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To: "Kenneth Cox" <kenstir@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: Advice configuring ServeRAID 8k for performance
References: <op.vgy9k5ov5ru9c3@kent60.office.vivox.com>
Date: Fri, 06 Aug 2010 00:27:43 +0200
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From: "Pierre C" <lists@peufeu.com>
Message-ID: <op.vgzkohd0eorkce@apollo13>
In-Reply-To: <op.vgy9k5ov5ru9c3@kent60.office.vivox.com>
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> 1) Should I switch to RAID 10 for performance?  I see things like "RAID  
> 5 is bad for a DB" and "RAID 5 is slow with <= 6 drives" but I see  
> little on RAID 6.

As others said, RAID6 is RAID5 + a hot spare.

Basically when you UPDATE a row, at some point postgres will write the  
page which contains that row.

RAID10 : write the page to all mirrors.
RAID5/6 : write the page to the relevant disk. Read the corresponding page  
 from all disks (minus one), compute parity, write parity.

As you can see one small write will need to hog all drives in the array.  
RAID5/6 performance for small random writes is really, really bad.

Databases like RAID10 for reads too because when you need some random data  
you can get it from any of the mirrors, so you get increased parallelism  
on reads too.

> with good redundancy.  My current performance is 85MB/s write, 151 MB/s  
> reads

FYI, I get 200 MB/s sequential out of the software RAID5 of 3 cheap  
desktop consumer SATA drives in my home multimedia server...