[PATCH] block/stat.txt

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

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.

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).