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Don't use "cp -i" in the example WAL archive_command.

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This is a dangerous example to provide because on machines with GNU cp,
it will silently do the wrong thing and risk archive corruption.  Worse,
during the 9.0 cycle somebody "improved" the discussion by removing the
warning that used to be there about that, and instead leaving the
impression that the command would work as desired on most Unixen.
It doesn't.  Try to rectify the damage by providing an example that is safe
most everywhere, and then noting that you can try cp -i if you want but
you'd better test that.

In back-patching this to all supported branches, I also added an example
command for Windows, which wasn't provided before 9.0.
This commit is contained in:
Tom Lane 2011-06-17 19:13:12 -04:00
parent 7975f2baa9
commit ae7fc61bc3
1 changed files with 13 additions and 12 deletions
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25
doc/src/sgml/backup.sgml
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@ -577,7 +577,7 @@ tar -cf backup.tar /usr/local/pgsql/data
character in the command. The simplest useful command is something
like:
<programlisting>
archive_command = 'cp -i %p /mnt/server/archivedir/%f &lt;/dev/null' # Unix
archive_command = 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f' # Unix
archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
</programlisting>
which will copy archivable WAL segments to the directory
@ -586,7 +586,7 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
<literal>%p</> and <literal>%f</> parameters have been replaced,
the actual command executed might look like this:
<programlisting>
cp -i pg_xlog/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065 &lt;/dev/null
test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_xlog/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065
</programlisting>
A similar command will be generated for each new file to be archived.
</para>
@ -615,18 +615,19 @@ cp -i pg_xlog/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900
preserve the integrity of your archive in case of administrator error
(such as sending the output of two different servers to the same archive
directory).
</para>
<para>
It is advisable to test your proposed archive command to ensure that it
indeed does not overwrite an existing file, <emphasis>and that it returns
nonzero status in this case</>. On many Unix platforms, <command>cp
-i</> causes copy to prompt before overwriting a file, and
<literal>&lt; /dev/null</> causes the prompt (and overwriting) to
fail. If your platform does not support this behavior, you should
add a command to test for the existence of the archive file. For
example, something like:
<programlisting>
archive_command = 'test ! -f /mnt/server/archivedir/%f &amp;&amp; cp %p /mnt/server/archivedir/%f'
</programlisting>
works correctly on most Unix variants.
nonzero status in this case</>.
The example command above for Unix ensures this by including a separate
<command>test</> step. On some Unix platforms, <command>cp</> has
switches such as <option>-i</> that can be used to do the same thing
less verbosely, but you should not rely on these without verifying that
the right exit status is returned. (In particular, GNU <command>cp</>
will return status zero when <option>-i</> is used and the target file
already exists, which is <emphasis>not</> the desired behavior.)
</para>
<para>