RAID-5 and database servers

Eberhard Moenkeberg emoenke at gwdg.de
Tue Mar 9 17:31:46 CST 2010


Hi,

On Tue, 9 Mar 2010, 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.

It is not this bad.
Raid5 needs to read the old block and the parity block, to "calculate out" 
the old block from the parity, to "calculate in" the new block into 
parity, write the new block, write the parity block.

So a raid5 write does 2 reads from different disks and two writes to 
different disks, plus calculation.

A raid1 write just does 2 writes to different disks.
Raid1 can have a double throughput advantage during read - just using 
disk1 AND disk2 - if the controller supports it.


Viele Gruesse
Eberhard Moenkeberg (emoenke at gwdg.de, em at kki.org)

-- 
Eberhard Moenkeberg
Arbeitsgruppe IT-Infrastruktur
E-Mail: emoenke at gwdg.de      Tel.: +49 (0)551 201-1551
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