We recently upgraded a very large database (~550 GB) from 8.1.4 to 8.2.4 via a pg_dump and pg_restore. (Note that the restore took several days.) We had accepted the default settings:
vacuum_freeze_min_age = 100 million
autovacuum_freeze_max_age = 200 million
Due to our very high transaction rate, it appears that a database-wide vacuum kicked off approximately 2 weeks after the restore. (Aside: after reading the docs and considering our system characteristics, I know now that our autovacuum_freeze_max_age should be more like 2 billion. However on this machine I haven't changed the config settings yet.) Also, I believe, that due to the bulk of our data having the same "age" after the restore, the db-wide vacuum had *a lot* of rows to mark with the FrozenXID.
The good thing is that the db-wide vacuum, which ran for a long time, was reasonably non-intrusive to other database activity (somewhat, but reasonable for the short term). The other good thing was that concurrent autovacuum processes were still vacuuming/analyzing tables as necessary.
The bad thing, which I don't totally understand from reading the docs, is that another db-wide vacuum kicked off exactly 24 hours after the first db-wide vacuum kicked off, before the first one had finished. (Note that these vacuums seem to go through the tables alphabetically.) I managed to explain this to myself in that there were still rows in tables not yet touched by the first db-wide vacuum that could have XIDs older than autovacuum_freeze_max_age. Fine, so two db-wide vacuums were now taking place, one behind the other.
The first db-wide vacuum finished approximately 36 hours after it started. At this point I was convinced that the second db-wide vacuum would run to completion with little or no work to do and all would be good. The thing I can't explain is why a third db-wide vacuum kicked off exactly 24 hours (again) after the second db-wide vacuum kicked off (and the second vacuum still running).
Wouldn't the first db-wide vacuum have marked any rows that needed it with the FrozenXID? Why would a third db-wide vacuum kick off so soon after the first db-wide vacuum had completed? Surely there haven't been 100 million more transactions in the last two days?
Can someone explain what is going on here? I can't quite figure it out based on the docs.
Thanks,
Steve