From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | John Naylor <john(dot)naylor(at)enterprisedb(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: Overhauling "Routine Vacuuming" docs, particularly its handling of freezing |
Date: | 2023-05-01 17:09:06 |
Message-ID: | CAH2-Wz=8pqiy5FFO+1CMzB-fouO+uX0bS3DP41mWXkraea-auw@mail.gmail.com |
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Lists: | pgsql-hackers |
On Mon, May 1, 2023 at 9:16 AM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Mon, May 1, 2023 at 9:08 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > I disagree. If you start the cluster in single-user mode, you can
> > actually wrap it around, unless something has changed that I don't
> > know about.
>
> This patch relies on John's other patch which strongly discourages the
> use of single-user mode. Were it not for that, I might agree.
Also, it's not clear that the term "wraparound" even describes what
happens when you corrupt the database by violating the "no more than
~2.1 billion XIDs distance between any two unfrozen XIDs" invariant in
single-user mode. What specific thing will have wrapped around? It's
possible (and very likely) that every unfrozen XID in the database is
from the same 64-XID-wise epoch.
I don't think that we need to say very much about this scenario (and
nothing at all about the specifics in "Routine Vacuuming"), so maybe
it doesn't matter much. But I maintain that it makes most sense to
describe this scenario as a violation of the "no more than ~2.1
billion XIDs distance between any two unfrozen XIDs" invariant, while
leaving the term "wraparound" out of it completely. That terms has way
too much baggage.
--
Peter Geoghegan
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