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Re: Opinions on Raid

From: Ron <rjpeace(at)earthlink(dot)net>
To: "Joe Uhl" <joeuhl(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Opinions on Raid
Date: 2007-02-27 16:05:39
Message-ID: E1HM4pm-0004QM-Oi@elasmtp-kukur.atl.sa.earthlink.net (view raw or flat)
Thread:
Lists: pgsql-performance
At 08:12 AM 2/27/2007, Joe Uhl wrote:
>We have been running Postgres on a 2U server with 2 disks configured in
>raid 1 for the os and logs and 4 disks configured in raid 10 for the
>data.  I have since been told raid 5 would have been a better option
>given our usage of Dell equipment and the way they handle raid 10.  I
>have just a few general questions about raid with respect to Postgres:
>
>[1] What is the performance penalty of software raid over hardware raid?
>  Is it truly significant?  We will be working with 100s of GB to 1-2 TB
>of data eventually.
The real CPU overhead when using SW RAID is when using any form of SW 
RAID that does XOR operations as part of writes (RAID 5, 6, 50, ..., 
etc).  At that point, you are essentially hammering on the CPU just 
as hard as you would on a dedicated RAID controller... ...and the 
dedicated RAID controller probably has custom HW helping it do this 
sort of thing more efficiently.
That being said, SW RAID 5 in this sort of scenario can be reasonable 
if you =dedicate= a CPU core to it.  So in such a system, your "n" 
core box is essentially a "n-1" core box because you have to lock a 
core to doing nothing but RAID management.
Religious wars aside, this actually can work well.  You just have to 
understand and accept what needs to be done.

SW RAID 1, or 10, or etc should not impose a great deal of CPU 
overhead, and often can be =faster= than a dedicated RAID controller.

SW RAID 5 etc in usage scenarios involving far more reads than writes 
and light write loads can work quite well even if you don't dedicate 
a core to RAID management, but you must be careful about workloads 
that are, or that contain parts that are, examples of the first 
scenario I gave.  If you have any doubts about whether you are doing 
too many writes, dedicate a core to RAID stuff as in the first scenario.


>[2] How do people on this list monitor their hardware raid?  Thus far we
>have used Dell and the only way to easily monitor disk status is to use
>their openmanage application.  Do other controllers offer easier means
>of monitoring individual disks in a raid configuration?  It seems one
>advantage software raid has is the ease of monitoring.
Many RAID controller manufacturers and storage product companies 
offer reasonable monitoring / management tools.

3ware AKA AMCC has a good reputation in this area for their cards.
So does Areca.
I personally do not like Adaptec's SW for this purpose, but YMMV.
LSI Logic has had both good and bad SW in this area over the years.

Dell, HP, IBM, etc's offerings in this area tend to be product line 
specific.  I'd insist on  some sort of  "try before you buy" if the 
ease of use / quality of the SW matters to your overall purchase decision.

Then there are the various CSSW and OSSW packages that contain this 
functionality or are dedicated to it.  Go find some reputable reviews.
(HEY LURKERS FROM Tweakers.net:  ^^^ THAT"S AN ARTICLE IDEA ;-) )

Cheers,
Ron 


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