topic Re: Write latency in Disk Enclosures

Write latency

Timo Steylen — Fri, 21 Aug 2009 13:17:52 GMT

In EVAPerf i collected the read and write latencies of the fysical disks.
What does us mean? microsecond?
i collected an average write latency (the physical diskgroup) of 14000 with a lot of peaks up to 60.000. Is this too high (i think this is 60 ms)? What causes a high write latency?

Re: Write latency

Víctor Cespón — Mon, 24 Aug 2009 09:07:31 GMT

No, all latencies are reported in ms.
Check here: http://h71028.www7.hp.com/ERC/downloads/5983-1674EN.pdf

page 12

You should have latencies of a few dozens of ms, not several thousands.

What's the drive queue depth?

Re: Write latency

Timo Steylen — Mon, 24 Aug 2009 09:28:09 GMT

the queue depths are from 0 to 1.

If i take a look to the VDISK latencies then it looks normal (write latency < 2ms).
But the latency on the phisycal disk group has an average of 13ms with peaks up to 60ms. Is this the sum of all write latencies from the vdisks? Is this normal?

Re: Write latency

Víctor Cespón — Mon, 24 Aug 2009 10:20:48 GMT

Aaah, that's more beliable. The physical disk latencies are always high, 20 - 40 ms. The disk group latency is lower due to the existence of a cache on the controller.

An I/O operation is considered done once the data is in the cache, no need to wait for the disks. So usually the write latency is 1 - 5 ms. Of course the time it takes to really write that data to the disks is higher, but that's not perceived from the outside.

Unless you have a disk with incredible high latencies (several hundred ms), the numbers you're seeing are normal.

Re: Write latency

Timo Steylen — Mon, 24 Aug 2009 10:26:41 GMT

That explains a lot! So it's normal behaviour. Many thanks!!