1=============================================== 2Block layer statistics in /sys/block/<dev>/stat 3=============================================== 4 5This file documents the contents of the /sys/block/<dev>/stat file. 6 7The stat file provides several statistics about the state of block 8device <dev>. 9 10Q. 11 Why are there multiple statistics in a single file? Doesn't sysfs 12 normally contain a single value per file? 13 14A. 15 By having a single file, the kernel can guarantee that the statistics 16 represent a consistent snapshot of the state of the device. If the 17 statistics were exported as multiple files containing one statistic 18 each, it would be impossible to guarantee that a set of readings 19 represent a single point in time. 20 21The stat file consists of a single line of text containing 11 decimal 22values separated by whitespace. The fields are summarized in the 23following table, and described in more detail below. 24 25 26=============== ============= ================================================= 27Name units description 28=============== ============= ================================================= 29read I/Os requests number of read I/Os processed 30read merges requests number of read I/Os merged with in-queue I/O 31read sectors sectors number of sectors read 32read ticks milliseconds total wait time for read requests 33write I/Os requests number of write I/Os processed 34write merges requests number of write I/Os merged with in-queue I/O 35write sectors sectors number of sectors written 36write ticks milliseconds total wait time for write requests 37in_flight requests number of I/Os currently in flight 38io_ticks milliseconds total time this block device has been active 39time_in_queue milliseconds total wait time for all requests 40discard I/Os requests number of discard I/Os processed 41discard merges requests number of discard I/Os merged with in-queue I/O 42discard sectors sectors number of sectors discarded 43discard ticks milliseconds total wait time for discard requests 44=============== ============= ================================================= 45 46read I/Os, write I/Os, discard I/0s 47=================================== 48 49These values increment when an I/O request completes. 50 51read merges, write merges, discard merges 52========================================= 53 54These values increment when an I/O request is merged with an 55already-queued I/O request. 56 57read sectors, write sectors, discard_sectors 58============================================ 59 60These values count the number of sectors read from, written to, or 61discarded from this block device. The "sectors" in question are the 62standard UNIX 512-byte sectors, not any device- or filesystem-specific 63block size. The counters are incremented when the I/O completes. 64 65read ticks, write ticks, discard ticks 66====================================== 67 68These values count the number of milliseconds that I/O requests have 69waited on this block device. If there are multiple I/O requests waiting, 70these values will increase at a rate greater than 1000/second; for 71example, if 60 read requests wait for an average of 30 ms, the read_ticks 72field will increase by 60*30 = 1800. 73 74in_flight 75========= 76 77This value counts the number of I/O requests that have been issued to 78the device driver but have not yet completed. It does not include I/O 79requests that are in the queue but not yet issued to the device driver. 80 81io_ticks 82======== 83 84This value counts the number of milliseconds during which the device has 85had I/O requests queued. 86 87time_in_queue 88============= 89 90This value counts the number of milliseconds that I/O requests have waited 91on this block device. If there are multiple I/O requests waiting, this 92value will increase as the product of the number of milliseconds times the 93number of requests waiting (see "read ticks" above for an example). 94