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
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