Re: Online base backup from the hot-standby

On Tue, Oct 25, 2011 at 12:33 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> One problem with this whole FPW-tracking is that pg_lesslog makes it fail.
> I'm not sure what we need to do about that - maybe just add a warning to the
> docs. But it leaves a bit bad feeling in my mouth. Usually we try to make
> features work orthogonally, without dependencies to other settings. Now this
> feature requires that full_page_writes is turned on in the master, and also
> that you don't use pg_lesslog to compress the WAL segments or your base
> backup might be corrupt.

Right, pg_lesslog users cannot use the documented procedure. They need to
do more complex one;

1. Execute pg_start_backup() in the master, and save its return value.
2. Wait until the backup starting checkpoint record has been replayed
in the standby. You can do this by comparing the return value of
pg_start_backup() with pg_last_replay_location().
3. Do the documented standby-only backup procedure.
4. Execute pg_stop_backup() in the master.

This is complicated, but I'm not sure how we can simplify it. Anyway we can
document this procedure for pg_lesslog users. We should?

> The procedure to take a backup from the standby
> seems more complicated than taking it on the master - there are more steps
> to follow.

Extending pg_basebackup so that it can take a backup from the standby would
make the procedure simple to a certain extent, I think. Though a user
still needs
to enable FPW in the master and must not use pg_lesslog.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center