On Sun, Dec 25, 2011 at 5:08 AM, Simon Riggs <si...@2ndquadrant.com> wrote: > On Sat, Dec 24, 2011 at 8:06 PM, Greg Stark <st...@mit.edu> wrote: >> On Sat, Dec 24, 2011 at 4:06 PM, Simon Riggs <si...@2ndquadrant.com> wrote: >>> Checksums merely detect a problem, whereas FPWs correct a problem if >>> it happens, but only in crash situations. >>> >>> So this does nothing to remove the need for FPWs, though checksum >>> detection could be used for double write buffers also. >> >> This is missing the point. If you have a torn page on a page that is >> only dirty due to hint bits then the checksum will show a spurious >> checksum failure. It will "detect" a problem that isn't there. > > It will detect a problem that *is* there, but one you are classifying > it as a non-problem because it is a correctable or acceptable bit > error.
I don't agree with this. We don't WAL-log hint bit changes precisely because it's OK if they make it to disk and it's OK if they don't. Given that, I don't see how we can say that writing out only half of a page that has had hint bit changes is a problem. It's not. (And if it is, then we ought to WAL-log all such changes regardless of whether CRCs are in use.) -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
