Re: Inconsistent DB data in Streaming Replication

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. 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.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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