Re: [HACKERS] Docs for archive_cleanup_command are poor

On Sat, Oct 9, 2010 at 10:04 AM, Brendan Jurd <direvus(at)gmail(dot)com> wrote:
> Hi folks,
>
> I have just set up HS+SR for the first time, and for the most part,
> the docs were excellent.  The one exception for me was the discussion
> of archive_cleanup_command.  This is a pretty important part of
> constructing a healthy standby server, and IMO the docs don't give it
> the treatment it deserves.
>
> Under "25.2.4. Setting Up a Standby Server", we have:
>
> "You can use archive_cleanup_command to prune the archive of files no
> longer needed by the standby."
>
> ... then a few paragraphs later ...
>
> "If you're using a WAL archive, its size can be minimized using the
> archive_cleanup_command  option to remove files that are no longer
> required by the standby server. Note however, that if you're using the
> archive for backup purposes, you need to retain files needed to
> recover from at least the latest base backup, even if they're no
> longer needed by the standby."
>
> So there are a couple of brief mentions of what
> archive_cleanup_command is for, but nothing about how it works, no
> exampes of how to use it, and no links at all.  Contrast how we deal
> with archive_command, restore_command and primary_conninfo.
>
> I'd like to suggest a few ways we could improve on this:
>
> 1. Remove the former paragraph.  It's stranded out there on its own in
> the middle of some unrelated text, and doesn't say anything of
> substance not also said in the latter paragraph.
>
> 2. Include an example archive_cleanup_command in the recovery.conf
> example snippet.
>
> 3. Link to 26.1 which actually explains how a_c_c works.
>
> 4. Mention, and link to, pg_archivecleanup from both 25.2.4 and 26.1.
> This is the utility that most newcomers to WAL archiving will want to
> use, so it's rather weird of us not to advertise it.
>
> I'm willing to write a patch for this, but I thought I'd raise the
> suggestions on-list first, before getting too invested.  So, please
> comment if you have an opinion on this.

Agreed.

And, ISTM that we should mention that we must not just specify
pg_archivecleanup in archive_cleanup_command when there are multiple
standby servers. This is because, in that case, we must calculate
the oldest restart point in those standbys and delete the archived
WAL files according to that point.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center