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 17 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 44flush I/Os requests number of flush I/Os processed 45flush ticks milliseconds total wait time for flush requests 46=============== ============= ================================================= 47 48read I/Os, write I/Os, discard I/0s 49=================================== 50 51These values increment when an I/O request completes. 52 53flush I/Os 54========== 55 56These values increment when an flush I/O request completes. 57 58Block layer combines flush requests and executes at most one at a time. 59This counts flush requests executed by disk. Not tracked for partitions. 60 61read merges, write merges, discard merges 62========================================= 63 64These values increment when an I/O request is merged with an 65already-queued I/O request. 66 67read sectors, write sectors, discard_sectors 68============================================ 69 70These values count the number of sectors read from, written to, or 71discarded from this block device. The "sectors" in question are the 72standard UNIX 512-byte sectors, not any device- or filesystem-specific 73block size. The counters are incremented when the I/O completes. 74 75read ticks, write ticks, discard ticks, flush ticks 76=================================================== 77 78These values count the number of milliseconds that I/O requests have 79waited on this block device. If there are multiple I/O requests waiting, 80these values will increase at a rate greater than 1000/second; for 81example, if 60 read requests wait for an average of 30 ms, the read_ticks 82field will increase by 60*30 = 1800. 83 84in_flight 85========= 86 87This value counts the number of I/O requests that have been issued to 88the device driver but have not yet completed. It does not include I/O 89requests that are in the queue but not yet issued to the device driver. 90 91io_ticks 92======== 93 94This value counts the number of milliseconds during which the device has 95had I/O requests queued. 96 97time_in_queue 98============= 99 100This value counts the number of milliseconds that I/O requests have waited 101on this block device. If there are multiple I/O requests waiting, this 102value will increase as the product of the number of milliseconds times the 103number of requests waiting (see "read ticks" above for an example). 104