Re: PostgreSQL on ZFS: performance tuning

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.

> work_mem = 500MB
> maintenance_work_mem = 2GB
> min_wal_size = 4GB
> max_wal_size = 8GB
> checkpoint_completion_target = 0.9

You probably need to increase the checkpoint_timeout too.

> wal_buffers = 16MB
> default_statistics_target = 500
> random_page_cost = 1
> log_lock_waits = on
> log_directory = 'pg_log'
> log_filename = 'postgresql-%Y-%m-%d_%H%M%S.log'
> log_destination = 'csvlog'
> logging_collector = on
> log_min_duration_statement = 10000
> shared_preload_libraries = 'pg_stat_statements'
> track_activity_query_size = 10000
> track_io_timing = on
>
>
> zfs-stats -A
> ------------------------------------------------------------------------
> ZFS Subsystem Report Thu Jul 28 21:58:46 2016
> ------------------------------------------------------------------------
> ARC Summary: (HEALTHY)
> Memory Throttle Count: 0
> ARC Misc:
> Deleted: 14.92b
> Recycle Misses: 7.01m
> Mutex Misses: 4.72m
> Evict Skips: 1.28b
> ARC Size: 53.27% 32.59 GiB
> Target Size: (Adaptive) 53.28% 32.60 GiB
> Min Size (Hard Limit): 12.50% 7.65 GiB
> Max Size (High Water): 8:1 61.18 GiB
> ARC Size Breakdown:
> Recently Used Cache Size: 92.83% 30.26 GiB
> Frequently Used Cache Size: 7.17% 2.34 GiB
> ARC Hash Breakdown:
> Elements Max: 10.36m
> Elements Current: 78.09% 8.09m
> Collisions: 9.63b
> Chain Max: 26
> Chains: 1.49m
> ------------------------------------------------------------------------
>
> zfs-stats -E
> ------------------------------------------------------------------------
> ZFS Subsystem Report Thu Jul 28 21:59:57 2016
> ------------------------------------------------------------------------
> ARC Efficiency: 49.85b
> Cache Hit Ratio: 70.94% 35.36b
> Cache Miss Ratio: 29.06% 14.49b
> Actual Hit Ratio: 66.32% 33.06b
> Data Demand Efficiency: 84.85% 25.39b
> Data Prefetch Efficiency: 17.85% 12.90b
> CACHE HITS BY CACHE LIST:
> Anonymously Used: 4.10% 1.45b
> Most Recently Used: 37.82% 13.37b
> Most Frequently Used: 55.67% 19.68b
> Most Recently Used Ghost: 0.58% 203.42m
> Most Frequently Used Ghost: 1.84% 649.83m
> CACHE HITS BY DATA TYPE:
> Demand Data: 60.92% 21.54b
> Prefetch Data: 6.51% 2.30b
> Demand Metadata: 32.56% 11.51b
> Prefetch Metadata: 0.00% 358.22k
> CACHE MISSES BY DATA TYPE:
> Demand Data: 26.55% 3.85b
> Prefetch Data: 73.13% 10.59b
> Demand Metadata: 0.31% 44.95m
> Prefetch Metadata: 0.00% 350.48k
>
> zfs-stats -Z
> ------------------------------------------------------------------------
> ZFS Subsystem Report Thu Jul 28 22:02:46 2016
> ------------------------------------------------------------------------
> File-Level Prefetch: (HEALTHY)
> DMU Efficiency: 49.97b
> Hit Ratio: 55.85% 27.90b
> Miss Ratio: 44.15% 22.06b
> Colinear: 22.06b
> Hit Ratio: 0.04% 7.93m
> Miss Ratio: 99.96% 22.05b
> Stride: 17.85b
> Hit Ratio: 99.61% 17.78b
> Miss Ratio: 0.39% 69.46m
> DMU Misc:
> Reclaim: 22.05b
> Successes: 0.05% 10.53m
> Failures: 99.95% 22.04b
> Streams: 10.14b
> +Resets: 0.10% 9.97m
> -Resets: 99.90% 10.13b
> Bogus: 0
>
>
> Notes\concerns:
>
> - primarycache=metadata (recommended in most articles) produces a
> significant performance degradation (in SELECT queries);

Those articles are wrong. PostgreSQL relies of filesystem cache, so it
needs primarycache=all.

>
> - from what I can see, Postgres uses memory too carefully. I would like
> somehow to force it to keep accessed data in memory as long as possible.
> Instead I often see that even frequently accessed data is pushed out of
> memory cache for no apparent reasons.
>

This is probably a consequence of the primarycache misconfiguration.

>
> Do I miss something important in my configs? Are there any double
> writes\reads somewhere because of OS\ZFS\Postgres caches? How to avoid
> them?
>
> Please share your experience\tips. Thanks.
>
>

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