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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2016-10-20 00:10:27 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2016-10-20 00:10:27 -0300
commitd0b2f91bede3bd5e3d24dd6803e56eee959c1797 (patch)
tree7fee4ab0509879c373c4f2cbd5b8a5be5b4041ee /Documentation/block/writeback_cache_control.txt
parente914f8eb445e8f74b00303c19c2ffceaedd16a05 (diff)
Linux-libre 4.8.2-gnupck-4.8.2-gnu
Diffstat (limited to 'Documentation/block/writeback_cache_control.txt')
-rw-r--r--Documentation/block/writeback_cache_control.txt28
1 files changed, 14 insertions, 14 deletions
diff --git a/Documentation/block/writeback_cache_control.txt b/Documentation/block/writeback_cache_control.txt
index 59e0516cb..8a6bdada5 100644
--- a/Documentation/block/writeback_cache_control.txt
+++ b/Documentation/block/writeback_cache_control.txt
@@ -20,11 +20,11 @@ a forced cache flush, and the Force Unit Access (FUA) flag for requests.
Explicit cache flushes
----------------------
-The REQ_FLUSH flag can be OR ed into the r/w flags of a bio submitted from
+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_FLUSH flag can be
+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.
@@ -41,21 +41,21 @@ signaled after the data has been committed to non-volatile storage.
Implementation details for filesystems
--------------------------------------
-Filesystems can simply set the REQ_FLUSH and REQ_FUA bits and do not have to
+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_FLUSH and REQ_FUA flags
+the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags
may both be set on a single bio.
Implementation details for make_request_fn based block drivers
--------------------------------------------------------------
-These drivers will always see the REQ_FLUSH and REQ_FUA bits as they sit
+These drivers will always see the REQ_PREFLUSH and REQ_FUA bits as they sit
directly below the submit_bio interface. 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_FLUSH bit set. For real device
-drivers that do not have a volatile cache the REQ_FLUSH and REQ_FUA bits
-on non-empty bios can simply be ignored, and REQ_FLUSH requests without
+to be implemented for bios with the REQ_PREFLUSH bit set. For real device
+drivers that do not have a volatile cache the REQ_PREFLUSH and REQ_FUA bits
+on non-empty bios can simply be ignored, and REQ_PREFLUSH requests without
data can be completed successfully without doing any work. Drivers for
devices with volatile caches need to implement the support for these
flags themselves without any help from the block layer.
@@ -65,17 +65,17 @@ Implementation details for request_fn based block drivers
--------------------------------------------------------------
For devices that do not support volatile write caches there is no driver
-support required, the block layer completes empty REQ_FLUSH requests before
-entering the driver and strips off the REQ_FLUSH and REQ_FUA bits from
+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. For devices with volatile write caches the
driver needs to tell the block layer that it supports flushing caches by
doing:
blk_queue_write_cache(sdkp->disk->queue, true, false);
-and handle empty REQ_FLUSH requests in its prep_fn/request_fn. Note that
-REQ_FLUSH requests with a payload are automatically turned into a sequence
-of an empty REQ_FLUSH request followed by the actual write by the block
+and handle empty REQ_OP_FLUSH requests in its prep_fn/request_fn. Note that
+REQ_PREFLUSH requests with a payload are automatically turned into a sequence
+of an empty REQ_OP_FLUSH request followed by the actual write by the block
layer. For devices that also support the FUA bit the block layer needs
to be told to pass through the REQ_FUA bit using:
@@ -83,4 +83,4 @@ to be told to pass through the REQ_FUA bit using:
and the driver must handle write requests that have the REQ_FUA bit set
in prep_fn/request_fn. If the FUA bit is not natively supported the block
-layer turns it into an empty REQ_FLUSH request after the actual write.
+layer turns it into an empty REQ_OP_FLUSH request after the actual write.