From: Bruce Momjian <pgman@candle.pha.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Mark Kirkwood <markir@coretech.co.nz>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: pgsql-admin@postgresql.org
Subject: Re: [HACKERS] Point in Time Recovery
Date: Wed, 21 Jul 2004 23:06:22 -0400 (EDT)
Message-ID: <200407220306.i6M36MU11209@candle.pha.pa.us> (raw)
In-Reply-To: <2110.1090463955@sss.pgh.pa.us>
I think we should push the partially complete WAL file to the archive
location before shutdown. I talked to you or Jan about it and you (or
Jan) wouldn't bite either, but I think when someone shuts down, they
assume they have things fully archived and can recover fully with a
previous backup and the archive files.
When you are running and finally fill up the WAL file it would then
overwrite the one in the archive but I think that is OK. Maybe we would
need to give it a special file extension so we only use it when we don't
have a full version.
---------------------------------------------------------------------------
Tom Lane wrote:
> Mark Kirkwood <markir@coretech.co.nz> writes:
> > Here is one for the 'idiot proof' category:
> > 1) initdb and set archive_command
> > 2) shutdown
> > 3) do a backup
> > 4) startup and run some transactions
> > 5) shutdown and remove PGDATA
> > 6) restore backup
> > 7) startup
>
> > Obviously this does not work as the backup is performed with the
> > database shutdown.
>
> Huh? It works fine.
>
> The bit you may be missing is that if you blow away $PGDATA including
> pg_xlog/, you won't be able to recover past whatever you have in your WAL
> archive area. The archive is certainly not going to include the current
> partially-filled WAL segment, and it might be missing a few earlier
> segments if the archival process isn't speedy. So you need to keep
> those recent segments in pg_xlog/ if you want to recover to current time
> or near-current time.
>
> I'm becoming more and more convinced that we should bite the bullet and
> move pg_xlog/ to someplace that is not under $PGDATA. It would just
> make things a whole lot more reliable, both for backup and to deal with
> scenarios like yours above. I tried to talk Bruce into this on the
> phone the other day, but he wouldn't bite. I still think it's a good
> idea though. It would
> (1) eliminate the problem that a tar backup of $PGDATA would restore
> stale copies of xlog segments, because the tar wouldn't include
> pg_xlog in the first place.
> (2) eliminate the problem that a naive "rm -rf $PGDATA" would blow away
> xlog segments that you still need.
>
> A possible compromise is that we should strongly suggest that pg_xlog
> be pushed out to another place and symlinked if you are going to use
> WAL archiving. That's already considered good practice for performance
> if you have a separate disk spindle to put WAL on. It'll just have
> to be good practive for WAL archiving too.
>
> regards, tom lane
>
> ---------------------------(end of broadcast)---------------------------
> TIP 7: don't forget to increase your free space map settings
>
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-admin@postgresql.org
Cc: pgman@candle.pha.pa.us, tgl@sss.pgh.pa.us, markir@coretech.co.nz, simon@2ndquadrant.com
Subject: Re: [HACKERS] Point in Time Recovery
In-Reply-To: <200407220306.i6M36MU11209@candle.pha.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox