Re: PostgreSQL on ZFS: performance tuning

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.

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Karl Denninger
karl(at)denninger(dot)net <mailto:karl(at)denninger(dot)net>
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