mirror of
https://github.com/postgres/postgres.git
synced 2025-09-02 04:21:28 +03:00
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 @@
|
||||
<!-- $PostgreSQL: pgsql/doc/src/sgml/pgfreespacemap.sgml,v 2.2 2007/12/10 05:32:51 tgl Exp $ -->
|
||||
<!-- $PostgreSQL: pgsql/doc/src/sgml/pgfreespacemap.sgml,v 2.3 2008/09/30 10:52:09 heikki Exp $ -->
|
||||
|
||||
<sect1 id="pgfreespacemap">
|
||||
<title>pg_freespacemap</title>
|
||||
@@ -9,183 +9,66 @@
|
||||
|
||||
<para>
|
||||
The <filename>pg_freespacemap</> module provides a means for examining the
|
||||
free space map (FSM). It provides two C functions:
|
||||
<function>pg_freespacemap_relations</function> and
|
||||
<function>pg_freespacemap_pages</function> that each return a set of
|
||||
records, plus two views <structname>pg_freespacemap_relations</structname>
|
||||
and <structname>pg_freespacemap_pages</structname> that wrap the functions
|
||||
for convenient use.
|
||||
free space map (FSM). It provides a function called
|
||||
<function>pg_freespacemap</function>, or two overloaded functions, to be
|
||||
precise. The functions show the value recorded in the free space map for
|
||||
a given page, or for all pages in the relation.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
By default public access is revoked from the functions and views, just in
|
||||
case there are security issues lurking.
|
||||
By default public access is revoked from the functions, just in case
|
||||
there are security issues lurking.
|
||||
</para>
|
||||
|
||||
<sect2>
|
||||
<title>The <filename>pg_freespacemap</> views</title>
|
||||
<title>Functions</title>
|
||||
|
||||
<variablelist>
|
||||
<varlistentry>
|
||||
<term>
|
||||
<function>pg_freespacemap(rel regclass IN, blkno bigint IN) returns int2</function>
|
||||
</term>
|
||||
|
||||
<listitem>
|
||||
<para>
|
||||
Returns the amount of free space on the page of the relation, specified
|
||||
by <literal>blkno</>, according to the FSM.
|
||||
(blkno).
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
|
||||
<varlistentry>
|
||||
<term>
|
||||
<function>pg_freespacemap(rel regclass IN, blkno OUT int4, avail OUT int2)</function>
|
||||
</term>
|
||||
|
||||
<listitem>
|
||||
<para>
|
||||
Displays the the amount of free space on each page of the relation,
|
||||
according to the FSM. A set of <literal>(blkno int4, avail int2)</>
|
||||
tuples is returned, one tuple for each page in the relation.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
</variablelist>
|
||||
|
||||
<para>
|
||||
The definitions of the columns exposed by the views are:
|
||||
</para>
|
||||
|
||||
<table>
|
||||
<title><structname>pg_freespacemap_relations</> 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>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>
|
||||
|
||||
|
||||
Reference in New Issue
Block a user