On 01/02/2011 09:35 AM, Heikki Linnakangas wrote:
On 02.01.2011 00:40, Josh Berkus wrote:
On 1/1/11 5:59 AM, Stefan Kaltenbrunner wrote:
well you keep saying that but to be honest I cannot really even see a
usecase for me - what is "only a random one of a set of servers is sync
at any time and I don't really know which one".
My usecases would al involved 2 sync standbys and 1 or more async ones.
but the second sync one would be in a different datacenter and I NEED to
protect against a datacenter failure which your proposals says I cannot
do :(
As far as I know, *nobody* has written the bookkeeping code to actually
track which standbys have ack'd. We need to get single-ack synch
standby merged, tested and working before we add anything as complicated
as "each standby on this list must ack". That means that it's extremely
unlikely for 9.1 at this point.
The bookkeeping will presumably consist of an XLogRecPtr in shared
memory for each standby, tracking how far the standby has acknowledged.
At commit, you scan the standby slots in shared memory and check that
the required standbys have acknowledged your commit record. The
bookkeeping required is the same whether or not we support a list of
standbys that must ack or just one.
Frankly, if Simon hadn't already submitted code, I'd be pushing for
single-standby-only for 9.1, instead of "any one".
Yes, we are awfully late, but let's not panic.
BTW, there's a bunch of replication related stuff that we should work to
close, that are IMHO more important than synchronous replication. Like
making the standby follow timeline changes, to make failovers smoother,
and the facility to stream a base-backup over the wire. I wish someone
worked on those...
yeah I agree that those two are much more of a problem for the general
user base. Whatever people think about our current system - it is very
easy to configure(in terms of knobs to toggle) but extremely hard to get
set up and dealt with during failovers(and I know nobody who got it
right the first few times or has not fucked up one thing in the process).
Syncrep is importantant but I would argue that getting those two fixed
is even more so ;)
Stefan
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