On Tue, Dec 1, 2009 at 8:30 AM, Heikki Linnakangas <heikki.linnakan...@enterprisedb.com> wrote: > Bruce Momjian wrote: >> What might be interesting is to report CRC mismatches if the database >> was shut down cleanly previously; I think in those cases we shouldn't >> have torn pages. > > Unfortunately that's not true. You can crash, leading to a torn page, > and then start up the database and shut it down cleanly. The torn page > is still there, even though the last shutdown was a clean one.
Thinking through this, as I understand it, in order to prevent this problem, you'd need to be able to predict at recovery time which pages might have been torn by the unclean shutdown. In order to do that, you'd need to know which pages were waiting to be written to disk at the time of the shutdown. For ordinary page modifications, that's not a problem, because there will be WAL records for those pages that need to be replayed, and we could recompute the CRC at the same time. But for hint bit changes, there's no persistent state that would tell us which hint bits were in the midst of being flipped when the system went down, so the only way to make sure all the CRCs are correct would be to rescan every page in the entire cluster and recompute every CRC. Is that right? ...Robert -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers