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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>.
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
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
read I/Os, write I/Os, discard I/0s
===================================
These values increment when an I/O request completes.
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
======================================
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).
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