(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">