On 2013-10-24 13:51:52 -0700, Josh Berkus wrote: > On 10/24/2013 01:14 PM, Heikki Linnakangas wrote: > > One extra WAL record whenever a hint bit is set on a page, for the first > > time after a checkpoint. In other words, a WAL record needs to be > > written in the same circumstances as with page checksums, but the WAL > > records are much smaller as they don't need to contain a full page > > image, just the block number of the changed block. > > > > Or maybe we'll write the full page image after all, like with page > > checksums, just without calculating the checksums. It might be tricky to > > skip the full-page image, because then a subsequent change of the page > > (which isn't just a hint-bit update) needs to somehow know it needs to > > take a full page image even though a WAL record for it was already written. > > I think it would be worth estimating what this actually looks like in > terms of log write quantity. My inclication is to say that if it > increases log writes less than 10%, we don't need to provide an option > to turn it off.
It entirely depends on your workload. If it happens to be something like: INSERT INTO table (lots_of_data); CHECKPOINT; SELECT * FROM TABLE; i.e. there's a checkpoint between loading the data and reading it - not exactly all that uncommon - we'll need to log something for every page. That can be rather noticeable. Especially as I think it will be rather hard to log anything but a real FPI. I really don't think everyone will want this. I am absolutely not against providing an option to log enough information to make pg_rewind work, but I think providing a command to do *safe* *planned* failover will help in many more. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
