Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts

Hi,

On 2014-05-30 14:16:31 +0200, Andres Freund wrote:
> When upgrading a < 9.3 cluster pg_upgrade doesn't bother to keep the old
> multixacts around because they won't be read after the upgrade (and
> aren't compatible). It just resets the new cluster's nextMulti to the
> old + 1.
> Unfortunately that means that there'll be a offsets/0000 file created by
> initdb around. Sounds harmless enough, but that'll actually cause
> problems if the old cluster had a nextMulti that's bigger than that
> page.
>
> When vac_truncate_clog() calls TruncateMultiXact() that'll scan
> pg_multixact/offsets to find the earliest existing segment. That'll be
> 0000. If the to-be-truncated data is older than the last existing
> segment it returns. Then it'll try to determine the last required data
> in members/ by accessing the oldest data in offsets/.
>
> Unfortunately, due to the existing 0000/ segment, that means it'll
> sometimes try to access a nonexistant offsets/ file. Preventing vacuum
> et al from succeeding.

For the sake of search engines, the error that triggers is:
ERROR: could not access status of transaction 2072053907
DETAIL: Could not open file "pg_multixact/offsets/7B81": No such file or directory.

Greetings,

Andres Freund

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