On Mon, Aug 5, 2013 at 12:53:24PM -0400, Tom Lane wrote: > Andres Freund <and...@2ndquadrant.com> writes: > > On 2013-08-05 12:18:25 -0400, Bruce Momjian wrote: > >> I am unclear why we don't need a lock around _each_ GUC, i.e. if two > >> sessions try to modify the same GUC at the same time. And if we need a > >> lock, seems we can have just one and write all the settings to one file > >> --- it is not like we have trouble doing locking, though this is > >> cluster-wide locking. > > > If you have two sessions modifying the same variable, one is going to > > win and overwrite the other's setting with or without locking around > > GUCs unless you error out if somebody else holds the lock. > > The point of a lock is just to ensure that the end result is one valid > state or the other, and not something corrupt. We would certainly need a > lock if we write to a single file. With file-per-GUC, we could possibly > dispense with a lock if we depend on atomic file rename(); though whether > it's wise to assume that for Windows is unclear. (Note that we ought to > write a temp file and rename it into place anyway, to avoid possibly > corrupting the existing file on out-of-disk-space. The only thing that > needs discussion is whether to add an explicit lock around that.)
So my larger question is why a single-guc-per-file avoids corruption while having all the gucs in a single file does not. It seems the later reduces the probability of lost updates, but does not eliminate it. Also, should I be concerned that everyone removed my pg_upgrade question in their replies. :-O -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
