Re: Patch for fail-back without fresh backup

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> 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
>>>
>>> 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.

With Regards,
Amit Kapila.