Odd RAID Performance Issue

[Ivan Voras]

On 13/02/2012 15:48, Stephen Sanders wrote:
> We've an application that logs data on one very large raid6 array
> and updates/accesses a database on another smaller raid5 array.

You would be better off with RAID10 for a database (or anything which
does random IO).

> Both arrays are connected to the same PCIe 3ware RAID controller.   The
> system has 2 six core 3Ghz processors and 24 GB of RAM.  The system is
> running FreeBSD 8.1.

Did you do any additional OS tuning? Do you use UFS or ZFS?

> The problem we're encountering is that the disk subsystem appears to
> 'pause' periodically.   It looks as if this is a result of disk read/write
> operations from the database array taking a very long time to complete
> (up to 8 sec).

You should be able to monitor this with "iostat -x 1" (or whatever
number of seconds instead of "1") - the last three columns should tell
you if the device(s) are extraordinarily busy, and the r/s and w/s
columns should tell you what the real IOPS rate is. You should probably
post a sample output from this command when the problem appears.



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