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Re: [ADMIN] H/W RAID 5 on slower disks versus no

From: "Josh Berkus" <josh(at)agliodbs(dot)com>
To: "Bjoern Metzdorf" <bm(at)turtle-entertainment(dot)de>,"scott(dot)marlowe" <scott(dot)marlowe(at)ihs(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: [ADMIN] H/W RAID 5 on slower disks versus no
Date: 2002-11-21 21:21:16
Message-ID: web-1836149@davinci.ethosmedia.com (view raw or flat)
Thread:
Lists: pgsql-adminpgsql-performance
Bjoern,

> Good to know.
> 
> What do you think is faster: 3 drives in raid 1 or 3 drives in raid
> 5?

My experience?  Raid 1.  But that depends on other factors as well;
your controller (software controllers use system RAM and thus lower
performance), what kind of reads you're getting and how often.  IMHO,
RAID 5 is faster for sequential reads (lareg numbers of records on
clustered indexes), RAID 1 for random reads.

And keep in mind: RAID 5 is *bad* for data writes.  In my experience,
database data-write performance on RAID 5 UW SCSI is as slow as IDE
drives, particulary for updating large numbers of records, *unless* the
updated records are sequentially updated and clustered.

But in a multi-user write-often setup, RAID 5 will slow you down and
RAID 1 is better.

Did that help?

-Josh Berkus

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