MD1000 and PERC5/E I/O bottleneck

Peter Verhoeven Peter at morrisint.com.au
Sun Jun 3 20:21:52 CDT 2007


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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