Re: Patch for fail-back without fresh backup

On Tuesday, June 18, 2013 12:18 AM Sawada Masahiko wrote:
> On Sun, Jun 16, 2013 at 2:00 PM, Amit kapila <amit.kapila@huawei.com>
> wrote:
> > On Saturday, June 15, 2013 8:29 PM Sawada Masahiko wrote:
> > On Sat, Jun 15, 2013 at 10:34 PM, Amit kapila
> <amit.kapila@huawei.com> wrote:
> >>
> >> On Saturday, June 15, 2013 1:19 PM Sawada Masahiko wrote:
> >> On Fri, Jun 14, 2013 at 10:15 PM, Amit Kapila
> <amit.kapila@huawei.com> 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@mail.gmail.com
> >>>>>>
> >>>>>> 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?

With Regards,
Amit Kapila.