Re: PostgreSQL on ZFS: performance tuning - Mailing list pgsql-performance

On 9/27/2016 16:38, Tomas Vondra wrote:

On 09/27/2016 06:00 PM, Torsten Zuehlsdorff wrote:

On 29.07.2016 08:30, Tomas Vondra wrote:

On 07/29/2016 08:04 AM, trafdev wrote:

Hi.

I have an OLAP-oriented DB (light occasional bulk writes and heavy
aggregated selects over large periods of data) based on Postgres 9.5.3.

Server is a FreeBSD 10.3 with 64GB of RAM and 2x500GB SSD (root on ZFS,
mirror).

The largest table is 13GB (with a 4GB index on it), other tables are 4,
2 and less than 1GB.

After reading a lot of articles and "howto-s" I've collected following
set of tweaks and hints:


ZFS pools creation:
zfs create zroot/ara/sqldb
zfs create -o recordsize=8k -o primarycache=all zroot/ara/sqldb/pgsql


zfs get primarycache,recordsize,logbias,compression
zroot/ara/sqldb/pgsql
NAME                   PROPERTY      VALUE         SOURCE
zroot/ara/sqldb/pgsql  primarycache  all           local
zroot/ara/sqldb/pgsql  recordsize    8K            local
zroot/ara/sqldb/pgsql  logbias       latency       local
zroot/ara/sqldb/pgsql  compression   lz4           inherited from zroot

L2ARC is disabled
VDEV cache is disabled


pgsql -c "mkdir /ara/sqldb/pgsql/data_ix"
pgsql -c "initdb --locale=en_US.UTF-8 -E UTF-8 -D /ara/sqldb/pgsql/data"


/etc/sysctl.conf
vfs.zfs.metaslab.lba_weighting_enabled=0


postgresql.conf:
listen_addresses = '*'
max_connections = 100
shared_buffers = 16GB
effective_cache_size = 48GB

It may not be a problem for your workload, but this effective_cache_size
value is far too high.

May i asked why? ZFS in default caches your size of RAM minus 1 GB.
Getting the shared buffer from the 64 GB RAM i would asume 47 GB
would be a better value. But this would not be far too high. So
please can you explain this?

Because it's not a global value, but an estimate of how much RAM is available as a cache for a single query. So if you're running 10 queries at the same time, they'll have to share the memory.

It's a bit trickier as there's often a fair amount of cross-backend sharing (backends accessing the same data, so it's likely one backend loads data into cache, and then other backends access it too).

It also ignores that memory may get allocated for other reasons - some queries may allocate quite a bit of memory for sorts/aggregations, so not only is

   effective_cache_size = RAM - shared_buffers

excessive as it ignores the per-query nature, but also because it neglects these other allocations.

regards

You may well find that with lz4 compression a 128kb record size on that filesystem is materially faster -- it is here for most workloads under Postgres.

--
Karl Denninger
karl@denninger.net
The Market Ticker
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