diff options
Diffstat (limited to 'Documentation/block/writeback_cache_control.txt')
| -rw-r--r-- | Documentation/block/writeback_cache_control.txt | 28 |
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. |