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Re: Avoiding adjacent checkpoint records

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Peter Geoghegan <peter(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Avoiding adjacent checkpoint records
Date: 2012-06-08 06:44:24
Message-ID: CA+U5nMKEjvqv0jHcsioUf=f5L_XJbLOXuUHcFT36p8x-+rq7jg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 8 June 2012 05:01, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Peter Geoghegan <peter(at)2ndquadrant(dot)com> writes:
>> On 7 June 2012 18:03, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> On Thu, Jun 7, 2012 at 12:52 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>>> Clearly, delaying checkpoint indefinitely would be a high risk choice.
>>>> But they won't be delayed indefinitely, since changes cause WAL
>>>> records to be written and that would soon cause another checkpoint.
>
>>> But that's exactly the problem - it might not be soon at all.
>
>> Your customer's use-case seems very narrow, and your complaint seems
>> unusual to me, but couldn't you just get the customer to force
>> checkpoints in a cronjob or something? CheckPointStmt will force,
>> provided !RecoveryInProgress() .
>
> I think both you and Simon have completely missed the point.  This
> is not a "use case" in the sense of someone doing it deliberately.
> This is about data redundancy, ie, if you lose your WAL through some
> unfortunate mishap, are you totally screwed or is there a reasonable
> chance that the data is on-disk in the main data store?  I would guess
> that the incidents Robert has been talking about were cases where EDB
> were engaged to do crash recovery, and were successful precisely because
> PG wasn't wholly dependent on the WAL copy of the data.

Apart from the likely point that hint bits exist on disk...

> This project has always put reliability first.  It's not clear to me why
> we would compromise that across-the-board in order to slightly reduce
> idle load in replication configurations.  Yeah, it's probably not a
> *large* compromise ... but it is a compromise, and one that doesn't
> seem to me to be very well-advised.  We can fix the idle-load issue
> without compromising on this basic goal; it will just take more than
> a ten-line patch to do it.

So now the standard for my patches is that I must consider what will
happen if the xlog is deleted?

Tell me such a rule is applied uniformly to all patches then I would be happy.


I will revoke, not because of the sense in this argument but because
you personally ask for it.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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