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Suggested-by: Damien Le Moal <dlemoal@kernel.org> Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Damien Le Moal <dlemoal@kernel.org> Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com> Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Link: https://lore.kernel.org/r/20240619154623.450048-3-hch@lst.de Signed-off-by: Jens Axboe <axboe@kernel.dk>
96 lines
3.8 KiB
ReStructuredText
96 lines
3.8 KiB
ReStructuredText
==========================================
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Explicit volatile write back cache control
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==========================================
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Introduction
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------------
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Many storage devices, especially in the consumer market, come with volatile
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write back caches. That means the devices signal I/O completion to the
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operating system before data actually has hit the non-volatile storage. This
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behavior obviously speeds up various workloads, but it means the operating
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system needs to force data out to the non-volatile storage when it performs
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a data integrity operation like fsync, sync or an unmount.
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The Linux block layer provides two simple mechanisms that let filesystems
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control the caching behavior of the storage device. These mechanisms are
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a forced cache flush, and the Force Unit Access (FUA) flag for requests.
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Explicit cache flushes
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----------------------
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The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from
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the filesystem and will make sure the volatile cache of the storage device
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has been flushed before the actual I/O operation is started. This explicitly
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guarantees that previously completed write requests are on non-volatile
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storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be
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set on an otherwise empty bio structure, which causes only an explicit cache
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flush without any dependent I/O. It is recommend to use
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the blkdev_issue_flush() helper for a pure cache flush.
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Forced Unit Access
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------------------
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The REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the
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filesystem and will make sure that I/O completion for this request is only
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signaled after the data has been committed to non-volatile storage.
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Implementation details for filesystems
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--------------------------------------
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Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to
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worry if the underlying devices need any explicit cache flushing and how
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the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags
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may both be set on a single bio.
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Feature settings for block drivers
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----------------------------------
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For devices that do not support volatile write caches there is no driver
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support required, the block layer completes empty REQ_PREFLUSH requests before
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entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from
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requests that have a payload.
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For devices with volatile write caches the driver needs to tell the block layer
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that it supports flushing caches by setting the
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BLK_FEAT_WRITE_CACHE
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flag in the queue_limits feature field. For devices that also support the FUA
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bit the block layer needs to be told to pass on the REQ_FUA bit by also setting
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the
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BLK_FEAT_FUA
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flag in the features field of the queue_limits structure.
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Implementation details for bio based block drivers
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--------------------------------------------------
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For bio based drivers the REQ_PREFLUSH and REQ_FUA bit are simply passed on to
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the driver if the driver sets the BLK_FEAT_WRITE_CACHE flag and the driver
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needs to handle them.
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*NOTE*: The REQ_FUA bit also gets passed on when the BLK_FEAT_FUA flags is
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_not_ set. Any bio based driver that sets BLK_FEAT_WRITE_CACHE also needs to
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handle REQ_FUA.
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For remapping drivers the REQ_FUA bits need to be propagated to underlying
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devices, and a global flush needs to be implemented for bios with the
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REQ_PREFLUSH bit set.
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Implementation details for blk-mq drivers
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-----------------------------------------
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When the BLK_FEAT_WRITE_CACHE flag is set, REQ_OP_WRITE | REQ_PREFLUSH requests
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with a payload are automatically turned into a sequence of a REQ_OP_FLUSH
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request followed by the actual write by the block layer.
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When the BLK_FEAT_FUA flags is set, the REQ_FUA bit is simply passed on for the
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REQ_OP_WRITE request, else a REQ_OP_FLUSH request is sent by the block layer
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after the completion of the write request for bio submissions with the REQ_FUA
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bit set.
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