Commit af563942 authored by Jens Axboe's avatar Jens Axboe
Browse files

block: add some comments around the bio read-write flags

Signed-off-by: default avatarJens Axboe <>
parent 6feef531
......@@ -123,13 +123,23 @@ struct bio {
* bio bi_rw flags
* bit 0 -- read (not set) or write (set)
* bit 0 -- data direction
* If not set, bio is a read from device. If set, it's a write to device.
* bit 1 -- rw-ahead when set
* bit 2 -- barrier
* Insert a serialization point in the IO queue, forcing previously
* submitted IO to be completed before this oen is issued.
* bit 3 -- fail fast, don't want low level driver retries
* bit 4 -- synchronous I/O hint: the block layer will unplug immediately
* Note that this does NOT indicate that the IO itself is sync, just
* that the block layer will not postpone issue of this IO by plugging.
* bit 5 -- metadata request
* Used for tracing to differentiate metadata and data IO. May also
* get some preferential treatment in the IO scheduler
* bit 6 -- discard sectors
* Informs the lower level device that this range of sectors is no longer
* used by the file system and may thus be freed by the device. Used
* for flash based storage.
#define BIO_RW 0 /* Must match RW in req flags (blkdev.h) */
#define BIO_RW_AHEAD 1 /* Must match FAILFAST in req flags */
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