Robert Haas wrote:
> On Thu, Jan 19, 2017 at 10:06 AM, Alvaro Herrera
> <> wrote:
> > Peter Eisentraut wrote:
> >
> >> Also, I wonder whether we should not in vacuum.c change the order of the
> >> calls of SetTransactionIdLimit() and SetMultiXactIdLimit() as well, just
> >> to keep everything consistent.
> >
> > I am wary of doing that.  The current coding is well battle-tested by
> > now, but doing things in the opposite order, not at all.  Pending some
> > testing to show that there is no problem with a change, I would leave
> > things as they are.  Probably said testing is too onerous for the
> > benefit (which is just a little consistency).  What I fear is: what
> > happens if a concurrent checkpoint reads the values between the two
> > operations, and a crash occurs?  I think that the checkpoint might save
> > the updated values, so after crash recovery the truncate would not be
> > executed, possibly leaving files around.  Leaving files around might be
> > dangerous for multixacts at least (it's probably harmless for xids).
> Don't both SLRUs eventually wrap?  If so, leaving file around seems
> dangerous for either in equal measure.

Well, multixact uses the whole 2^32 space as valid, while xid only uses
half of it.  I think you would have to crash on every checkpoint just
between those two points for two billion xacts for there to be a problem
for xids.  Anyway this just reinforces my argument that we shouldn't
move those calls -- moving only the commit_ts one is good.

Álvaro Herrera      
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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