On Wed, Jul 28, 2010 at 3:16 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> (1) You're assuming that the page will be zeroes on the slave without
> having forced it to be so. A really obvious case where this fails
> is where we're doing crash-and-restart on the master: a later action
> could have modified the page away from the all-zero state. (In
> principle that's OK but I think this might break torn-page protection.)
Hmm, yeah, that does seem like it has the potential to be bad. I
think this is sufficient reason to go with fix #1.
> (2) On filesystems that support holes, the page will not have storage,
> whereas it (probably) does on the master. This could lead to a
> divergence in behavior later, ie slave runs out of disk space at a
> different point than the master.
I can't get excited about this one.
> (3) The position of the nominal EOF can drive choices about which page
> to put new tuples in, specifically thats where RelationGetBufferForTuple
> will go if FSM has no information. This could result in unexpected
> divergence in behavior after the slave goes live compared to what the
> master would have done. Maybe that's OK but it seems better to avoid
> it if we can, especially when you think about crash-and-restart on the
> master as opposed to a separate slave.
You're still going to have that in the "normal" (not altering the
tablespace) extension case, which is presumably far more common.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company
