŒ&Implementation details for filesystems”h]”hŒ&Implementation details for filesystems”…””}”(hj?  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hj<  hžhhŸh¶h K*ubhÉ)”}”(hX  Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to
worry if the underlying devices need any explicit cache flushing and how
the Forced Unit Access is implemented.  The REQ_PREFLUSH and REQ_FUA flags
may both be set on a single bio.”h]”hX  Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to
worry if the underlying devices need any explicit cache flushing and how
the Forced Unit Access is implemented.  The REQ_PREFLUSH and REQ_FUA flags
may both be set on a single bio.”…””}”(hjM  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h K,hj<  hžhubeh}”(h]”Œ&implementation-details-for-filesystems”ah ]”h"]”Œ&implementation details for filesystems”ah$]”h&]”uh1h¡hh£hžhhŸh¶h K*ubh¢)”}”(hhh]”(h§)”}”(hŒ"Feature settings for block drivers”h]”hŒ"Feature settings for block drivers”…””}”(hjf  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hjc  hžhhŸh¶h K2ubhÉ)”}”(hŒÿFor devices that do not support volatile write caches there is no driver
support required, the block layer completes empty REQ_PREFLUSH requests before
entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from
requests that have a payload.”h]”hŒÿFor devices that do not support volatile write caches there is no driver
support required, the block layer completes empty REQ_PREFLUSH requests before
entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from
requests that have a payload.”…””}”(hjt  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h K4hjc  hžhubhÉ)”}”(hŒFor devices with volatile write caches the driver needs to tell the block layer
that it supports flushing caches by setting the”h]”hŒFor devices with volatile write caches the driver needs to tell the block layer
that it supports flushing caches by setting the”…””}”(hj‚  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h K9hjc  hžhubhŒblock_quote”“”)”}”(hŒBLK_FEAT_WRITE_CACHE
”h]”hÉ)”}”(hŒBLK_FEAT_WRITE_CACHE”h]”hŒBLK_FEAT_WRITE_CACHE”…””}”(hj–  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h K<hj’  ubah}”(h]”h ]”h"]”h$]”h&]”uh1j  hŸh¶h K<hjc  hžhubhÉ)”}”(hŒ¢flag in the queue_limits feature field.  For devices that also support the FUA
bit the block layer needs to be told to pass on the REQ_FUA bit by also setting
the”h]”hŒ¢flag in the queue_limits feature field.  For devices that also support the FUA
bit the block layer needs to be told to pass on the REQ_FUA bit by also setting
the”…””}”(hjª  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h K>hjc  hžhubj‘  )”}”(hŒBLK_FEAT_FUA
”h]”hÉ)”}”(hŒBLK_FEAT_FUA”h]”hŒBLK_FEAT_FUA”…””}”(hj¼  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h KBhj¸  ubah}”(h]”h ]”h"]”h$]”h&]”uh1j  hŸh¶h KBhjc  hžhubhÉ)”}”(hŒ9flag in the features field of the queue_limits structure.”h]”hŒ9flag in the features field of the queue_limits structure.”…””}”(hjÐ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h KDhjc  hžhubeh}”(h]”Œ"feature-settings-for-block-drivers”ah ]”h"]”Œ"feature settings for block drivers”ah$]”h&]”uh1h¡hh£hžhhŸh¶h K2ubh¢)”}”(hhh]”(h§)”}”(hŒ2Implementation details for bio based block drivers”h]”hŒ2Implementation details for bio based block drivers”…””}”(hjé  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hjæ  hžhhŸh¶h KGubhÉ)”}”(hŒ¯For bio based drivers the REQ_PREFLUSH and REQ_FUA bit are simply passed on to
the driver if the driver sets the BLK_FEAT_WRITE_CACHE flag and the driver
needs to handle them.”h]”hŒ¯For bio based drivers the REQ_PREFLUSH and REQ_FUA bit are simply passed on to
the driver if the driver sets the BLK_FEAT_WRITE_CACHE flag and the driver
needs to handle them.”…””}”(hj÷  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h KIhjæ  hžhubhÉ)”}”(hŒ¨*NOTE*: The REQ_FUA bit also gets passed on when the BLK_FEAT_FUA flags is
_not_ set.  Any bio based driver that sets BLK_FEAT_WRITE_CACHE also needs to
handle REQ_FUA.”h]”(hŒemphasis”“”)”}”(hŒ*NOTE*”h]”hŒNOTE”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j	  hj  ubhŒ¢: The REQ_FUA bit also gets passed on when the BLK_FEAT_FUA flags is
_not_ set.  Any bio based driver that sets BLK_FEAT_WRITE_CACHE also needs to
handle REQ_FUA.”…””}”(hj  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h KMhjæ  hžhubhÉ)”}”(hŒ¦For remapping drivers the REQ_FUA bits need to be propagated to underlying
devices, and a global flush needs to be implemented for bios with the
REQ_PREFLUSH bit set.”h]”hŒ¦For remapping drivers the REQ_FUA bits need to be propagated to underlying
devices, and a global flush needs to be implemented for bios with the
REQ_PREFLUSH bit set.”…””}”(hj#  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h KQhjæ  hžhubeh}”(h]”Œ2implementation-details-for-bio-based-block-drivers”ah ]”h"]”Œ2implementation details for bio based block drivers”ah$]”h&]”uh1h¡hh£hžhhŸh¶h KGubh¢)”}”(hhh]”(h§)”}”(hŒ)Implementation details for blk-mq drivers”h]”hŒ)Implementation details for blk-mq drivers”…””}”(hj<  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hj9  hžhhŸh¶h KVubhÉ)”}”(hŒÒWhen the BLK_FEAT_WRITE_CACHE flag is set, REQ_OP_WRITE | REQ_PREFLUSH requests
with a payload are automatically turned into a sequence of a REQ_OP_FLUSH
request followed by the actual write by the block layer.”h]”hŒÒWhen the BLK_FEAT_WRITE_CACHE flag is set, REQ_OP_WRITE | REQ_PREFLUSH requests
with a payload are automatically turned into a sequence of a REQ_OP_FLUSH
request followed by the actual write by the block layer.”…””}”(hjJ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h KXhj9  hžhubhÉ)”}”(hŒôWhen the BLK_FEAT_FUA flags is set, the REQ_FUA bit is simply passed on for the
REQ_OP_WRITE request, else a REQ_OP_FLUSH request is sent by the block layer
after the completion of the write request for bio submissions with the REQ_FUA
bit set.”h]”hŒôWhen the BLK_FEAT_FUA flags is set, the REQ_FUA bit is simply passed on for the
REQ_OP_WRITE request, else a REQ_OP_FLUSH request is sent by the block layer
after the completion of the write request for bio submissions with the REQ_FUA
bit set.”…””}”(hjX  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÈhŸh¶h K\hj9  hžhubeh}”(h]”Œ)implementation-details-for-blk-mq-drivers”ah ]”h"]”Œ)implementation details for blk-mq drivers”ah$]”h&]”uh1h¡hh£hžhhŸh¶h KVubeh}”(h]”Œ*explicit-volatile-write-back-cache-control”ah ]”h"]”Œ*explicit volatile write back cache control”ah$]”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
halt_level”KŒexit_status_level”KŒdebug”NŒwarning_stream”NŒ	traceback”ˆŒinput_encoding”Œ	utf-8-sig”Œinput_encoding_error_handler”Œstrict”Œoutput_encoding”Œutf-8”Œoutput_encoding_error_handler”j™  Œerror_encoding”Œutf-8”Œerror_encoding_error_handler”Œbackslashreplace”Œlanguage_code”Œen”Œrecord_dependencies”NŒconfig”NŒ	id_prefix”hŒauto_id_prefix”Œid”Œdump_settings”NŒdump_internals”NŒdump_transforms”NŒdump_pseudo_xml”NŒexpose_internals”NŒstrict_visitor”NŒ_disable_config”NŒ_source”h¶Œ_destination”NŒ_config_files”]”Œ7/var/lib/git/docbuild/linux/Documentation/docutils.conf”aŒfile_insertion_enabled”ˆŒraw_enabled”KŒline_length_limit”M'Œpep_references”NŒpep_base_url”Œhttps://peps.python.org/”Œpep_file_url_template”Œpep-%04d”Œrfc_references”NŒrfc_base_url”Œ&https://datatracker.ietf.org/doc/html/”Œ	tab_width”KŒtrim_footnote_reference_space”‰Œsyntax_highlight”Œlong”Œsmart_quotes”ˆŒsmartquotes_locales”]”Œcharacter_level_inline_markup”‰Œdoctitle_xform”‰Œdocinfo_xform”KŒsectsubtitle_xform”‰Œimage_loading”Œlink”Œembed_stylesheet”‰Œcloak_email_addresses”ˆŒsection_self_link”‰Œenv”NubŒreporter”NŒindirect_targets”]”Œsubstitution_defs”}”Œsubstitution_names”}”Œrefnames”}”Œrefids”}”Œnameids”}”(js  jp  hëhèj  j  j9  j6  j`  j]  jã  jà  j6  j3  jk  jh  uŒ	nametypes”}”(js  ‰hë‰j  ‰j9  ‰j`  ‰jã  ‰j6  ‰jk  ‰uh}”(jp  h£hèh·j  hîj6  j  j]  j<  jà  jc  j3  jæ  jh  j9  uŒfootnote_refs”}”Œcitation_refs”}”Œautofootnotes”]”Œautofootnote_refs”]”Œsymbol_footnotes”]”Œsymbol_footnote_refs”]”Œ	footnotes”]”Œ	citations”]”Œautofootnote_start”KŒsymbol_footnote_start”K Œ
id_counter”Œcollections”ŒCounter”“”}”…”R”Œparse_messages”]”Œtransform_messages”]”Œtransformer”NŒinclude_log”]”Œ
decoration”Nhžhub.