From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | Heikki Linnakangas <hlinnaka(at)iki(dot)fi> |
Cc: | pgsql-hackers(at)postgresql(dot)org, gregburek(at)heroku(dot)com |
Subject: | Re: Phantom segment upon promotion causing troubles. |
Date: | 2017-06-21 21:29:53 |
Message-ID: | 20170621212953.kh6po7gkpe3am3dx@alap3.anarazel.de |
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Lists: | pgsql-hackers |
Hi,
On 2017-06-20 16:11:32 +0300, Heikki Linnakangas wrote:
> On 06/19/2017 10:30 AM, Andres Freund wrote:
> > Greg Burek from Heroku (CCed) reported a weird issue on IM, that was
> > weird enough to be interesting. What he'd observed was that he promoted
> > some PITR standby, and early clones of that node work, but later clones
> > did not, failing to read some segment.
> >
> > The problems turns out to be the following: [explanation]
>
> Good detective work!
Thanks.
> > The minimal fix here is presumably not to use XLByteToPrevSeg() in
> > RemoveXlogFile(), but XLByteToSeg(). I don't quite see what purpose it
> > serves here - I don't think it's ever needed.
>
> Agreed, I don't see a reason for it either.
Pushed. And found like three other things while investigating :/
> > There seems to be a larger question ehre though: Why does
> > XLogFileReadAnyTLI() probe all timelines even if they weren't a parent
> > at that period? That seems like a bad idea, especially in more
> > complicated scenarios where some precursor timeline might live for
> > longer than it was a parent? ISTM XLogFileReadAnyTLI() should check
> > which timeline a segment ought to come from, based on the historY?
>
> Yeah. I've had that thought for years as well, but there has never been any
> pressing reason to bite the bullet and rewrite it, so I haven't gotten
> around to it.
Heh. Still seems like something we should tackle - but it'd not be
urgent enough to backpatch, so it doesn't quite seem like something to
tackle *just now* :/
- Andres
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