Tom Lane wrote: > Bruce Momjian <br...@momjian.us> writes: > > OK, here is another idea, maybe crazy: > > > When we read in a page that has an invalid CRC, we check the page to see > > which hint bits are _not_ set, and we try setting them to see if can get > > a matching CRC. If there no missing hint bits and the CRC doesn't > > match, we know the page is corrupt. If two hint bits are missing, we > > can try setting one and both of them and see if can get a matching CRC. > > If we can, the page is OK, if not, it is corrupt. > > > Now if 32 hint bits are missing, but could be based on transaction > > status, then we would need 2^32 possible hint bit combinations, so we > > can't do the test and we just assume the page is valid. > > A typical page is going to have something like 100 tuples, so > potentially 2^400 combinations to try. I don't see this being > realistic from that standpoint. What's much worse is that to even > find the potentially missing hint bits, you need to make very strong > assumptions about the validity of the rest of the page. > > The suggestions that were made upthread about moving the hint bits > could resolve the second objection, but once you do that you might > as well just exclude them from the CRC and eliminate the guessing.
OK, crazy idea #3. What if we had a per-page counter of the number of hint bits set --- that way, we would only consider a CRC check failure to be corruption if the count matched the hint bit count on the page. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + If your life is a hard drive, Christ can be your backup. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers