From: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
---|---|
To: | Markus Bertheau <mbertheau(dot)pg(at)googlemail(dot)com> |
Cc: | pgsql-bugs(at)postgresql(dot)org |
Subject: | Re: Incomplete docs for restore_command for hot standby |
Date: | 2008-02-21 21:18:32 |
Message-ID: | 1203628712.4229.53.camel@ebony.site |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-bugs pgsql-patches |
On Thu, 2008-02-21 at 08:01 +0600, Markus Bertheau wrote:
> (I sent this to -docs already, but it didn't get through for some reason.)
>
> >From the current 8.3 docs:
>
> Section 24.3.3.1 states about restore_command:
>
> "The command will be asked for file names that are not present in the
> archive; it must return nonzero when so asked."
>
> Section 24.4.1 further states:
>
> "The magic that makes the two loosely coupled servers work together is
> simply a restore_command used on the standby that waits for the next
> WAL file to become available from the primary."
>
> It is not clear from the first paragraph, whether the non-existing
> file that restore_command is being asked for is a not-yet-generated
> WAL file or something different. If it was a not-yet-generated WAL
> file, restore_command for replication would have to wait for it to
> appear. If it was something different, restore_command for replication
> would have to return an error right away. (Because else it would hang
> indefinitely, waiting for a file that is not going to appear). Yet I
> couldn't find hints in the documentation as to how these two cases can
> be detected by restore_command, i.e. how restore_command should tell a
> request for a WAL file from a request for a non-WAL file.
The two sentences aren't mutually exclusive, especially when you
consider they are discussing two different use cases. Why not read up on
pg_standby anyway?
> Practice (http://archives.postgresql.org/sydpug/2006-10/msg00001.php)
> shows that this is a problem, and people use unproved heuristics
> ('history' substring in the requested file name).
Old email written during beta. Read at your own peril.
> Additionally, 24.3.3 contains slightly misleading information:
>
> "It is important that the command return nonzero exit status on
> failure. The command will be asked for log files that are not present
> in the archive; it must return nonzero when so asked. This is not an
> error condition."
>
> This suggests that all non-existing files that restore_command will be
> asked for are log files. One could therefore reasonably assume that
> restore_command for replication should wait on all non-existing files.
> 24.3.3.1 later corrects this by stating that not only log files may be
> requested, but nevertheless.
If you have some suggested changes, I'd be happy to hear them.
Probably additions are better than just changes though.
--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com
From | Date | Subject | |
---|---|---|---|
Next Message | David Lee | 2008-02-21 23:34:12 | BUG #3979: SELECT DISTINCT slow even on indexed column |
Previous Message | Vincent D'Haene | 2008-02-21 20:53:42 | Re: BUG #3951: SELECT ... WHERE Param = ? does not work if Param is of type bytea |
From | Date | Subject | |
---|---|---|---|
Next Message | Alvaro Herrera | 2008-02-21 21:21:21 | Re: fix in --help output |
Previous Message | Zdenek Kotala | 2008-02-21 21:16:15 | Re: fix in --help output |