Thread: Postgres 9.3 tuning advice

Hello all,

Iam running a postgresql 9.0.13 master/slave instance in a write heavy
workload.

The hardware is a Dell 720, E5530 - 8 core, 128GB RAM. The database (around
250g with indexes/bloat etc) is sitting on flashcache device with 2 fusion-
io PCIe MLC SSDs as frontend and a MD3200 based RAID10 (14*1TB SATA disks)
as backend. OS is centos 6.2 with kernel 2.6.32-431.20.3.el6.x86_64.

Currently, the performance related configuration is mostly default i,e
shared_buffers,
effective_cache_size. The only directive that seems different is
checkpoint_segments = 96

Iam moving to postgresql 9.3 shortly and planning to tune the above
directives as below..

effective_cache_size = 100GB # free+buffers is pretty consistent around 110
to 120GB and pg_oscache_total is around 80GB consistently
checkpoint_segments = 32 # 96 seems to long and all flushes seem to be only
due to checkpoint_timeout

Additionally iam turning off THB defrag as suggested by some posts on the
lists. Though, My initial pgbench testing doesn't seem to indicate any
issues with THB defrag turned on/off.

Iam not sure about shared_buffers and wal_buffers - iam inclined to leave
them to defaults. But based on this article
(http://rhaas.blogspot.in/2012/03/tuning-sharedbuffers-and-walbuffers.html)
it looks there will be some advantages in tuning it

What would be a good value for shared_buffers and wal_buffers ?

Please let me know if additional information will help.

TIA
dushy

dushy wrote:
> Iam running a postgresql 9.0.13 master/slave instance in a write heavy
> workload.
> 
> The hardware is a Dell 720, E5530 - 8 core, 128GB RAM. The database (around
> 250g with indexes/bloat etc) is sitting on flashcache device with 2 fusion-
> io PCIe MLC SSDs as frontend and a MD3200 based RAID10 (14*1TB SATA disks)
> as backend. OS is centos 6.2 with kernel 2.6.32-431.20.3.el6.x86_64.
> 
> Currently, the performance related configuration is mostly default i,e
> shared_buffers,
> effective_cache_size. The only directive that seems different is
> checkpoint_segments = 96
> 
> Iam moving to postgresql 9.3 shortly and planning to tune the above
> directives as below..
> 
> effective_cache_size = 100GB # free+buffers is pretty consistent around 110
> to 120GB and pg_oscache_total is around 80GB consistently
> checkpoint_segments = 32 # 96 seems to long and all flushes seem to be only
> due to checkpoint_timeout
> 
> Additionally iam turning off THB defrag as suggested by some posts on the
> lists. Though, My initial pgbench testing doesn't seem to indicate any
> issues with THB defrag turned on/off.
> 
> Iam not sure about shared_buffers and wal_buffers - iam inclined to leave
> them to defaults. But based on this article
> (http://rhaas.blogspot.in/2012/03/tuning-sharedbuffers-and-walbuffers.html)
> it looks there will be some advantages in tuning it
> 
> What would be a good value for shared_buffers and wal_buffers ?
> 
> Please let me know if additional information will help.

The frequently heard advice for setting shared_buffers is 25% of RAM, but
with memory as big as that that may be too much (it can lead to checkpoint
I/O spikes and greater overhead in managing shared buffers).
Try with something like 8 or 16 GB.
Ideally you should test, use pg_buffercache to inspect shared buffers
and see what setting works best for you.

Set wal_buffers to 16MB so that a whole WAL segment will fit.

The best tuning strategy would be to stuff another 128 GB RAM into
the machine and have your DB in RAM.

Yours,
Laurenz Albe

On Wed, Aug 13, 2014 at 9:44 AM, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:
> Set wal_buffers to 16MB so that a whole WAL segment will fit.

No need, wal_buffers is automatically tuned now. If your
shared_buffers is 512MB or larger, wal_buffers will be 16MB.

Regards,
Marti