Lines Matching refs:the

8 Many storage devices, especially in the consumer market, come with volatile
9 write back caches.  That means the devices signal I/O completion to the
10 operating system before data actually has hit the non-volatile storage.  This
11 behavior obviously speeds up various workloads, but it means the operating
12 system needs to force data out to the non-volatile storage when it performs
16 control the caching behavior of the storage device.  These mechanisms are
17 a forced cache flush, and the Force Unit Access (FUA) flag for requests.
23 The REQ_FLUSH flag can be OR ed into the r/w flags of a bio submitted from
24 the filesystem and will make sure the volatile cache of the storage device
25 has been flushed before the actual I/O operation is started.  This explicitly
27 storage before the flagged bio starts. In addition the REQ_FLUSH flag can be
30 the blkdev_issue_flush() helper for a pure cache flush.
36 The REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the
38 signaled after the data has been committed to non-volatile storage.
44 Filesystems can simply set the REQ_FLUSH and REQ_FUA bits and do not have to
45 worry if the underlying devices need any explicit cache flushing and how
46 the Forced Unit Access is implemented.  The REQ_FLUSH and REQ_FUA flags
53 These drivers will always see the REQ_FLUSH and REQ_FUA bits as they sit
54 directly below the submit_bio interface.  For remapping drivers the REQ_FUA
56 to be implemented for bios with the REQ_FLUSH bit set.  For real device
57 drivers that do not have a volatile cache the REQ_FLUSH and REQ_FUA bits
60 devices with volatile caches need to implement the support for these
61 flags themselves without any help from the block layer.
68 support required, the block layer completes empty REQ_FLUSH requests before
69 entering the driver and strips off the REQ_FLUSH and REQ_FUA bits from
70 requests that have a payload.  For devices with volatile write caches the
71 driver needs to tell the block layer that it supports flushing caches by
78 of an empty REQ_FLUSH request followed by the actual write by the block
79 layer.  For devices that also support the FUA bit the block layer needs
80 to be told to pass through the REQ_FUA bit using:
84 and the driver must handle write requests that have the REQ_FUA bit set
85 in prep_fn/request_fn.  If the FUA bit is not natively supported the block
86 layer turns it into an empty REQ_FLUSH request after the actual write.