Check Disk Performance in Linux

We use iostat command to check the disk performance.

Reports that we get from ‘iostat’ are really useful but I myself had a little bit of trouble when trying to interpret the results while using the the first time.

I usually use the iostat command with the following switches:

iostat –d –x <interval>

Where in…

-d = gets rid of the CPU stats so that we can easily concentrate on the I/O only

-x = some additional info like ‘await’ and ‘svctm’ (will discuss them later)

<interval> = this is time in seconds, so every number of <interval> seconds you will get a new ‘iostat’ report

Let’s now see a sample output of ‘iostat’:

If we look at stats above usually we would look at %util and if we see close to 100% it can identify the problem for a single disk setup, but not in a usual multi-disks scenario.

Columns that we look at it in order to identify the problem will be:

syvctm: The average service time (in milliseconds) for I/O requests that were issued to the device

await: The average time (in milliseconds) for I/O requests issued to the device to be served. This includes the time spent by the requests in queue and the time spent servicing them.

This basically means:

 await = syvctm + wait time in queue

Now using the above we can have a basic rule to identify an overloaded setup:

…if you can see a lot of difference in values for ‘syvctm’ and ‘await’ every now and then, that can tell you about I/O requests being going into long waits and this should help you identify the problem.