MD1000 and PERC5/E I/O bottleneck

Eric Yablonowitz eyablon at tripadvisor.com
Sun Jun 3 20:36:57 CDT 2007


RAID-1 (inherently 2 disks) offers much less write throughput than a  
7 disk RAID-5 on the PERC-5 in my tests.  RAID-10 on 6 disks was  
closer, but still (somewhat surprisingly) slower than RAID-5 on 7 disks.

On Jun 3, 2007, at 9:21 PM, Peter Verhoeven wrote:

> Can you afford to go with RAID-1 instead of RAID 5?
>
>
> -----Original Message-----
> From: linux-poweredge-bounces at dell.com
> [mailto:linux-poweredge-bounces at dell.com] On Behalf Of Eric  
> Yablonowitz
> Sent: Saturday, 2 June 2007 9:03 AM
> To: linux-poweredge at dell.com
> Subject: MD1000 and PERC5/E I/O bottleneck
>
> Hello,
>
> I am trying to tune the I/O for a PostgreSQL DB server, and I'm  
> running
> into a bottleneck.
>
> Hardware:
>
> Dell PE-6950 w/ 32GB RAM and 4 dual core Opterons PERC-5/E RAID  
> card in
> slot 3 (PCIe-8x slot) MD1000 array w/ 15 x 146GB 15K-RPM SAS HDDs
>
> Software:
>
> CentOS 5 (2.6.18-8.1.4.el5 x86_64)
> bonnie++ for benchmarking
>
> I am most concerned about WRITE I/O performance as my intention is to
> keep the DB small enough to stay inside of the OS cache (making reads
> unimportant once the cache is seeded).  It seems no matter what I do I
> cannot get better than about 280MB/s write throughput.
>
> I configure the MD1000 with 2 7-HDD RAID 5 virtual disks (VDs).
> Here's what I get:
>
> running one bonnie++ process on one virtual disk - 184MB/s running two
> bonnie++ processes on one virtual disk - 210MB/s Running three+
> processes starts to degrade performance...
>
> Ok, so it would seem that a single 7 spindle RAID 5 VD can handle  
> around
> 210MB/s of writes.  Fair enough.  But I have two of these VDs.  What
> happens if I run bonnie++ on the second VD simultaneously?  My
> assumption was that I should be able to sustain
> similar throughput on both arrays simultaneously.   After all, the
> buses involved (SAS and PCIe) are nowhere near saturation at these
> rates.  But here's what I get:
>
> running one bonnie++ process on each virtual disk - 138MB/s per  
> process
> (276MB/s total throughput) running two bonnie++ processes on each
> virtual disk - 64MB/s per process (128MB/s per VD and 256MB/s total
> throughput)
>
> So these results imply that there is a bottleneck somewhere that is
> preventing me from getting anywhere near what my 14 spindles can  
> handle.
> Any thoughts on what this bottleneck might be?
>
> Thanks,
> Eric
>
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