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Add reliability docs about storage/memory corruptions.

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Add section to the Reliability section about what is and is not protected for
various file types.
Further edits welcome.

Designed to allow 1-2 line change when/if checksums are committed.

Inspired by docs written by Jeff Davis, though completely different from his
patch.
This commit is contained in:
Simon Riggs 2013-03-18 22:38:07 +00:00
parent e39feb1006
commit 2266db392c
1 changed files with 44 additions and 0 deletions
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doc/src/sgml/wal.sgml
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@ -177,6 +177,50 @@
(BBU) disk controllers do not prevent partial page writes unless
they guarantee that data is written to the BBU as full (8kB) pages.
</para>
<para>
<productname>PostgreSQL</> also protects against some kinds of data corruption
on storage devices that may occur because of hardware errors or media failure over time,
such as reading/writing garbage data.
<itemizedlist>
<listitem>
<para>
Each individual record in a WAL file is protected by a CRC-32 (32-bit) check
that allows us to tell if record contents are correct. The CRC value
is set when we write each WAL record and checked during crash recovery,
archive recovery and replication.
</para>
</listitem>
<listitem>
<para>
Data pages are not currently checksummed, though full page images recorded
in WAL records will be protected. Data pages have a 16-bit field available
for future use with a data page checksum feature.
</para>
</listitem>
<listitem>
<para>
Internal data structures such as pg_clog, pg_subtrans, pg_multixact,
pg_serial, pg_notify, pg_stat, pg_snapshots, pg_twophase are not directly
checksummed, nor are pages protected by full page writes. However, where
such data structures are persistent, WAL records are written that allow
recent changes to be accurately rebuilt at crash recovery and those
WAL records are protected as discussed above.
</para>
</listitem>
<listitem>
<para>
Temporary data files used in larger SQL queries for sorts,
materializations and intermediate results are not currently checksummed,
nor will WAL records be written for changes to those files.
</para>
</listitem>
</itemizedlist>
</para>
<para>
<productname>PostgreSQL</> does not protect against correctable memory errors
and it is assumed you will operate using RAM that uses industry standard
Error Correcting Codes (ECC) or better protection.
</para>
</sect1>
<sect1 id="wal-intro">