Linux-PowerEdge Digest, Vol 58, Issue 18

[John LLOYD]

> From: Brian Mathis <brian.mathis at gmail.com>
> Subject: Tuning RAID performance: stripe alignment, etc...
> To: linux-poweredge at dell.com
> Message-ID:
> 	<183c528b0905111938pa46acc3r1f8e95a7529e0575 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> I have a new R710 server that will be running CentOS 5 with a PERC6/i
> controller and (4) 450GB 15K SAS drives configured as a RAID10 (plus a
> hot spare).  This system will be a database server so I/O performance
> is top priority.  I've been doing a lot of searching on how to get the
> best performance with regard to correct PV and stride alignment using
> Linux and LVM, but so far I've only been able to find a few vague
> guides and emails discussing the issue, but no real information on how
> to ensure everything is aligned properly.
> 
> I suspect this is somewhat server-specific, which is why I'm posting
> here.  Anyone have recommendations on how best to set this server up
> for best performance?

Since you are already using RAID10 (mirrored pairs of disks which are
striped together) you are pretty well already in the right zone.  The
6/i does not (afaik) have a battery so you will have to put the system
on a UPS, and force the RAID into write-back caching.  Without this you
will see a difference in throughput.

In my experience, aligning stripes for RAID-0 or RAID-1 does not help
much; it is useful on RAID-5 tho.  Remember that stripe size is for
allocation purposes only, it has no effect on actual I/O transfer size.

Other tuning tips: configure just the "fast" part of the disk for
database and leave the rest empty, or, at the least, unused except for
off-peak hours.  (Disks are fastest in the first few tracks where the
bit recording density is highest, that is, at the outer edge of the
surface).  Simply partition the first part for use by database and the
rest not at all.

For database work, memory is king, so configure the largest practical
amount of RAM for the database software.  For Oracle, 2/3 of RAM. 

If you have the time, and a useful synthetic benchmark, just for kicks
try configuring your four disks as a JBOD and use LVM or MD to make your
RAID-10.  You are likely to get somewhat different performance than
using the PERC for RAID.



--John