Re: choosing RAID level for xlogs

The other point that is well made is that with enough drives you will
max out the PCI bus before you max out the drives.  64-bit 66Mhz can
do about 400MB/sec, which can be acheived by two 3 drive stripes (6
drive in RAID 10).  A true PCI-X card can do better, but can your
controller?  Remember, U320 is only 320MB/channel...

Alex Turner
NetEconomist

On 8/16/05, Anjan Dave <adave@vantage.com> wrote:
> Thanks, everyone. I got some excellent replies, including some long explanations. Appreciate the time you guys took
outfor 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)
couldbenefit from by filling it up and writing out the complete stripe, but come to think of it, it's shared among the
twoStorage Processors, all the LUNs, not just the ones holding the pg_xlog directory. The other thing (with Clariion)
isthe write cache mirroring. Write isn't signalled complete to the host until the cache content is mirrored across the
otherSP (and vice-versa), which is a good thing, but this operation could potentially become a bottleneck with very
highload on the SPs. 
>
> Also, one would have to fully trust the controller/manufacturer's claim on signalling the write completion. And,
performanceis 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@globexplorer.com]
>         Sent: Tue 8/16/2005 6:22 PM
>         To: Anjan Dave; pgsql-performance@postgresql.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
belowis 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@postgresql.org on behalf of Anjan Dave
>         Sent:   Mon 8/15/2005 1:35 PM
>         To:     pgsql-performance@postgresql.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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