[Beowulf] LVM performance problems

[Michael Will]

What model server is this? 

What level raid is the software raid volume, how many drives and what
type?

How did you exactly establish the performance number?

I have done performance benchmarking with LVM over hardware raid luns
had have not been able to
show a significant performance degradation, and it is actually faster
when you stripe across
several raid-protected luns. 

Some general disk benchmarking rules:

Write at least twice as large files as you have RAM (so that's 8G
minimum in your case) 
so caching does not inflate the numbers. Also don't forget to measure
the time for it to be synced to disk. 

Example for large block I/O large file benchmarking on a 4G system:

time dd if=/dev/zero of=/mnt/targetfilesystem/largefile bs=1M count=8192
time sync

If you divide 8192 by the aggragate of the seconds used by both commands
then you have a good MB/s estimate.

For read you can just do:
time dd of=/dev/null if=/mnt/targetfilesystem/largefile bs=1M count=8192

To do some more intense testing including mixed read/write, I use
bonnie++. If you want to turn it into a real
science and get very specific data for specific access patterns, you
might want to look at iozone.

Michael
-----Original Message-----
From: beowulf-bounces at beowulf.org [mailto:beowulf-bounces at beowulf.org]
On Behalf Of Judd Tracy
Sent: Friday, May 04, 2007 6:41 AM
To: beowulf at beowulf.org
Subject: [Beowulf] LVM performance problems

I am trying to bring up a small file server and am noticing some serious
performance issues when using LVM.  I created a software raid /dev/md0
which I can read at ~195MB/s using the raw raid device, but as soon as I
put LVM on top of it the read speeds drop to ~95MB/s using a raw lvm
partition without a filesystem.  When I use and xfs filesystem on top of
the lvm partition it drops down to ~40MB/s.  This seems like a
processing power issue, but the machine is a dual processor opteron
system with 4GB of ram in it.  Does anyone have some insight into why I
might be having such problems with the system.

Judd Tracy
Institute for Simulation and Training
University of Central Florida
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