Re: Patch for fail-back without fresh backup

On Tuesday, June 18, 2013, Amit Kapila wrote:

> On Tuesday, June 18, 2013 12:18 AM Sawada Masahiko wrote:
> > On Sun, Jun 16, 2013 at 2:00 PM, Amit kapila <amit(dot)kapila(at)huawei(dot)com<javascript:;>
> >
> > wrote:
> > > On Saturday, June 15, 2013 8:29 PM Sawada Masahiko wrote:
> > > On Sat, Jun 15, 2013 at 10:34 PM, Amit kapila
> > <amit(dot)kapila(at)huawei(dot)com <javascript:;>> wrote:
> > >>
> > >> On Saturday, June 15, 2013 1:19 PM Sawada Masahiko wrote:
> > >> On Fri, Jun 14, 2013 at 10:15 PM, Amit Kapila
> > <amit(dot)kapila(at)huawei(dot)com <javascript:;>> wrote:
> > >>> On Friday, June 14, 2013 2:42 PM Samrat Revagade wrote:
> > >>>> Hello,
> > >>>
> > >>>>>> 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-
> > 6OzRaew5pWhk7yQtb
> > >>>>>> JgWrFu513s+Q(at)mail(dot)gmail(dot)com <javascript:;>
> > >>>>>>
> > >>>>>> Let me again summarize the problem we are trying to address.
> > >>>>>>
> > >>>>>>
> > >>>>> How will you take care of extra WAL on old master during
> > recovery. If it
> > >>>>> plays the WAL which has not reached new-master, it can be a
> > problem.
> > >>
> > >>>> you means that there is possible that old master's data ahead of
> > new
> > >>>> master's data.
> > >>
> > >>> I mean to say is that WAL of old master can be ahead of new
> > master. I understood that
> > >>> data files of old master can't be ahead, but I think WAL can be
> > ahead.
> > >>
> > >>>> so there is inconsistent data between those server when fail back.
> > right?
> > >>>> if so , there is not possible inconsistent. because if you use GUC
> > option
> > >>>> as his propose (i.g., failback_safe_standby_mode = remote_flush),
> > >>>> when old master is working fine, all file system level changes
> > aren't
> > >>>> done before WAL replicated.
> > >>
> > >>> Would the propose patch will take care that old master's WAL is
> > also not ahead in some way?
> > >>> If yes, I think i am missing some point.
> > >
> > >> yes it will happen that old master's WAL ahead of new master's WAL
> > as you said.
> > >> but I think that we can solve them by delete all WAL file when old
> > >> master starts as new standby.
> > >
> > > I think ideally, it should reset WAL location at the point where new
> > master has forrked off.
> > > In such a scenario it would be difficult for user who wants to get a
> > dump of some data in
> > > old master which hasn't gone to new master. I am not sure if such a
> > need is there for real users, but if it
> > > is there, then providing this solution will have some drawbacks.
>
> > I think that we can dumping data before all WAL files deleting. All
> > WAL files deleting is done when old master starts as new standby.
>
> Can we dump data without starting server?
>
> Sorry I made a mistake. We can't it.

this proposing patch need to be able to also handle such scenario in
future.

Regards,

---
Sawada Masahiko

--
Regards,

-------
Sawada Masahiko