Re: choosing RAID level for xlogs

Yes, that's true, though, I am a bit confused because the Clariion array
document I am reading talks about how the write cache can eliminate the
RAID5 Write Penalty for sequential and large IOs...resulting in better
sequential write performance than RAID10.

anjan


-----Original Message-----
From: mudfoot@rawbw.com [mailto:mudfoot@rawbw.com]
Sent: Tuesday, August 16, 2005 2:00 PM
To: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] choosing RAID level for xlogs

Quoting Anjan Dave <adave@vantage.com>:

> Hi,
>
>
>
> One simple question. For 125 or more checkpoint segments
> (checkpoint_timeout is 600 seconds, shared_buffers are at 21760 or
> 170MB) on a very busy database, what is more suitable, a separate 6
disk
> RAID5 volume, or a RAID10 volume? Databases will be on separate
> spindles. Disks are 36GB 15KRPM, 2Gb Fiber Channel. Performance is
> paramount, but I don't want to use RAID0.
>

RAID10 -- no question.  xlog activity is overwhelmingly sequential 8KB
writes.
In order for RAID5 to perform a write, the host (or controller) needs to
perform
extra calculations for parity.  This turns into latency.  RAID10 does
not
perform those extra calculations.

>
>
> PG7.4.7 on RHAS 4.0
>
>
>
> I can provide more info if needed.
>
>
>
> Appreciate some recommendations!
>
>
>
> Thanks,
>
> Anjan
>
>
>
>
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