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Document that BBU's do not allow partial page writes to be safely turned 

off unless they guarantee that all writes to the BBU arrive in 8kB chunks.

Per discussion with Greg Smith
This commit is contained in:
Bruce Momjian 2010-12-22 21:12:00 -05:00
parent 2a0f13a765
commit 7a1ca8977f
1 changed files with 6 additions and 4 deletions
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10
doc/src/sgml/wal.sgml
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@ -167,10 +167,12 @@
<productname>PostgreSQL</> periodically writes full page images to
permanent WAL storage <emphasis>before</> modifying the actual page on
disk. By doing this, during crash recovery <productname>PostgreSQL</> can
restore partially-written pages from WAL. If you have a battery-backed disk
controller or file-system software that prevents partial page writes
(e.g., ZFS), you can safely turn off this page imaging by turning off the
<xref linkend="guc-full-page-writes"> parameter.
restore partially-written pages from WAL. If you have file-system software
that prevents partial page writes (e.g., ZFS), you can turn off
this page imaging by turning off the <xref
linkend="guc-full-page-writes"> parameter. Battery-Backed Unit
(BBU) disk controllers do not prevent partial page writes unless
they guarantee that data is written to the BBU as full (8kB) pages.
</para>
</sect1>