| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Mark Dilger <mark(dot)dilger(at)enterprisedb(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Peter Geoghegan <pg(at)bowt(dot)ie>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: new heapcheck contrib module |
| Date: | 2020-07-30 20:47:06 |
| Message-ID: | 20200730204706.fnvwv247czrkglzx@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
Hi,
On 2020-07-30 13:18:01 -0700, Mark Dilger wrote:
> Per tuple, tuple_is_visible() potentially checks whether the xmin or xmax committed via TransactionIdDidCommit. I am worried about concurrent truncation of clog entries causing I/O errors on SLRU lookup when performing that check. The three strategies I had for dealing with that were taking the XactTruncationLock (formerly known as CLogTruncationLock, for those reading this thread from the beginning), locking out vacuum, and the idea upthread from Andres about setting PROC_IN_VACUUM and such. Maybe I'm being dense and don't need to worry about this. But I haven't convinced myself of that, yet.
I think it's not at all ok to look in the procarray or clog for xids
that are older than what you're announcing you may read. IOW I don't
think it's OK to just ignore the problem, or try to work around it by
holding XactTruncationLock.
Greetings,
Andres Freund
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