"Zeugswetter Andreas SB SD" <ZeugswetterA@spardat.at> writes:
>> Do we need a checkpoint after the archiving
>> starts but before the backup begins?
> No.
Actually yes. You have to start at a checkpoint record when replaying
the log, so if no checkpoint occurred between starting to archive WAL
and starting the tar backup, you have a useless backup.
It would be reasonable to issue a CHECKPOINT just before starting the
backup as part of the standard operating procedure for taking PITR
dumps. We need not require this, but it would help to avoid this
particular sort of mistake; and of course it might save a little bit of
replay effort if the backup is ever used.
As far as the business about copying pg_control first goes: there is
another way to think about it, which is to copy pg_control to another
place that will be included in your backup. For example the standard
backup procedure could be
1. [somewhat optional] Issue CHECKPOINT and wait till it finishes.
2. cp $PGDATA/global/pg_control $PGDATA/pg_control.dump
3. tar cf /dev/mt $PGDATA
4. do something to record ending WAL position
If we standardized on this way, then the tar archive would automatically
contain the pre-backup checkpoint position in ./pg_control.dump, and
there is no need for any special assumptions about the order in which
tar processes things.
However, once you decide to do things like that, there is no reason why
the copied file has to be an exact image of pg_control. I claim it
would be more useful if the copied file were plain text so that you
could just "cat" it to find out the starting WAL position; that would
let you determine without any special tools what range of WAL archive
files you are going to need to bring back from your archives.
This is pretty much the same chain of reasoning that Bruce and I went
through yesterday to come up with the idea of putting a label file
inside the tar backups. We concluded that it'd be worth putting
both the backup starting time and the checkpoint WAL position into
the label file --- the starting time isn't needed for restore but
might be really helpful as documentation, if you needed to verify
which dump file was which.
regards, tom lane
