On Thu, Jul 19, 2012 at 12:43:23PM -0400, Robert Haas wrote: > On Thu, Jul 19, 2012 at 12:02 PM, Bruce Momjian <br...@momjian.us> wrote: > > On Thu, Jul 19, 2012 at 09:41:29AM -0400, Robert Haas wrote: > >> On Thu, Jul 19, 2012 at 2:38 AM, Bruce Momjian <br...@momjian.us> wrote: > >> > No, the point is they run pg_upgrade on the stopped primary and stopped > >> > standbys. Are those the same? I am not really sure. > >> > >> Of course not. > > > > OK, but why? When the clusters are stopped they are the same, you are > > running the same initdb on both matchines, and running the same > > pg_upgrade. What would cause the difference, other than the Database > > System Identifier, which we can deal with? I don't think we can > > guarantee they are the same, but what would guarantee they are > > different? > > There isn't any guarantee that they are different. There's just no > guarantee that they are the same, which is enough to make this idea a > non-starter. > > In general, it's pretty easy to understand that if you perform the > same series of inserts, updates, and deletes on two systems, you might > not end up with the exact same binary contents. There are a lot of > reasons for this: any concurrent activity whatsoever - even the exact > timing of autovacuum - can cause the same tuples can end up in > different places in the two systems. Now, admittedly, in the case of > pg_upgrade, you're restoring the dump using a single process with > absolutely no concurrent activity and even autovacuum disabled, so the > chances of ending up with entirely identical binary contents are > probably higher than average. But even there you could have > checkpoints trigger at slightly different times while restoring the > dumps, and of course checkpoints take buffer locks, and so now a HOT > prune might happen on one machine but get skipped on the other one > because the checkpointer has dropped the lock but not the pin, and now > you're hosed. > > Even if you could control for that particular possibility, there are > surely others now and there will be more in the future.
I think the checkpoint issue is the ideal killer --- thanks. -- 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 (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers