diff options
Diffstat (limited to 'doc/src/sgml/maintenance.sgml')
| -rw-r--r-- | doc/src/sgml/maintenance.sgml | 33 |
1 files changed, 25 insertions, 8 deletions
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml index f84ad7557d9..b5b9da7f8a9 100644 --- a/doc/src/sgml/maintenance.sgml +++ b/doc/src/sgml/maintenance.sgml @@ -496,9 +496,25 @@ When that happens, <command>VACUUM</command> will eventually need to perform an <firstterm>aggressive vacuum</firstterm>, which will freeze all eligible unfrozen XID and MXID values, including those from all-visible but not all-frozen pages. - In practice most tables require periodic aggressive vacuuming. + </para> + + <para> + If a table is building up a backlog of all-visible but not all-frozen + pages, a normal vacuum may choose to scan skippable pages in an effort to + freeze them. Doing so decreases the number of pages the next aggressive + vacuum must scan. These are referred to as <firstterm>eagerly + scanned</firstterm> pages. Eager scanning can be tuned to attempt to freeze + more all-visible pages by increasing <xref + linkend="guc-vacuum-max-eager-freeze-failure-rate"/>. Even if eager + scanning has kept the number of all-visible but not all-frozen pages to a + minimum, most tables still require periodic aggressive vacuuming. However, + any pages successfully eager frozen may be skipped during an aggressive + vacuum, so eager freezing may minimize the overhead of aggressive vacuums. + </para> + + <para> <xref linkend="guc-vacuum-freeze-table-age"/> - controls when <command>VACUUM</command> does that: all-visible but not all-frozen + controls when a table is aggressively vacuumed. All all-visible but not all-frozen pages are scanned if the number of transactions that have passed since the last such scan is greater than <varname>vacuum_freeze_table_age</varname> minus <varname>vacuum_freeze_min_age</varname>. Setting @@ -626,10 +642,12 @@ SELECT datname, age(datfrozenxid) FROM pg_database; </tip> <para> - <command>VACUUM</command> normally only scans pages that have been modified - since the last vacuum, but <structfield>relfrozenxid</structfield> can only be - advanced when every page of the table - that might contain unfrozen XIDs is scanned. This happens when + While <command>VACUUM</command> scans mostly pages that have been + modified since the last vacuum, it may also eagerly scan some + all-visible but not all-frozen pages in an attempt to freeze them, but + the <structfield>relfrozenxid</structfield> will only be advanced when + every page of the table that might contain unfrozen XIDs is scanned. + This happens when <structfield>relfrozenxid</structfield> is more than <varname>vacuum_freeze_table_age</varname> transactions old, when <command>VACUUM</command>'s <literal>FREEZE</literal> option is used, or when all @@ -931,8 +949,7 @@ vacuum insert threshold = vacuum base insert threshold + vacuum insert scale fac If the <structfield>relfrozenxid</structfield> value of the table is more than <varname>vacuum_freeze_table_age</varname> transactions old, an aggressive vacuum is performed to freeze old tuples and advance - <structfield>relfrozenxid</structfield>; otherwise, only pages that have been modified - since the last vacuum are scanned. + <structfield>relfrozenxid</structfield>. </para> <para> |