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

From: "scott(dot)marlowe" <scott(dot)marlowe(at)ihs(dot)com>
To: Bjoern Metzdorf <bm(at)turtle-entertainment(dot)de>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on
Date: 2002-11-21 22:37:47
Message-ID: Pine.LNX.4.33.0211211533390.23898-100000@css120.ihs.com (view raw or flat)
Thread:
Lists: pgsql-adminpgsql-performance
On Thu, 21 Nov 2002, Bjoern Metzdorf wrote:

> > Generally RAID 5.  RAID 1 is only faster if you are doing a lot of
> > parellel reads.  I.e. you have something like 10 agents reading at the
> > same time.  RAID 5 also works better under parallel load than a single
> > drive.
> 
> yep, but write performance sucks.

Well, it's not all that bad.  After all, you only have to read the parity 
stripe and data stripe (two reads) update the data stripe, xor the new 
data stripe against the old parity stripe, and write both.  In RAID 1 you 
have to read the old data stripe, update it, and then write it to two 
drives.  So, generally speaking, it's not that much more work on RAID 5 
than 1.  My experience has been that RAID5 is only about 10 to 20% percent 
slower than RAID1 in writing, if that.

> > The fastest of course, is multidrive RAID0.  But there's no redundancy.
> 
> With 4 drives I'd always go for raid 10, fast and secure
> 
> > Oddly, my testing doesn't show any appreciable performance increase in
> > linux by layering RAID5 or 1 over RAID0 or vice versa, something that
> > is usually faster under most setups.
> 
> Is this with linux software raid? raid10 is not significantly faster? cant
> believe that...

Yep, Linux software raid.  It seems like it doesn't parallelize well.  
That's with several different setups.  I've tested it on a machine a dual 
Ultra 40/80 controller and 6 Ultra wide 10krpm SCSI drives, and no matter 
how I arrange the drives, 50, 10, 01, 05, the old 1 or 5 setups are just 
about as fast.


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