On 11/11/2011 04:54 AM, Rafael Martinez wrote:
Your explanation in 2) sounds like a good candidate for the problem we had. As I said in june, I think we need to improve the documentation in this area. A note in the documentation about what you have explained in 2) with maybe some hints about how to find out if this is happening will be a great improvement.
A new counter was added to pg_stat_bgwriter in PostgreSQL 9.1 that tracks when the problem I described happens. It's hard to identify it specifically without a source code change of some sort. Initially I added new logging to the server code to identify the issue before the new counter was there. The only thing you can easily look at that tends to correlate well with the worst problems here is the output from turning log_checkpoint on. Specifically, the "sync" times going way up is a sign there's a problem with write speed.
As for the documentation, not much has really changed from when you brought this up on the docs list. The amount of WAL files that can be created by a "short-term peak" is unlimited, which is why there's no better limit listed than that. Some of the underlying things that make the problem worse are operating system level issues, not ones in the database itself; the PostgreSQL documentation doesn't try to wander too far into that level. There are also a large number of things you can do at the application level that will generate a lot of WAL activity. It would be impractical to list all of them in the checkpoint documentation though.
On reviewing this section of the docs again, one thing that we could do is make the "WAL Configuration" section talk more about log_checkpoints and interpreting its output. Right now there's no mention of that parameter in the section that talks about parameters to configure; there really should be.
-- Greg Smith 2ndQuadrant US greg@xxxxxxxxxxxxxxx Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
