Jonah H. Harris escribió: > On Thu, Oct 2, 2008 at 1:44 PM, Alvaro Herrera > <[EMAIL PROTECTED]> wrote: > > How about when a hint bit is set and the page is not already dirty, set > > the checksum to the "always valid" value? The problem I have with this > > idea is that there would be lots of pages excluded from the CRC checks, > > a non-trivial percentage of the time. > > I don't like that because it trades-off corruption detection (the > whole point of this feature) for a slight performance improvement.
I agree that giving up corruption detection is not such a hot idea, but what I'm intending to get back is not performance but correctness (in this case protection from the torn page problem) > > Maybe we could mix this with Simon's approach to counting hint bit > > setting, and calculate a valid CRC on the page every n-th non-logged > > change. > > I still think we should only calculate checksums on the actual write. Well, if we could trade off a bit of performance for correctness, I would give up on that :-) However, you're right that this tradeoff is not what we're having here. > And, this still seems to have an issue with WAL, unless Simon's > original idea somehow included recording hint bit settings/dirtying > the page in WAL. I have to admit I don't remember exactly how it worked :-) I think the idea was avoiding setting the page dirty until a certain number of hint bit setting operations had been done (which I think means it's not useful for the present purpose). -- Alvaro Herrera http://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
