Robert Haas wrote: > On Thu, Jan 19, 2017 at 10:06 AM, Alvaro Herrera > <alvhe...@2ndquadrant.com> 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 https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
