Re: Patch for fail-back without fresh backup

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.

With Regards,
Amit Kapila.