Re: block-level incremental backup

On Sat, Aug 31, 2019 at 7:59 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Thu, Aug 29, 2019 at 10:41 AM Jeevan Ladhe
> <jeevan(dot)ladhe(at)enterprisedb(dot)com> wrote:
> > Due to the inherent nature of pg_basebackup, the incremental backup also
> > allows taking backup in tar and compressed format. But, pg_combinebackup
> > does not understand how to restore this. I think we should either make
> > pg_combinebackup support restoration of tar incremental backup or
> restrict
> > taking the incremental backup in tar format until pg_combinebackup
> > supports the restoration by making option '--lsn' and '-Ft' exclusive.
> >
> > It is arguable that one can take the incremental backup in tar format,
> extract
> > that manually and then give the resultant directory as input to the
> > pg_combinebackup, but I think that kills the purpose of having
> > pg_combinebackup utility.
>
> I don't agree. You're right that you would have to untar (and
> uncompress) the backup to run pg_combinebackup, but you would also
> have to do that to restore a non-incremental backup, so it doesn't
> seem much different. It's true that for an incremental backup you
> might need to untar and uncompress multiple prior backups rather than
> just one, but that's just the nature of an incremental backup. And,
> on a practical level, if you want compression, which is pretty likely
> if you're thinking about incremental backups, the way to get that is
> to use tar format with -z or -Z.
>
> It might be interesting to teach pg_combinebackup to be able to read
> tar-format backups, but I think that there are several variants of the
> tar format, and I suspect it would need to read them all. If someone
> un-tars and re-tars a backup with a different tar tool, we don't want
> it to become unreadable. So we'd either have to write our own
> de-tarring library or add an external dependency on one.

Are we using any tar library in pg_basebackup.c? We already have the
capability
in pg_basebackup to do that.

> I don't
> think it's worth doing that at this point; I definitely don't think it
> needs to be part of the first patch.
>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>
>
>

--
Ibrar Ahmed