I think there are 2 questions in Dmitry's email.
1) How to minimize the workload on the master while still having a low time
lag between master and slave
and
2) How to coordinate or synchronize the master/slave so the application can
reliably update the master and query the slave for the just-updated data
without getting old/stale data.
On 1, it sounds like the workload is directly proportional to the number of
direct slaves the master has and somewhat proportional to the
sync_interval/time lag setting. Correct me if I'm wrong here.
This second question is one that I have also been struggling with. We would
like to direct all inserts/updates/deletes to the master but send all
selects to the slave(s). However, our app routinely makes updates and
immediately queries back the (just-updated) data. Any suggestions on how to
do this without a bogus and unreliable wait/sleep call? Does Dmitry's idea
make sense??
Thanks,
Bill
----- Original Message -----
From: "Christopher Browne" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Cc: <[email protected]>
Sent: Sunday, May 13, 2007 6:57 PM
Subject: Re: [Slony1-general] How to reduce the time data appears in the
slave?
"Dmitry Koterov" <[EMAIL PROTECTED]> writes:
So, if anybody reads from sl_log_status, it blocks until anything is
written to sl_log_status. Seems it will be fully transparent for
Slony, but decreases the amount of polling queries. (It is an idea
only, practical implementation may need to be more detailed.) What
do you think about it?
It smells to me as though you're trying to optimize the wrong problem.
The "pg_listener problem" is one that changes in 1.2 *mostly*
rectifies; the costs associated with NOTIFY/LISTEN should be dropped
to a dull roar in 1.2. (There are further optimizations in CVS HEAD,
but it shouldn't be *highly* relevant.)
What hasn't been touched, and in a sense, can't be, is to reduce the
load of processing the queries against the "master" node that pull
replication data to feed to the subscribers.
I suspect that it is THOSE queries that are causing you trouble, as
opposed to anything about NOTIFY/LISTEN.
If you have nine nodes all feeding off the master, that's going to put
considerable load on the master node. And THAT would easily cause
things to slow down on the master, and cause replicas to lag.
The fewer nodes that feed directly from the origin, the less load that
you put on the origin, and the easier it should be able to feed them.
--
select 'cbbrowne' || '@' || 'ca.afilias.info';
<http://dba2.int.libertyrms.com/>
Christopher Browne
(416) 673-4124 (land)
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