On Tue, Jul 17, 2012 at 6:56 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > So I went to fix this in the obvious way (attached), but while testing > it I found that the number of buffers_backend events reported during > a regression test run barely changed; which surprised the heck out of > me, so I dug deeper. The cause turns out to be extremely scary: > ForwardFsyncRequest isn't getting called at all in the bgwriter process, > because the bgwriter process has a pendingOpsTable. So it just queues > its fsync requests locally, and then never acts on them, since it never > runs any checkpoints anymore.
:-( > This implies that nobody has done pull-the-plug testing on either HEAD > or 9.2 since the checkpointer split went in (2011-11-01), because even > a modicum of such testing would surely have shown that we're failing to > fsync a significant fraction of our write traffic. > > Furthermore, I would say that any performance testing done since then, > if it wasn't looking at purely read-only scenarios, isn't worth the > electrons it's written on. In particular, any performance gain that > anybody might have attributed to the checkpointer splitup is very > probably hogwash. I don't think anybody thought that was going to result in a direct performance gain, but I agree the performance testing needs to be redone. I suspect that the impact on my testing is limited, because I do mostly pgbench testing, and the lost fsync requests were probably duplicated by non-lost fsync requests from backend writes. But I agree that it needs to be redone once this is fixed. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers