Re: Hot Standby 0.2.1

Looking at the startup sequence now.

I see that you modified ExtendSUBTRANS so that it doesn't wipe out
previously set values if it's called with out-of-order xids. I guess
that works, although I think it can leave pages unzeroed if it's called
with a large enough gap between xids, so that the previous one was on
e.g page 10 and the next one on page 12. Page 11 would not be zeroed,
AFAICS. Not sure if such big gaps in the xid sequence are possible, but
seems so if you have a very large max_connections setting and a lot of
subtransactions.

Nevertheless, I'd feel better if we kept ExtendSUBTRANS unmodified, and
instead arranged things so that ExtendSUBTRANS is always called in
xid-order. We can call it from RecordKnownAssignedTransactionIds, which
handles the out-of-order problem for other purposes already.

I think we have similar problem with clog. ExtendCLOG is currently not
called during recovery, so isn't it possible for the problem with
subtransaction commits that's described in the comments in StartupCLOG
to arise during hot standby? Again, I think we should call ExtendCLOG()
in RecordKnownAssignedTransactionIds. RecordKnownAssignedTransactionIds
is the hot standby version of GetNewTransactionId(), so to speak.

If we go with that, I think we'll need to call StartupSUBTRANS/CLOG
earlier in the startup sequence too, when we establish the startup
snapshot and let backends in.

Thoughts?

Other stuff:

- I removed the new DBConsistentUpToLSN() function and moved its
functionality into XLogNeedsFlush(). Since XLogFlush() updates
minRecoveryPoint instead of flushing WAL during recovery, it makes sense
for XLogNeedsFlush() to correspondingly check if minRecoveryPoint needs
to be updated when in recovery. That's a better definition for the call
in bufmgr.c too.

- I went ahead with the changes with RecoveryInfoLock and tracking the
number of held AccessExclusive locks in lmgr.c instead of proc array.

Can we find a better name for "loggable locks"? It means locks that need
to be WAL logged when acquired, for hot standby, and "loggable lock"
doesn't sound right for that. "Loggable" implies that it can be logged,
but it really means that it *must* be logged.

Keep an eye on my git repository for latest changes.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com