Re: Patch for fail-back without fresh backup

On 14.06.2013 12:11, Samrat Revagade wrote:
> We have already started a discussion on pgsql-hackers for the problem of
> taking fresh backup during the failback operation here is the link for that:
>
> http://www.postgresql.org/message-id/CAF8Q-Gxg3PQTf71NVECe-6OzRaew5pWhk7yQtbJgWrFu513s+Q@mail.gmail.com
>
> Let me again summarize the problem we are trying to address.
>
> When the master fails, last few WAL files may not reach the standby. But
> the master may have gone ahead and made changes to its local file system
> after flushing WAL to the local storage. So master contains some file
> system level changes that standby does not have. At this point, the data
> directory of master is ahead of standby's data directory.
>
> Subsequently, the standby will be promoted as new master. Later when the
> old master wants to be a standby of the new master, it can't just join the
> setup since there is inconsistency in between these two servers. We need to
> take the fresh backup from the new master. This can happen in both the
> synchronous as well as asynchronous replication.

Did you see the thread on the little tool I wrote called pg_rewind?

http://www.postgresql.org/message-id/519DF910.4020609@vmware.com

It solves that problem, for both clean and unexpected shutdown. It needs
some more work and a lot more testing, but requires no changes to the
backend. Robert Haas pointed out in that thread that it has a problem
with hint bits that are not WAL-logged, but it will still work if you
also enable the new checksums feature, which forces hint bit updates to
be WAL-logged. Perhaps we could add a GUC to enable hint bits to be
WAL-logged, regardless of checksums, to make pg_rewind work.

I think that's a more flexible approach to solve this problem. It
doesn't require an online feedback loop from the standby to master, for
starters.

- Heikki