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Rewrite the FSM. Instead of relying on a fixed-size shared memory segment, the
free space information is stored in a dedicated FSM relation fork, with each relation (except for hash indexes; they don't use FSM). This eliminates the max_fsm_relations and max_fsm_pages GUC options; remove any trace of them from the backend, initdb, and documentation. Rewrite contrib/pg_freespacemap to match the new FSM implementation. Also introduce a new variant of the get_raw_page(regclass, int4, int4) function in contrib/pageinspect that let's you to return pages from any relation fork, and a new fsm_page_contents() function to inspect the new FSM pages.
This commit is contained in:
Heikki Linnakangas
@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/acronyms.sgml,v 1.5 2008/03/18 16:05:07 mha Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/acronyms.sgml,v 1.6 2008/09/30 10:52:09 heikki Exp $ -->
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<appendix id="acronyms">
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<title>Acronyms</title>
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@ -216,7 +216,7 @@
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<term><acronym>FSM</acronym></term>
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<listitem>
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<para>
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<link linkend="runtime-config-resource-fsm">Free Space Map</link>
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<link linkend="storage-fsm">Free Space Map</link>
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</para>
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</listitem>
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</varlistentry>
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@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.190 2008/08/25 19:03:37 tgl Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.191 2008/09/30 10:52:09 heikki Exp $ -->
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<chapter Id="runtime-config">
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<title>Server Configuration</title>
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@ -896,80 +896,6 @@ SET ENABLE_SEQSCAN TO OFF;
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2 id="runtime-config-resource-fsm">
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<title>Free Space Map</title>
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<indexterm>
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<primary>free space map</primary>
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</indexterm>
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<para>
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These parameters control the size of the shared <firstterm>free space
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map</> (<acronym>FSM</>), which tracks the locations of unused space in the database.
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An undersized free space map can cause the database to consume
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increasing amounts of disk space over time, because free space that
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is not in the map cannot be re-used; instead <productname>PostgreSQL</>
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will request more disk space from the operating system when it needs
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to store new data.
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The last few lines displayed by a database-wide <command>VACUUM VERBOSE</>
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command can help in determining if the current settings are adequate.
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A <literal>NOTICE</> message is also printed during such an operation
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if the current settings are too low.
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</para>
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<para>
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Increasing these parameters might cause <productname>PostgreSQL</>
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to request more <systemitem class="osname">System V</> shared
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memory than your operating system's default configuration
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allows. See <xref linkend="sysvipc"> for information on how to
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adjust those parameters, if necessary.
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</para>
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<variablelist>
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<varlistentry id="guc-max-fsm-pages" xreflabel="max_fsm_pages">
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<term><varname>max_fsm_pages</varname> (<type>integer</type>)</term>
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<indexterm>
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<primary><varname>max_fsm_pages</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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Sets the maximum number of disk pages for which free space will
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be tracked in the shared free-space map. Six bytes of shared memory
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are consumed for each page slot. This setting must be at least
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16 * <varname>max_fsm_relations</varname>. The default is chosen
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by <application>initdb</> depending on the amount of available memory,
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and can range from 20k to 200k pages.
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This parameter can only be set at server start.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="guc-max-fsm-relations" xreflabel="max_fsm_relations">
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<term><varname>max_fsm_relations</varname> (<type>integer</type>)</term>
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<indexterm>
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<primary><varname>max_fsm_relations</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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Sets the maximum number of relations (tables and indexes) for which
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free space will be tracked in the shared free-space map. Roughly
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seventy bytes of shared memory are consumed for each slot.
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The default is one thousand relations.
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This parameter can only be set at server start.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<note>
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<para>
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See the <xref linkend="sql-vacuum" endterm="sql-vacuum-title">
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command for information on setting this parameter.
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</para>
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</note>
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</sect2>
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<sect2 id="runtime-config-resource-kernel">
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@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/pageinspect.sgml,v 1.3 2007/12/10 05:32:51 tgl Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/pageinspect.sgml,v 1.4 2008/09/30 10:52:09 heikki Exp $ -->
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<sect1 id="pageinspect">
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<title>pageinspect</title>
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@ -19,7 +19,7 @@
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<variablelist>
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<varlistentry>
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<term>
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<function>get_raw_page(text, int) returns bytea</function>
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<function>get_raw_page(relname text, forknum int, blkno int) returns bytea</function>
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</term>
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<listitem>
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@ -27,13 +27,28 @@
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<function>get_raw_page</function> reads the specified block of the named
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table and returns a copy as a <type>bytea</> value. This allows a
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single time-consistent copy of the block to be obtained.
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<literal>forknum</literal> should be 0 for the main data fork, or 1 for
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the FSM.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>page_header(bytea) returns record</function>
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<function>get_raw_page(relname text, blkno int) returns bytea</function>
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</term>
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<listitem>
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<para>
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A shorthand of above, for reading from the main fork. Equal to
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<literal>get_raw_page(relname, 0, blkno)</literal>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>page_header(page bytea) returns record</function>
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</term>
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<listitem>
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@ -63,7 +78,7 @@ test=# SELECT * FROM page_header(get_raw_page('pg_class', 0));
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<varlistentry>
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<term>
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<function>heap_page_items(bytea) returns setof record</function>
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<function>heap_page_items(page bytea) returns setof record</function>
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</term>
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<listitem>
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@ -90,7 +105,7 @@ test=# SELECT * FROM heap_page_items(get_raw_page('pg_class', 0));
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<varlistentry>
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<term>
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<function>bt_metap(text) returns record</function>
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<function>bt_metap(relname text) returns record</function>
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</term>
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<listitem>
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||||
@ -113,7 +128,7 @@ fastlevel | 0
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<varlistentry>
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<term>
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<function>bt_page_stats(text, int) returns record</function>
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<function>bt_page_stats(relname text, blkno int) returns record</function>
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</term>
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<listitem>
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||||
@ -141,7 +156,7 @@ btpo_flags | 3
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<varlistentry>
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<term>
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||||
<function>bt_page_items(text, int) returns setof record</function>
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<function>bt_page_items(relname text, blkno int) returns setof record</function>
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</term>
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<listitem>
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@ -164,6 +179,26 @@ test=# SELECT * FROM bt_page_items('pg_cast_oid_index', 1);
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</programlisting>
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</listitem>
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||||
</varlistentry>
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<varlistentry>
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<term>
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||||
<function>fsm_page_contents(page bytea) returns text</function>
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</term>
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||||
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||||
<listitem>
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||||
<para>
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||||
<function>fsm_page_contents</function> shows the internal node structure
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||||
of a FSM page. The output is a multi-line string, with one line per
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node in the binary tree within the page. Only those nodes that are not
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zero are printed. The so-called "next" pointer, which points to the
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next slot to be returned from the page, is also printed.
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||||
</para>
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<para>
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See <filename>src/backend/storage/freespace/README</> for more
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information on the structure of an FSM page.
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</para>
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</listitem>
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||||
</varlistentry>
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||||
</variablelist>
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</sect2>
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@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/pgfreespacemap.sgml,v 2.2 2007/12/10 05:32:51 tgl Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/pgfreespacemap.sgml,v 2.3 2008/09/30 10:52:09 heikki Exp $ -->
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<sect1 id="pgfreespacemap">
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<title>pg_freespacemap</title>
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@ -9,183 +9,66 @@
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<para>
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The <filename>pg_freespacemap</> module provides a means for examining the
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free space map (FSM). It provides two C functions:
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<function>pg_freespacemap_relations</function> and
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<function>pg_freespacemap_pages</function> that each return a set of
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records, plus two views <structname>pg_freespacemap_relations</structname>
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and <structname>pg_freespacemap_pages</structname> that wrap the functions
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for convenient use.
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free space map (FSM). It provides a function called
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<function>pg_freespacemap</function>, or two overloaded functions, to be
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precise. The functions show the value recorded in the free space map for
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a given page, or for all pages in the relation.
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</para>
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<para>
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By default public access is revoked from the functions and views, just in
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case there are security issues lurking.
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By default public access is revoked from the functions, just in case
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there are security issues lurking.
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</para>
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<sect2>
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<title>The <filename>pg_freespacemap</> views</title>
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<title>Functions</title>
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<variablelist>
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<varlistentry>
|
||||
<term>
|
||||
<function>pg_freespacemap(rel regclass IN, blkno bigint IN) returns int2</function>
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</term>
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<listitem>
|
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<para>
|
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Returns the amount of free space on the page of the relation, specified
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by <literal>blkno</>, according to the FSM.
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(blkno).
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</para>
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||||
</listitem>
|
||||
</varlistentry>
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||||
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<varlistentry>
|
||||
<term>
|
||||
<function>pg_freespacemap(rel regclass IN, blkno OUT int4, avail OUT int2)</function>
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</term>
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<listitem>
|
||||
<para>
|
||||
Displays the the amount of free space on each page of the relation,
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according to the FSM. A set of <literal>(blkno int4, avail int2)</>
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tuples is returned, one tuple for each page in the relation.
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</para>
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||||
</listitem>
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||||
</varlistentry>
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||||
</variablelist>
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||||
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<para>
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The definitions of the columns exposed by the views are:
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||||
</para>
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||||
|
||||
<table>
|
||||
<title><structname>pg_freespacemap_relations</> Columns</title>
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||||
|
||||
<tgroup cols="4">
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||||
<thead>
|
||||
<row>
|
||||
<entry>Name</entry>
|
||||
<entry>Type</entry>
|
||||
<entry>References</entry>
|
||||
<entry>Description</entry>
|
||||
</row>
|
||||
</thead>
|
||||
<tbody>
|
||||
|
||||
<row>
|
||||
<entry><structfield>reltablespace</structfield></entry>
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||||
<entry><type>oid</type></entry>
|
||||
<entry><literal>pg_tablespace.oid</literal></entry>
|
||||
<entry>Tablespace OID of the relation</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>reldatabase</structfield></entry>
|
||||
<entry><type>oid</type></entry>
|
||||
<entry><literal>pg_database.oid</literal></entry>
|
||||
<entry>Database OID of the relation</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>relfilenode</structfield></entry>
|
||||
<entry><type>oid</type></entry>
|
||||
<entry><literal>pg_class.relfilenode</literal></entry>
|
||||
<entry>Relfilenode of the relation</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>avgrequest</structfield></entry>
|
||||
<entry><type>integer</type></entry>
|
||||
<entry></entry>
|
||||
<entry>Moving average of free space requests (NULL for indexes)</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>interestingpages</structfield></entry>
|
||||
<entry><type>integer</type></entry>
|
||||
<entry></entry>
|
||||
<entry>Count of pages last reported as containing useful free space</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>storedpages</structfield></entry>
|
||||
<entry><type>integer</type></entry>
|
||||
<entry></entry>
|
||||
<entry>Count of pages actually stored in free space map</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>nextpage</structfield></entry>
|
||||
<entry><type>integer</type></entry>
|
||||
<entry></entry>
|
||||
<entry>Page index (from 0) to start next search at</entry>
|
||||
</row>
|
||||
|
||||
</tbody>
|
||||
</tgroup>
|
||||
</table>
|
||||
|
||||
<table>
|
||||
<title><structname>pg_freespacemap_pages</> Columns</title>
|
||||
|
||||
<tgroup cols="4">
|
||||
<thead>
|
||||
<row>
|
||||
<entry>Name</entry>
|
||||
<entry>Type</entry>
|
||||
<entry>References</entry>
|
||||
<entry>Description</entry>
|
||||
</row>
|
||||
</thead>
|
||||
<tbody>
|
||||
|
||||
<row>
|
||||
<entry><structfield>reltablespace</structfield></entry>
|
||||
<entry><type>oid</type></entry>
|
||||
<entry><literal>pg_tablespace.oid</literal></entry>
|
||||
<entry>Tablespace OID of the relation</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>reldatabase</structfield></entry>
|
||||
<entry><type>oid</type></entry>
|
||||
<entry><literal>pg_database.oid</literal></entry>
|
||||
<entry>Database OID of the relation</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>relfilenode</structfield></entry>
|
||||
<entry><type>oid</type></entry>
|
||||
<entry><literal>pg_class.relfilenode</literal></entry>
|
||||
<entry>Relfilenode of the relation</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>relblocknumber</structfield></entry>
|
||||
<entry><type>bigint</type></entry>
|
||||
<entry></entry>
|
||||
<entry>Page number within the relation</entry>
|
||||
</row>
|
||||
<row>
|
||||
<entry><structfield>bytes</structfield></entry>
|
||||
<entry><type>integer</type></entry>
|
||||
<entry></entry>
|
||||
<entry>Free bytes in the page, or NULL for an index page (see below)</entry>
|
||||
</row>
|
||||
|
||||
</tbody>
|
||||
</tgroup>
|
||||
</table>
|
||||
|
||||
<para>
|
||||
For <structname>pg_freespacemap_relations</structname>, there is one row
|
||||
for each relation in the free space map.
|
||||
<structfield>storedpages</structfield> is the number of pages actually
|
||||
stored in the map, while <structfield>interestingpages</structfield> is the
|
||||
number of pages the last <command>VACUUM</> thought had useful amounts of
|
||||
free space.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
If <structfield>storedpages</structfield> is consistently less than
|
||||
<structfield>interestingpages</> then it'd be a good idea to increase
|
||||
<varname>max_fsm_pages</varname>. Also, if the number of rows in
|
||||
<structname>pg_freespacemap_relations</structname> is close to
|
||||
<varname>max_fsm_relations</varname>, then you should consider increasing
|
||||
<varname>max_fsm_relations</varname>.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
For <structname>pg_freespacemap_pages</structname>, there is one row for
|
||||
each page in the free space map. The number of rows for a relation will
|
||||
match the <structfield>storedpages</structfield> column in
|
||||
<structname>pg_freespacemap_relations</structname>.
|
||||
The values stored in the free space map are not exact. They're rounded
|
||||
to precision of 1/256th of BLCKSZ (32 bytes with default BLCKSZ), and
|
||||
they're not kept fully up-to-date as tuples are inserted and updated.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
For indexes, what is tracked is entirely-unused pages, rather than free
|
||||
space within pages. Therefore, the average request size and free bytes
|
||||
within a page are not meaningful, and are shown as NULL.
|
||||
space within pages. Therefore, the values are not meaningful, just
|
||||
whether a page is full or empty.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Because the map is shared by all the databases, there will normally be
|
||||
entries for relations not belonging to the current database. This means
|
||||
that there may not be matching join rows in <structname>pg_class</> for
|
||||
some rows, or that there could even be incorrect joins. If you are
|
||||
trying to join against <structname>pg_class</>, it's a good idea to
|
||||
restrict the join to rows having <structfield>reldatabase</> equal to
|
||||
the current database's OID or zero.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
When either of the views is accessed, internal free space map locks are
|
||||
taken for long enough to copy all the state data that the view will display.
|
||||
This ensures that the views produce a consistent set of results, while not
|
||||
blocking normal activity longer than necessary. Nonetheless there
|
||||
could be some impact on database performance if they are read often.
|
||||
NOTE: The interface was changed in version 8.4, to reflect the new FSM
|
||||
implementation introduced in the same version.
|
||||
</para>
|
||||
</sect2>
|
||||
|
||||
@ -193,45 +76,37 @@
|
||||
<title>Sample output</title>
|
||||
|
||||
<programlisting>
|
||||
regression=# SELECT c.relname, r.avgrequest, r.interestingpages, r.storedpages
|
||||
FROM pg_freespacemap_relations r INNER JOIN pg_class c
|
||||
ON r.relfilenode = c.relfilenode AND
|
||||
r.reldatabase IN (0, (SELECT oid FROM pg_database
|
||||
WHERE datname = current_database()))
|
||||
ORDER BY r.storedpages DESC LIMIT 10;
|
||||
relname | avgrequest | interestingpages | storedpages
|
||||
---------------------------------+------------+------------------+-------------
|
||||
onek | 256 | 109 | 109
|
||||
pg_attribute | 167 | 93 | 93
|
||||
pg_class | 191 | 49 | 49
|
||||
pg_attribute_relid_attnam_index | | 48 | 48
|
||||
onek2 | 256 | 37 | 37
|
||||
pg_depend | 95 | 26 | 26
|
||||
pg_type | 199 | 16 | 16
|
||||
pg_rewrite | 1011 | 13 | 13
|
||||
pg_class_relname_nsp_index | | 10 | 10
|
||||
pg_proc | 302 | 8 | 8
|
||||
(10 rows)
|
||||
postgres=# SELECT * FROM pg_freespace('foo');
|
||||
blkno | avail
|
||||
-------+-------
|
||||
0 | 0
|
||||
1 | 0
|
||||
2 | 0
|
||||
3 | 32
|
||||
4 | 704
|
||||
5 | 704
|
||||
6 | 704
|
||||
7 | 1216
|
||||
8 | 704
|
||||
9 | 704
|
||||
10 | 704
|
||||
11 | 704
|
||||
12 | 704
|
||||
13 | 704
|
||||
14 | 704
|
||||
15 | 704
|
||||
16 | 704
|
||||
17 | 704
|
||||
18 | 704
|
||||
19 | 3648
|
||||
(20 rows)
|
||||
|
||||
postgres=# SELECT * FROM pg_freespace('foo', 7);
|
||||
pg_freespace
|
||||
--------------
|
||||
1216
|
||||
(1 row)
|
||||
|
||||
regression=# SELECT c.relname, p.relblocknumber, p.bytes
|
||||
FROM pg_freespacemap_pages p INNER JOIN pg_class c
|
||||
ON p.relfilenode = c.relfilenode AND
|
||||
p.reldatabase IN (0, (SELECT oid FROM pg_database
|
||||
WHERE datname = current_database()))
|
||||
ORDER BY c.relname LIMIT 10;
|
||||
relname | relblocknumber | bytes
|
||||
--------------+----------------+-------
|
||||
a_star | 0 | 8040
|
||||
abstime_tbl | 0 | 7908
|
||||
aggtest | 0 | 8008
|
||||
altinhoid | 0 | 8128
|
||||
altstartwith | 0 | 8128
|
||||
arrtest | 0 | 7172
|
||||
b_star | 0 | 7976
|
||||
box_tbl | 0 | 7912
|
||||
bt_f8_heap | 54 | 7728
|
||||
bt_i4_heap | 49 | 8008
|
||||
(10 rows)
|
||||
</programlisting>
|
||||
</sect2>
|
||||
|
||||
@ -239,7 +114,9 @@ regression=# SELECT c.relname, p.relblocknumber, p.bytes
|
||||
<title>Author</title>
|
||||
|
||||
<para>
|
||||
Mark Kirkwood <email>markir@paradise.net.nz</email>
|
||||
Original version by Mark Kirkwood <email>markir@paradise.net.nz</email>.
|
||||
Rewritten in version 8.4 to suit new FSM implementation by Heikki
|
||||
Linnakangas <email>heikki@enterprisedb.com</email>
|
||||
</para>
|
||||
</sect2>
|
||||
|
||||
|
||||
@ -1,5 +1,5 @@
|
||||
<!--
|
||||
$PostgreSQL: pgsql/doc/src/sgml/ref/vacuum.sgml,v 1.51 2008/02/03 16:24:08 tgl Exp $
|
||||
$PostgreSQL: pgsql/doc/src/sgml/ref/vacuum.sgml,v 1.52 2008/09/30 10:52:10 heikki Exp $
|
||||
PostgreSQL documentation
|
||||
-->
|
||||
|
||||
@ -96,11 +96,7 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">
|
||||
<term><literal>VERBOSE</literal></term>
|
||||
<listitem>
|
||||
<para>
|
||||
Prints a detailed vacuum activity report for each table. Can be used
|
||||
to help determine appropriate settings for
|
||||
<xref linkend="guc-max-fsm-pages">,
|
||||
<xref linkend="guc-max-fsm-relations">, and
|
||||
<xref linkend="guc-default-statistics-target">.
|
||||
Prints a detailed vacuum activity report for each table.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
@ -1,4 +1,4 @@
|
||||
<!-- $PostgreSQL: pgsql/doc/src/sgml/release.sgml,v 1.585 2008/09/17 20:57:35 tgl Exp $ -->
|
||||
<!-- $PostgreSQL: pgsql/doc/src/sgml/release.sgml,v 1.586 2008/09/30 10:52:09 heikki Exp $ -->
|
||||
<!--
|
||||
|
||||
Typical markup:
|
||||
@ -6004,8 +6004,7 @@ current_date < 2017-11-17
|
||||
<para>
|
||||
Increase default values for <link
|
||||
linkend="guc-shared-buffers"><varname>shared_buffers</></link>
|
||||
and <link
|
||||
linkend="guc-max-fsm-pages"><varname>max_fsm_pages</></link>
|
||||
and <varname>max_fsm_pages</>
|
||||
(Andrew)
|
||||
</para>
|
||||
</listitem>
|
||||
|
||||
@ -1,4 +1,4 @@
|
||||
<!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.417 2008/09/23 09:20:34 heikki Exp $ -->
|
||||
<!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.418 2008/09/30 10:52:10 heikki Exp $ -->
|
||||
|
||||
<chapter Id="runtime">
|
||||
<title>Operating System Environment</title>
|
||||
@ -1117,16 +1117,6 @@ set semsys:seminfo_semmsl=32
|
||||
<entry>8200 (assuming 8 kB <symbol>XLOG_BLCKSZ</>)</entry>
|
||||
</row>
|
||||
|
||||
<row>
|
||||
<entry><xref linkend="guc-max-fsm-relations"></>
|
||||
<entry>70</>
|
||||
</row>
|
||||
|
||||
<row>
|
||||
<entry><xref linkend="guc-max-fsm-pages"></>
|
||||
<entry>6</>
|
||||
</row>
|
||||
|
||||
<row>
|
||||
<entry>Fixed space requirements</>
|
||||
<entry>770 kB</entry>
|
||||
|
||||
@ -1,4 +1,4 @@
|
||||
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.24 2008/08/05 12:09:30 mha Exp $ -->
|
||||
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.25 2008/09/30 10:52:10 heikki Exp $ -->
|
||||
|
||||
<chapter id="storage">
|
||||
|
||||
@ -130,7 +130,12 @@ there.
|
||||
<para>
|
||||
Each table and index is stored in a separate file, named after the table
|
||||
or index's <firstterm>filenode</> number, which can be found in
|
||||
<structname>pg_class</>.<structfield>relfilenode</>.
|
||||
<structname>pg_class</>.<structfield>relfilenode</>. In addition to the
|
||||
main file (aka. main fork), a <firstterm>free space map</> (see
|
||||
<xref linkend="storage-fsm">) that stores information about free space
|
||||
available in the relation, is stored in a file named after the filenode
|
||||
number, with the the _1 suffix. For example, if the table's filenode number
|
||||
is 12345, the FSM file is named <filename>12345_1</>.
|
||||
</para>
|
||||
|
||||
<caution>
|
||||
@ -367,6 +372,48 @@ comparison table, in which all the HTML pages were cut down to 7 kB to fit.
|
||||
|
||||
</sect1>
|
||||
|
||||
<sect1 id="storage-fsm">
|
||||
|
||||
<title>Free Space Map</title>
|
||||
|
||||
<indexterm>
|
||||
<primary>Free Space Map</primary>
|
||||
</indexterm>
|
||||
<indexterm><primary>FSM</><see>Free Space Map</></indexterm>
|
||||
|
||||
<para>
|
||||
A Free Space Map is stored with every heap and index relation, except for
|
||||
hash indexes, to keep track of available space in the relation. It's stored
|
||||
along the main relation data, in a separate FSM relation fork, named after
|
||||
relfilenode of the relation, but with a <literal>_1</> suffix. For example,
|
||||
if the relfilenode of a relation is 12345, the FSM is stored in a file called
|
||||
<filename>12345_1</>, in the same directory as the main relation file.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The Free Space Map is organized as a tree of <acronym>FSM</> pages. The
|
||||
bottom level <acronym>FSM</> pages stores the free space available on every
|
||||
heap (or index) page, using one byte to represent each heap page. The upper
|
||||
levels aggregate information from the lower levels.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Within each <acronym>FSM</> page is a binary tree, stored in an array with
|
||||
one byte per node. Each leaf node represents a heap page, or a lower level
|
||||
<acronym>FSM</> page. In each non-leaf node, the higher of its children's
|
||||
values is stored. The maximum value in the leaf nodes is therefore stored
|
||||
at the root.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
See <filename>src/backend/storage/freespace/README</> for more details on
|
||||
how the <acronym>FSM</> is structured, and how it's updated and searched.
|
||||
<xref linkend="pgfreespacemap"> contrib module can be used to view the
|
||||
information stored in free space maps.
|
||||
</para>
|
||||
|
||||
</sect1>
|
||||
|
||||
<sect1 id="storage-page-layout">
|
||||
|
||||
<title>Database Page Layout</title>
|
||||
|
||||
Reference in New Issue
Block a user