Re: PostgreSQL on ZFS: performance tuning

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

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services