Thursday, 17 June 2010

Smoothing replication

At first, streaming replication looked pretty raw though I've been doing a thorough review of the code. I guess that's probably how Hot Standby looked at first as well. Now, I'm getting quite excited at the way things are shaping up.

Originally the WALSender process waited for 200ms after sending each chunk of data. That's now been fixed so that WALSender will stream continually until no outstanding WAL data remains.

Also, the max chunk size has been reduced to 128kB, which is now the same default size used by DRBD.

On Monday, I noticed that the WALSender was sending WAL data before it had been fsynced. So in case of a crash that could cause a problem. That only really matters if you're using synchronous_commit = off or fsync = off. (Though fsync = off and replication just have no business being configured simultaneously).

Lots of smaller changes have also come through in recent months, so now we have the ability to get log messages when replication connects or disconnects. We also now have special messages if the replication connection is refused by the primary via the pg_hba.conf.

From here, it's looking like streaming replication is fast, efficient and low latency. It's also easy to use and easy to troubleshoot. It's an impressive community effort.

I'm also happy to say that almost all of the features of pg_standby have been assimilated into core now, so the only external module you'll need is something called pg_archivecleanup which is new in this release.

Next few months we're going to see designs emerge for synchronous replication. That is going to require some subtle coding to get good. It sounds like a couple of people are planning prototypes of how that should work.

No comments:

Post a Comment