Iostat
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iostat is a program that shows how many I/O operations happen in a specified timeframe.
Contents
Install it
To install iostat, install the package sysstat. For example with SUSE Linux you open a console and enter:
yast -i sysstat
Run it
Just call IOstat plus the time interval in seconds:
# iostat 10
Linux 2.6.34.7-0.5-desktop (tweedleburg) 12/11/10 _x86_64_ (8 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
0.67 0.00 0.17 0.07 0.00 99.09
Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sdb 1.35 5.76 28.55 1387011 6870465
sda 1.35 5.56 28.55 1337701 6870465
md126 4.26 11.32 28.38 2723674 6830024
You will get a new block every 10 seconds in this case.
Zoom in to one device, 1 second interval:
# iostat -xmt 1 -d /dev/sda
Linux 2.6.34.7-0.7-desktop (tweedleburg) 03/03/11 _x86_64_ (8 CPU)
03/03/11 08:46:33
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
sda 0.03 1.63 0.06 0.72 0.00 0.01 26.56 0.08 108.28 4.64 0.36
03/03/11 08:46:34
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
avgrq-sz
avgrq-sz is the average request size ("blocksize") in 512 byte sectors. You can influence it with dd's iflag/oflag parameter, for example:
dd if=/dev/dm-11 of=/dev/null iflag=direct bs=64K
Note there is a max block size under /sys/block/device/queue/max_sector_kb
