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 device <dev>.
- Why are there multiple statistics in a single file? Doesn't sysfs normally contain a single value per file?
- 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 |
| discard I/Os | requests | number of discard I/Os processed |
| discard merges | requests | number of discard I/Os merged with in-queue I/O |
| discard sectors | sectors | number of sectors discarded |
| discard ticks | milliseconds | total wait time for discard requests |
| flush I/Os | requests | number of flush I/Os processed |
| flush ticks | milliseconds | total wait time for flush requests |
read I/Os, write I/Os, discard I/0s
These values increment when an I/O request completes.
flush I/Os
These values increment when an flush I/O request completes.
Block layer combines flush requests and executes at most one at a time. This counts flush requests executed by disk. Not tracked for partitions.
read merges, write merges, discard merges
These values increment when an I/O request is merged with an already-queued I/O request.
read sectors, write sectors, discard_sectors
These values count the number of sectors read from, written to, or discarded from 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, discard ticks, flush 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.
in_flight
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.
io_ticks
This value counts the number of milliseconds during which the device has had I/O requests queued.
time_in_queue
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).
