Re: Updated backup APIs for non-exclusive backups - Mailing list pgsql-hackers

On Sun, 2018-11-25 at 13:50 -0500, Stephen Frost wrote:
> I don't see any compelling argument for trying to do something half-way
> any more today than I did two years ago when this was being discussed.

That may well be so.  It may be better to make users unhappy than to
make them very unhappy...

But I find the following points unconvincing:

> > I would say the typical use case for the exclusive backup method is
> > the following (and I have seen it often):
> > 
> > You have some kind of backup software that does file system backups
> > and is able to run a "pre-backup" and "post-backup" script.
> > The backup is triggered by the backup software.
> 
> Seeing it often doesn't make it a good solution.  Running just
> pre-backup and post-backup scripts and copying the filesystem isn't
> enough to perform an online PostgreSQL backup- the WAL needs to be
> collected as well, and you need to make sure that you have all of the
> WAL before the backup can be considered complete.

Yes, that's why "pg_stop_backup" has the "wait_for_archive" parameter.
So this is not a problem.

> On restore, you're
> going to need to create a recovery.conf (at least in released versions)
> which provides a restore command (needed even in HEAD today) to get the
> old WAL, so having to also create the backup_label file shouldn't be
> that difficult.

You write "recovery.conf" upon recovery, when you have the restored
backup, so you have it on a file system.  No problem adding a file then.

This is entirely different from adding a "backup_label" file to
a backup that has been taken by a backup software in some arbitrary
format in some arbitrary location (think snapshot).

So these two cases don't compare.

> Lastly, if you really want, you can extract out the data from
> pg_stop_backup in whatever your post-backup script is.

Come on, now.
You usually use backup techniques like that because you can't get
your large database backed up in the available time window otherwise.

> > Another thing that is problematic with non-exclusive backups is that
> > you have to write the backup_label file into the backup after the
> > backup has been taken.  With a technique like the above, you cannot
> > easily do that.
> 
> ... why not?  You need to create the recovery.conf anyway, and you need
> to be archiving WAL somewhere, so it certainly seems like you could put
> the backup_label there too.

As I said above, you don't add "recovery.conf" to the backup right away,
so these two cases don't compare.

> > I expect that that will make a lot of users unhappy.
> 
> If it means that they implement a better backup strategy, then it's
> probably a good thing, which is the goal of this.

I thought our goal is to provide convenient backup methods...


Ignoring "backup_label" on restart, as I suggested in my previous message,
probably isn't such a hot idea.

But what's wrong with retaining the exclusive backup method and just
sticking a big "Warning: this may cause a restart to fail after a crash"
on it?  That sure wouldn't be unsafe.

If somebody prefers a simpler backup method at the price of having to
manually restart the server after a crash, what's wrong with that?
Why not give them the choice?

I'd say that one could write a server start script that just removes
"backup_label", but I am sure someone will argue that then somebody
will restore a backup and then restart the server without first
recovering the database...

Yours,
Laurenz Albe