Re: Inconsistent DB data in Streaming Replication

On Wed, Apr 10, 2013 at 11:16 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> On 2013-04-10 10:10:31 -0400, Tom Lane wrote:
>> Amit Kapila <amit(dot)kapila(at)huawei(dot)com> writes:
>> > On Wednesday, April 10, 2013 3:42 PM Samrat Revagade wrote:
>> >> Sorry, this is incorrect. Streaming replication continuous, master is not
>> >> waiting, whenever the master writes the data page it checks that the WAL
>> >> record is written in standby till that LSN.
>>
>> > I am not sure it will resolve the problem completely as your old-master can
>> > have some WAL extra then new-master for same timeline. I don't remember
>> > exactly will timeline switch feature
>> > take care of this extra WAL, Heikki can confirm this point?
>> > Also I think this can serialize flush of data pages in checkpoint/bgwriter
>> > which is currently not the case.
>>
>> Yeah. TBH this entire discussion seems to be "let's cripple performance
>> in the normal case so that we can skip doing an rsync when resurrecting
>> a crashed, failed-over master". This is not merely optimizing for the
>> wrong thing, it's positively hazardous. After a fail-over, you should
>> be wondering whether it's safe to resurrect the old master at all, not
>> about how fast you can bring it back up without validating its data.
>> IOW, I wouldn't consider skipping the rsync even if I had a feature
>> like this.
>
> Agreed. Especially as in situations where you fall over in a planned
> way, e.g. for a hardware upgrade, you can avoid the need to resync with
> a littlebit of care.

It's really worth documenting that way.

> So its mostly in catastrophic situations this
> becomes a problem and in those you really should resync - and its a good
> idea not to use a normal rsync but a rsync --checksum or similar.

If database is very large, rsync --checksum takes very long. And I'm concerned
that most of data pages in master has the different checksum from those in the
standby because of commit hint bit. I'm not sure how rsync --checksum can
speed up the backup after failover.

Regards,

--
Fujii Masao