Re: How to setup disk spindles for best performance

On Aug 29, 2008, at 4:43 AM, Greg Smith wrote:

> On Thu, 21 Aug 2008, Christiaan Willemsen wrote:
>
>> Anyway, I'm going to return the controller, because it does not
>> scale very well with more that 4 disks in raid 10. Bandwidth is
>> limited to 350MB/sec, and IOPS scale badly with extra disks...
>
> How did you determine that upper limit?  Usually it takes multiple
> benchmark processes running at once in order to get more than 350MB/
> s out of a controller.  For example, if you look carefully at the
> end of
http://www.commandprompt.com/blogs/joshua_drake/2008/04/is_that_performance_i_smell_ext2_vs_ext3_on_50_spindles_testing_for_postgresql/

>  you can see that Joshua had to throw 8 threads at the disks in
> order to reach maximum bandwidth.

I used IOmeter to do some tests, with 50 worker thread doing jobs. I
can get more than 350 MB/sec, I'll have to use huge blocksizes
(something like 8 MB). Even worse is random read and 70%read, 50%
random tests. They don't scale at all when you add disks.  A 6 disk
raid 5 is exactly as fast as a 12 disk raid 10 :(

>> The idea for xlog + os on 4 disk raid 10 and the rest for the data
>> sound good
>
> I would just use a RAID1 pair for the OS, another pair for the xlog,
> and throw all the other disks into a big 0+1 set.  There is some
> value to separating the WAL from the OS disks, from both the
> performance and the management perspectives.  It's nice to be able
> to monitor the xlog write bandwidth rate under load easily for
> example.

Yes, that's about what I had in mind.

Kind regards,

Christiaan