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docs: Workload Tuning: Expand PostgreSQL section #601

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22 changes: 19 additions & 3 deletions docs/Performance and Tuning/Workload Tuning.rst
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Expand Up @@ -652,9 +652,23 @@ PostgreSQL

Make separate datasets for PostgreSQL's data and WAL. Set
``compression=lz4`` and ``recordsize=32K`` (64K also work well, as
does the 128K default) on both. Configure ``full_page_writes = off``
for PostgreSQL, as ZFS will never commit a partial write. For a database
with large updates, experiment with ``logbias=throughput`` on
does the 128K default) on both.
Previous advice before ``compression=lz4`` was to to set
``recordsize=8K`` to match PostgreSQL's 8K page size to avoid
write amplification, but compression often causes even bigger
performance improvements [#compression_larger_block_size]_,
and compression is only effective when given larger ``recordesize``.

Configure ``full_page_writes = off``
for PostgreSQL, as ZFS will never commit a partial write.
But do not do this if you set ``recordsize`` < 8K for some reason,
or if there is any chance that somebody may
copy this database config to a non-ZFS system,
as PostgreSQL can end up with corrupt data on power loss
if the filesystem does not provide at least
Postgres-page-sized atomic writes [#postgres_full_page_writes]_.

For a database with large updates, experiment with ``logbias=throughput`` on
PostgreSQL's data to avoid writing twice, but be aware that with this
setting smaller updates can cause severe fragmentation.

Expand Down Expand Up @@ -778,6 +792,8 @@ AIO should be used to maximize IOPS when using files for guest storage.

.. rubric:: Footnotes

.. [#compression_larger_block_size] <https://github.com/openzfs/openzfs-docs/pull/369>
.. [#postgres_full_page_writes] https://wiki.postgresql.org/wiki/Full_page_writes
.. [#ssd_iops] <http://www.anandtech.com/show/6489/playing-with-op>
.. [#mysql_basic] <https://www.patpro.net/blog/index.php/2014/03/09/2617-mysql-on-zfs-on-freebsd/>
.. [#sqlite_ps] <https://www.sqlite.org/pragma.html#pragma_page_size>
Expand Down