Re: choosing RAID level for xlogs

Thanks, everyone. I got some excellent replies, including some long explanations. Appreciate the time you guys took out for the responses.

The gist of it i take, is to use RAID10. I have 400MB+ of write cache on the controller(s), that the RAID5 LUN(s) could benefit from by filling it up and writing out the complete stripe, but come to think of it, it's shared among the two Storage Processors, all the LUNs, not just the ones holding the pg_xlog directory. The other thing (with Clariion) is the write cache mirroring. Write isn't signalled complete to the host until the cache content is mirrored across the other SP (and vice-versa), which is a good thing, but this operation could potentially become a bottleneck with very high load on the SPs.

Also, one would have to fully trust the controller/manufacturer's claim on signalling the write completion. And, performance is a priority over the drive space lost in RAID10 for me.

I can use 4 drives instead of 6.

Thanks,
Anjan

t-----Original Message-----
From: Gregory S. Williamson [mailto:gsw(at)globexplorer(dot)com]
Sent: Tue 8/16/2005 6:22 PM
To: Anjan Dave; pgsql-performance(at)postgresql(dot)org
Cc:
Subject: RE: [PERFORM] choosing RAID level for xlogs

I would be very cautious about ever using RAID5, despite manufacturers' claims to the contrary. The link below is authored by a very knowledgable fellow whose posts I know (and trust) from Informix land.

<http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt>

Greg Williamson
DBA
GlobeXplorer LLC

-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org on behalf of Anjan Dave
Sent: Mon 8/15/2005 1:35 PM
To: pgsql-performance(at)postgresql(dot)org
Cc:
Subject: [PERFORM] choosing RAID level for xlogs
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.

PG7.4.7 on RHAS 4.0

I can provide more info if needed.

Appreciate some recommendations!

Thanks,

Anjan

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