On 07.01.2012 12:14, Simon Riggs wrote:
page_checksums is an optional parameter, so you can turn it on or off on the standby as you wish. People frequently have a standby dedicated to HA and other standbys for queries. So this is all normal and natural.
There needs to be a well-documented way of turning it on/off. In particular, from off to on. If you ever flip it off even for a minute, pages with invalid checksums start to appear in the data directory, and if you then turn it on again, you start getting errors. Perhaps there needs to be a third setting, calculate-but-dont-verify, where CRCs are updated but existing CRCs are not expected to be correct. And a utility to scan through your database and fix any incorrect CRCs, so that after that's run in calculate-but-dont-verify mode, you can safely turn checksums on.
Even better would be a way to make that process robust so that you can't do a pilot error and turn page_checksums 'on' when it's not safe to do so. Otherwise when you get a checksum error, your first thought is going to be "hmm, I wonder if anyone ever turned page_checksums 'off' by accident anytime in the past, or if this is a genuine error".
page_checksums will default to 'off' in the final patch anyway, in my understanding.
That makes the need for such a facility even more important. Otherwise there's no safe way to ever switch it from off to on.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
