Re: Problem with PITR recovery

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > The problem is that we don't archive the partially written xlog file,
> > and in this case that xlog file contains the information needed to make
> > the tar file consistent.
>
> > Is this a known problem? Do we document this? If so, I can't find it.
>
> Yes, and yes. You did not follow the procedure:
>
> http://www.postgresql.org/docs/8.0/static/backup-online.html#BACKUP-PITR-RECOVERY
>
> In particular, step 2 says:
>
> : ... you need at the least to copy the contents of the pg_xlog
> : subdirectory of the cluster data directory, as it may contain logs which
> : were not archived before the system went down.
>
> Possibly this needs to be highlighted a little better.

I figured that part of the goal of PITR was that you could recover from
just the tar backup and archived WAL files --- using the pg_xlog
contents is nice, but not something we can require.

I understood the last missing WAL log would cause missing information,
but not that it would make the tar backup unusable.

It would be nice if we could force a new WAL file on pg_stop_backup()
and archive the WAL file needed to match the tar file. How hard would
that be?

I see in the docs:

To make use of this backup, you will need to keep around all the WAL
segment files generated at or after the starting time of the backup. To
aid you in doing this, the pg_stop_backup function creates a backup
history file that is immediately stored into the WAL archive area. This
file is named after the first WAL segment file that you need to have to
make use of the backup. For example, if the starting WAL file is
0000000100001234000055CD the backup history file will be named something
like 0000000100001234000055CD.007C9330.backup. (The second part of this
file name stands for an exact position within the WAL file, and can
ordinarily be ignored.) Once you have safely archived the backup dump
file, you can delete all archived WAL segments with names numerically
preceding this one.

I am not clear on what the "backup dump file" is? I assume it means
0000000100001234000055CD. It is called "WAL segment file" above. I
will rename that phrase to match the above terminology. Patch attached
and applied.

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