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