Franck Routier wrote:
Hi,
I am installing a database (postgresql) server.
I am considering two options:
- either setup two 6 disks raid10 arrays
- or setup three 4 disks raid10 arrays
You guessed I have 12 disks :)
Raw performance is better on 6 disks arrays, but having 3 arrays allows
me to setup 3 tablespaces and maybe to achieve better parallelism.
I am under the impression that I will get better results with requests
spread over 3 less effective arrays rather than two slightly more effive
one.
Does it make any sense, or am I totally missing the point ?
Thanks,
Franck
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Hi
No, it makes total sense.
Having just done a lot of work on optimising Postgresql:
How many distinct table spaces depends on your expected usage pattern -
ie if you have one table that is hammered for updates all the time you
might benefit from having the table storage on a physically separate
tablespace to any indexes for that table.
If you are hammering some tables for updates whilst querying other
tables at a high rate, placing the updating tables on a different
tablespace to the ones you are querying may benefit.
The nice thing about Postgresql 8.1 upwards (at least I haven't tried
this under 8.0) is that you can ALTER TABLE|INDEX to use a different
tablespace at run time on a live database, so experimentation is easy.
However, the single biggest improvement I found was to ensure that the
WAL is redirected to a otherwise quiescent disk.
In my case, I arranged two physically separate storage volumes thus:
VOL1: OS (/) + WAL
VOL2: DB storage (minus WAL) + /var/log
Taking /var/log off VOL1 rendered it fairly quiet after all applications
had started and having the WAL on a quiet volume gave me a tenfold
improvement in INSERT rate, so not insignificant.
If you are expecting heavy insert/update accesses, I would suggest you
take two disks off as a RAID1 mirror and devote them entirely to the WAL.
Cheers
Tim
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