Re: Advice configuring ServeRAID 8k for performance

On 06/08/10 06:28, Kenneth Cox wrote:
> I am using PostgreSQL 8.3.7 on a dedicated IBM 3660 with 24GB RAM
> running CentOS 5.4 x86_64. I have a ServeRAID 8k controller with 6
> SATA 7500RPM disks in RAID 6, and for the OLAP workload it feels*
> slow. I have 6 more disks to add, and the RAID has to be rebuilt in
> any case, but first I would like to solicit general advice. I know
> that's little data to go on, and I believe in the scientific method,
> but in this case I don't have the time to make many iterations.
>
> My questions are simple, but in my reading I have not been able to
> find definitive answers:
>
> 1) Should I switch to RAID 10 for performance? I see things like
> "RAID 5 is bad for a DB" and "RAID 5 is slow with <= 6 drives" but I
> see little on RAID 6. RAID 6 was the original choice for more usable
> space with good redundancy. My current performance is 85MB/s write,
> 151 MB/s reads (using dd of 2xRAM per
> http://www.westnet.com/~gsmith/content/postgresql/pg-disktesting.htm).
>

Normally I'd agree with the others and recommend RAID10 - but you say
you have an OLAP workload - if it is *heavily* read biased you may get
better performance with RAID5 (more effective disks to read from).
Having said that, your sequential read performance right now is pretty
low (151 MB/s - should be double this), which may point to an issue
with this controller. Unfortunately this *may* be important for an OLAP
workload (seq scans of big tables).

> 2) Should I configure the ext3 file system with noatime and/or
> data=writeback or data=ordered? My controller has a battery, the
> logical drive has write cache enabled (write-back), and the physical
> devices have write cache disabled (write-through).
>

Probably wise to use noatime. If you have a heavy write workload (i.e so
what I just wrote above does *not* apply), then you might find adjusting
the ext3 commit interval upwards from its default of 5 seconds can help
(I'm doing some testing at the moment and commit=20 seemed to improve
performance by about 5-10%).

> 3) Do I just need to spend more time configuring postgresql? My
> non-default settings were largely generated by pgtune-0.9.3:
>
> max_locks_per_transaction = 128 # manual; avoiding "out of shared
> memory"
> default_statistics_target = 100
> maintenance_work_mem = 1GB
> constraint_exclusion = on
> checkpoint_completion_target = 0.9
> effective_cache_size = 16GB
> work_mem = 352MB
> wal_buffers = 32MB
> checkpoint_segments = 64
> shared_buffers = 2316MB
> max_connections = 32
>

Possibly higher checkpoint_segments and lower wal_buffers (I recall
someone - maybe Greg suggesting that there was no benefit in having the
latter > 10MB). I wonder about setting shared_buffers higher - how large
is the database?

Cheers

Mark