Re: [PATCH] Patch to compute Max LSN of Data Pages

On Wed, Nov 14, 2012 at 5:35 PM, Amit Kapila <amit(dot)kapila(at)huawei(dot)com> wrote:
> On Tuesday, November 13, 2012 10:17 PM Fujii Masao wrote:
>> On Tue, Nov 13, 2012 at 1:23 PM, Amit kapila <amit(dot)kapila(at)huawei(dot)com>
>> wrote:
>> > On Monday, November 12, 2012 9:56 PM Alvaro Herrera wrote:
>> > Robert Haas escribió:
>> >> On Tue, Jul 31, 2012 at 8:09 AM, Amit kapila <amit(dot)kapila(at)huawei(dot)com>
>> wrote:
>> >
>> >>> >> I think I can see all of those things being potentially useful.
>> There
>> >>> >> are a couple of pending patches that will revise the WAL format
>
>> >>> I wonder if we shouldn't make this a separate utility, rather than
>> >>> something that is part of pg_resetxlog. Anyone have a thought on
>> that
>> >>> topic?
>> >
>> >> That thought did cross my mind too.
>> >
>> We might be able to use this utility to decide whether we need to take
>> a fresh backup from the master onto the standby, to start old master
>> as new standby after failover.
>>
>> When starting new standby after failover, any data page in the standby
>> must
>> not precede the master. Otherwise, the standby cannot catch up with the
>> master
>> consistently. But, the master might write the data page corresponding to
>> the WAL which has not been replicated to the standby yet. So, if
>> failover happens
>> before that WAL has been replicated, the data page in old master would
>> precede
>> new master (i.e., old standby), and in this case the backup is required.
>> OTOH,
>> if maximum LSN in data page in the standby is less than the master, the
>> backup
>> is not required.
>
> When new standby will start the replication (RequestXLogStreaming()), it
> will
> send the startpoint, so won't in above scenario that startpoint will be
> ahead of new master
> (or new master won't have that LSN) and replication will not be
> eastablished?

The startpoint is the heading LSN of the WAL file including the latest
checkpoint record. Yes, there can be the case where the startpoint is
ahead of new master. In this case, replication would fail to be established
because of lack of requested WAL file. OTOH, there can be the case
where new master has already been ahead of the startpoint.

> So now user may not be able to decide whether he needs to do incremental or
> full backup from new master,
> is this the case you are trying to point?

Sorry, I could not parse this comment. Could you elaborate your concern again?

Regards,

--
Fujii Masao