diff options
| -rw-r--r-- | include/linux/fs.h | 59 |
1 files changed, 59 insertions, 0 deletions
diff --git a/include/linux/fs.h b/include/linux/fs.h index 562d2855cf3..b535aec4406 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -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)) |