IO statistics

This page describes the IO statistics that is collected at the IO-scheduler level. It describes the information about the container's real work with disks. This is different from what shown by IO accounting.

Kernel interface

The stats are reported via the proc files. Currently it is available in kernels starting from 028stab069.1.

Files

• /proc/bc/$id/iostat

statistics for beancounter $id

• /proc/bc/iostat

statistics for all beancounters

Format

Files contains one row for each disk-beancounter pair.

Columns are:

N	name	type	description
1	disk	string	Disk device name, e.g. sda or hda, or a special queue (like fuse or flush)
2	ub id	integer	Beancounter id
3	state	char	currently unused (always '.')
4	busy queues	integer	The number of queues with requests (see below)
5	on dispatch	integer	currently unused (always '0')
6	activations count	integer	currently unused (always '0')
7	wait time	integer	Total time in waiting state in milliseconds
8	used time	integer	Total time in active state in milliseconds.
9	requests completed	integer	The number of completed requests
10	sectors transferred	integer	The number of 512 sectors transferred (includes both read and write)

New columns might be added at the end of row in future.

Separate stats exist for fuse and flush, that only report requests and sectors stats (others are always 0).

Example of parsing code: parse_proc_iostat() function in vzstat.c

I/O schedulers

Check available/active I/O schedulers for block device "sda":

# cat /sys/block/sda/queue/scheduler
noop deadline [cfq]

• for "cfq" I/O scheduler: a separate block device line is added in iostat proc file

# cat /proc/bc/100/iostat
flush 100 . 0 0 0 0 0 7389 1893968 0 0
fuse 100 . 0 0 0 0 0 0 0 0 0
sda 100 . 0 0 0 9000 1843380 245216 55845488 245028 188

• for "deadline" I/O scheduler: no additional per-device line is added, iops counters for such devices are added to "flush" line counters (iops limit works)

• for "noop" I/O scheduler: iops are not counted (iops limit does not work)

• for devices with no I/O scheduler (like logical devices, ceph rbd devices, etc): iops are not counted (iops limit does not work)

# cat /sys/block/dm-0/queue/scheduler
none

# cat /sys/block/rbd0/queue/scheduler
none

Queues

Each beancounter may have many queues with requests. Typically there's one queue for each task with synchronous (e.g. reads) requests and and the fixed amount of them for asynchronous requests (e.g. cached writes) for each beancounter.

Interpretation

Disk usage times

The disk usage should be reported in a top-like style. Consider the following code

read_iostat(&a);
sleep(interval);
read_iostat(&b);

Now the following numbers should be calculated and shown.

active  = sum(b.used_time - a.used_time) * 100 / interval;
waiting = sum(b.wait_time - a.wait_time) * 100 / interval;
idle    = 100 - (active + waiting);

The sum function sums up the times for all disk for the beancounter.

Additionally two more values should be shown for beancounter.

IO speed

The value

sum(b.transfered_sectors - a.transfered_sectors) * 512 / interval

denotes the speed of the IO performed by the beancounter.

Average request size

The value

(b.transfered_sectors - a.transfered_sectors)/(b.requests_completed - a.requests_completed)

denotes the average request size for a beancounter to a particular disk.

See also

• IO accounting
• I/O priorities
• I/O limits