On Thu, Nov 15, 2012 at 2:44 PM, Jeff Davis <pg...@j-davis.com> wrote: > The issue about external utilities is a bigger problem than I realized > at first. Originally, I thought that it was just a matter of code to > associate the checksum with the data. > > However, an external utility will never see a torn page while the system > is online (after recovery); but it *will* see an inconsistent view of > the checksum and the data if they are issued in separate write() calls. > So, the hazard of storing the checksum in a different place is not > equivalent to the existing hazard of a torn page.
I agree that the hazards are not equivalent, but I'm not sure I agree that an external utility will never see a torn page while the system is on-line. We have a bunch of code that essentially forces full_page_writes=on during a base backup even if it's normally off. I think that's necessary precisely because neither the 8kB write() nor the unknown-sized-read used by the external copy program are guaranteed to be atomic. -- 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
