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| -rw-r--r-- | doc/src/sgml/wal.sgml | 44 |
1 files changed, 44 insertions, 0 deletions
diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml index dbaadb6f158..bf1f7b1fca7 100644 --- a/doc/src/sgml/wal.sgml +++ b/doc/src/sgml/wal.sgml @@ -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"> |