Re: ZFS snapshots - to use pg_start_backup() and pg_stop_backup() or no?

On Sep 11, 2013, at 4:29 PM, Gregory Haase <haaseg(at)onefreevoice(dot)com> wrote:

> I was trying to figure out how to get the following syntax to work:
>
> echo "select pg_start_backup('zfs_snapshot'); \\! zfs snapshot zroot/zpgsql(at)test; \\ select pg_stop_backup();" | psql postgres

I do:

psql -c "select pg_start_backup('whatever');" && zfs snapshot pool/fs(at)sn && psql -c "select pg_stop_backup();"

That way no need to shell out from psql :)

>
> The above command successfully starts the backup and creates the snapshot but then fails to stop the backup. I've tried various combinations of \ and \\ here with different whitespace and I just can't seem to find a combination that works. I don't understand the proper use of \\ (described as the separator metacommand).

Keep in mind that echo "\\" will actually only echo '\' because \ is a shell escape as well...

>
> However, in my research, I noted that a bunch of people seem to just not even bother with pg_start_backup/pg_stop_backup and I guess aren't that worried about the crash recovery process if they need to perform a restore. I also find the omission of the start/stop backup functions from the File System Level Backup page: http://www.postgresql.org/docs/9.2/static/backup-file.html
>
> Is the pg_start_backup() and pg_stop_backup() even necessary?
>

If all of your Postgres files are part of *the same* consistent snapshot (i.e. are on one FS that gets snapshotted), then the start/stop backup should not be necessary. It will just look like a server crash instead.

pg_start_backup is used when you do not have filesystem snapshotting available, and is described in detail on the next manual page:

http://www.postgresql.org/docs/9.2/static/continuous-archiving.html