RAID-5 and database servers

Support @ Technologist support at technologist.si
Tue Mar 9 17:35:34 CST 2010


Sorry, I just replied to one msg not to the group..
anyway: for you Matt:

Mostly they recommend raid 1+0 or raid 1 + hotspares..
or raid5 with more then 4 discs (has that to do with smaller
datablocks/stripe or the higher troughput??).. otherwise performance can
be very bad..

At least that was in the recommendations of my postgres serverpart..

cheers..


Op dinsdag 09-03-2010 om 23:23 uur [tijdzone +0000], schreef Jefferson
Ogata:
> On 2010-03-09 23:12, Matt Domsch wrote:
> > On Tue, Mar 09, 2010 at 04:54:44PM -0600, John G. Heim wrote:
> >> Has anyone configured a database server with RAID-5? Is it really a bad idea 
> >> to do so? I asked last month for tips on configuring a DB server. I have 
> >> around $6K to spend. I am pretty much settled on getting 2 quad-core CPUs 
> >> and 32 Gb of RAM. But I'm still ignorant in terms of what to get for disk. 
> >> 1500 RPM, I know that. But is it better to do RAID-1 or RAID-5. I can't 
> >> figure out why RAID-1 would be better than RAID-5. I understand that with 
> >> RAID-5, a single database write might translate into writing 2 blocks (a 
> >> data block and a parity block). But doesn't RAID-1 *always* do an extra 
> >> write for every data block written?
> > 
> > RAID 5's problem isn't the extra write.  It's that to write a hunk
> > that's not a whole stripe width (64k * (num_drives - 1)) it has to
> > first read a whole stripe (num_drives-1), calculate the parity, and
> > then write to 2 disks.
> 
> Not really. It can recalculate parity for a single block using the 
> parity block, the new block, and the block it is about to overwrite, 
> regardless of how many disks are in the stripe.
> 
> In any case, RAID 5 (and even RAID 6) implementations are extremely fast 
> nowadays. What you should do is ask your database vendor what numbers 
> they expect in terms of read and write bandwidth and IOPS in order to 
> achieve your performance objective, and then use iozone or similar tools 
> to benchmark the RAID configurations you are considering.
> 
> Note that using direct I/O and/or asynchronous I/O may have a large 
> impact on performance, as well as available memory. The RAID level may 
> be essentially insignificant compared to these factors. Just try to make 
> your RAID block size equal to the database block size. And align your 
> partitions to the block size as well, or don't use partitions at all. 
> See this essay for further info on the latter:
> 
> http://insights.oetiker.ch/linux/raidoptimization/
> 
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