Re: Inconsistent DB data in Streaming Replication

2013-04-10 18:46 keltezéssel, Fujii Masao írta:
> 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.

"rsync --checksum" alone may not but "rsync --inplace" may speed up backup a lot.

>
> Regards,
>

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