Re: PostgreSQL on ZFS: performance tuning
От | Tomas Vondra |
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Тема | Re: PostgreSQL on ZFS: performance tuning |
Дата | |
Msg-id | 2a522e5a-c79b-9bd5-46c8-441e8e762a87@2ndquadrant.com обсуждение исходный текст |
Ответ на | Re: PostgreSQL on ZFS: performance tuning (Torsten Zuehlsdorff <mailinglists@toco-domains.de>) |
Ответы |
Re: PostgreSQL on ZFS: performance tuning
(Karl Denninger <karl@denninger.net>)
|
Список | pgsql-performance |
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
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