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Document and move the various READ/WRITE types

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It's a somewhat twisty maze of hints and behavioural modifiers, try
and clear it up a bit with some documentation.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Jens Axboe 2009-04-14 08:19:27 +02:00
parent f600abe2de
commit 48e70bc18a
1 changed files with 59 additions and 0 deletions
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59
include/linux/fs.h
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@ -87,6 +87,60 @@ struct inodes_stat_t {
*/
#define FMODE_NOCMTIME ((__force fmode_t)2048)
/*
* The below are the various read and write types that we support. Some of
* them include behavioral modifiers that send information down to the
* block layer and IO scheduler. Terminology:
*
* The block layer uses device plugging to defer IO a little bit, in
* the hope that we will see more IO very shortly. This increases
* coalescing of adjacent IO and thus reduces the number of IOs we
* have to send to the device. It also allows for better queuing,
* if the IO isn't mergeable. If the caller is going to be waiting
* for the IO, then he must ensure that the device is unplugged so
* that the IO is dispatched to the driver.
*
* All IO is handled async in Linux. This is fine for background
* writes, but for reads or writes that someone waits for completion
* on, we want to notify the block layer and IO scheduler so that they
* know about it. That allows them to make better scheduling
* decisions. So when the below references 'sync' and 'async', it
* is referencing this priority hint.
*
* With that in mind, the available types are:
*
* READ A normal read operation. Device will be plugged.
* READ_SYNC A synchronous read. Device is not plugged, caller can
* immediately wait on this read without caring about
* unplugging.
* READA Used for read-ahead operations. Lower priority, and the
* block layer could (in theory) choose to ignore this
* request if it runs into resource problems.
* WRITE A normal async write. Device will be plugged.
* SWRITE Like WRITE, but a special case for ll_rw_block() that
* tells it to lock the buffer first. Normally a buffer
* must be locked before doing IO.
* WRITE_SYNC_PLUG Synchronous write. Identical to WRITE, but passes down
* the hint that someone will be waiting on this IO
* shortly. The device must still be unplugged explicitly,
* WRITE_SYNC_PLUG does not do this as we could be
* submitting more writes before we actually wait on any
* of them.
* WRITE_SYNC Like WRITE_SYNC_PLUG, but also unplugs the device
* immediately after submission. The write equivalent
* of READ_SYNC.
* WRITE_ODIRECT Special case write for O_DIRECT only.
* SWRITE_SYNC
* SWRITE_SYNC_PLUG Like WRITE_SYNC/WRITE_SYNC_PLUG, but locks the buffer.
* See SWRITE.
* WRITE_BARRIER Like WRITE, but tells the block layer that all
* previously submitted writes must be safely on storage
* before this one is started. Also guarantees that when
* this write is complete, it itself is also safely on
* storage. Prevents reordering of writes on both sides
* of this IO.
*
*/
#define RW_MASK 1
#define RWA_MASK 2
#define READ 0
@ -102,6 +156,11 @@ struct inodes_stat_t {
(SWRITE | (1 << BIO_RW_SYNCIO) | (1 << BIO_RW_NOIDLE))
#define SWRITE_SYNC (SWRITE_SYNC_PLUG | (1 << BIO_RW_UNPLUG))
#define WRITE_BARRIER (WRITE | (1 << BIO_RW_BARRIER))
/*
* These aren't really reads or writes, they pass down information about
* parts of device that are now unused by the file system.
*/
#define DISCARD_NOBARRIER (1 << BIO_RW_DISCARD)
#define DISCARD_BARRIER ((1 << BIO_RW_DISCARD) | (1 << BIO_RW_BARRIER))