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><A
NAME="WAL-INTERNALS"
>29.5. WAL Internals</A
></H1
><P
> <ACRONYM
CLASS="ACRONYM"
>WAL</ACRONYM
> is automatically enabled; no action is
required from the administrator except ensuring that the
disk-space requirements for the <ACRONYM
CLASS="ACRONYM"
>WAL</ACRONYM
> logs are met,
and that any necessary tuning is done (see <A
HREF="wal-configuration.html"
>Section 29.4</A
>).
</P
><P
> <ACRONYM
CLASS="ACRONYM"
>WAL</ACRONYM
> logs are stored in the directory
<TT
CLASS="FILENAME"
>pg_xlog</TT
> under the data directory, as a set of
segment files, normally each 16 MB in size (but the size can be changed
by altering the <TT
CLASS="OPTION"
>--with-wal-segsize</TT
> configure option when
building the server). Each segment is divided into pages, normally
8 kB each (this size can be changed via the <TT
CLASS="OPTION"
>--with-wal-blocksize</TT
>
configure option). The log record headers are described in
<TT
CLASS="FILENAME"
>access/xlog.h</TT
>; the record content is dependent
on the type of event that is being logged. Segment files are given
ever-increasing numbers as names, starting at
<TT
CLASS="FILENAME"
>000000010000000000000000</TT
>. The numbers do not wrap,
but it will take a very, very long time to exhaust the
available stock of numbers.
</P
><P
> It is advantageous if the log is located on a different disk from the
main database files. This can be achieved by moving the
<TT
CLASS="FILENAME"
>pg_xlog</TT
> directory to another location (while the server
is shut down, of course) and creating a symbolic link from the
original location in the main data directory to the new location.
</P
><P
> The aim of <ACRONYM
CLASS="ACRONYM"
>WAL</ACRONYM
> is to ensure that the log is
written before database records are altered, but this can be subverted by
disk drives that falsely report a
successful write to the kernel,
when in fact they have only cached the data and not yet stored it
on the disk. A power failure in such a situation might lead to
irrecoverable data corruption. Administrators should try to ensure
that disks holding <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s
<ACRONYM
CLASS="ACRONYM"
>WAL</ACRONYM
> log files do not make such false reports.
(See <A
HREF="wal-reliability.html"
>Section 29.1</A
>.)
</P
><P
> After a checkpoint has been made and the log flushed, the
checkpoint's position is saved in the file
<TT
CLASS="FILENAME"
>pg_control</TT
>. Therefore, at the start of recovery,
the server first reads <TT
CLASS="FILENAME"
>pg_control</TT
> and
then the checkpoint record; then it performs the REDO operation by
scanning forward from the log position indicated in the checkpoint
record. Because the entire content of data pages is saved in the
log on the first page modification after a checkpoint (assuming
<A
HREF="runtime-config-wal.html#GUC-FULL-PAGE-WRITES"
>full_page_writes</A
> is not disabled), all pages
changed since the checkpoint will be restored to a consistent
state.
</P
><P
> To deal with the case where <TT
CLASS="FILENAME"
>pg_control</TT
> is
corrupt, we should support the possibility of scanning existing log
segments in reverse order — newest to oldest — in order to find the
latest checkpoint. This has not been implemented yet.
<TT
CLASS="FILENAME"
>pg_control</TT
> is small enough (less than one disk page)
that it is not subject to partial-write problems, and as of this writing
there have been no reports of database failures due solely to the inability
to read <TT
CLASS="FILENAME"
>pg_control</TT
> itself. So while it is
theoretically a weak spot, <TT
CLASS="FILENAME"
>pg_control</TT
> does not
seem to be a problem in practice.
</P
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