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