|Block layer statistics in /sys/block/<dev>/stat
|This file documents the contents of the /sys/block/<dev>/stat file.
|The stat file provides several statistics about the state of block
|Q. Why are there multiple statistics in a single file? Doesn't sysfs
| normally contain a single value per file?
|A. By having a single file, the kernel can guarantee that the statistics
| represent a consistent snapshot of the state of the device. If the
| statistics were exported as multiple files containing one statistic
| each, it would be impossible to guarantee that a set of readings
| represent a single point in time.
|The stat file consists of a single line of text containing 11 decimal
|values separated by whitespace. The fields are summarized in the
|following table, and described in more detail below.
|Name units description
|---- ----- -----------
|read I/Os requests number of read I/Os processed
|read merges requests number of read I/Os merged with in-queue I/O
|read sectors sectors number of sectors read
|read ticks milliseconds total wait time for read requests
|write I/Os requests number of write I/Os processed
|write merges requests number of write I/Os merged with in-queue I/O
|write sectors sectors number of sectors written
|write ticks milliseconds total wait time for write requests
|in_flight requests number of I/Os currently in flight
|io_ticks milliseconds total time this block device has been active
|time_in_queue milliseconds total wait time for all requests
|read I/Os, write I/Os
|These values increment when an I/O request completes.
|read merges, write merges
|These values increment when an I/O request is merged with an
|already-queued I/O request.
|read sectors, write sectors
|These values count the number of sectors read from or written to this
|block device. The "sectors" in question are the standard UNIX 512-byte
|sectors, not any device- or filesystem-specific block size. The
|counters are incremented when the I/O completes.
|read ticks, write ticks
|These values count the number of milliseconds that I/O requests have
|waited on this block device. If there are multiple I/O requests waiting,
|these values will increase at a rate greater than 1000/second; for
|example, if 60 read requests wait for an average of 30 ms, the read_ticks
|field will increase by 60*30 = 1800.
|This value counts the number of I/O requests that have been issued to
|the device driver but have not yet completed. It does not include I/O
|requests that are in the queue but not yet issued to the device driver.
|This value counts the number of milliseconds during which the device has
|had I/O requests queued.
|This value counts the number of milliseconds that I/O requests have waited
|on this block device. If there are multiple I/O requests waiting, this
|value will increase as the product of the number of milliseconds times the
|number of requests waiting (see "read ticks" above for an example).