Gregory Stark wrote: > "Bruce Momjian" <[EMAIL PROTECTED]> writes: > > > I think we need to think about when these CRCs would be read and > > written. It would be written when it hits the disk, hopefully by the > > background writer, and I think after a server crash, all pages would > > have to be read and checked. The good news is that both of these are > > non-critical paths. > > If you're protecting against torn pages then yes, if the system is shut down > uncleanly by a system crash or power failure you would in theory have to scan > every page of every table and index before starting up. > > But if the system was shut down uncleanly as the result of a Postgres crash or > fast shutdown of Postgres then that isn't an issue. And many users may prefer > to bring the system up as soon as possible as long as they know any corrupt > pages will be spotted and throw errors as soon as it's seen.
I don't think we should start up a system and only detect the errors later. > So I think you need a mode that only checks checksums when a page is read from > disk. That would protect against torn pages (but not necessarily before > bringing up the system) and against bad i/o hardware. > > Unfortunately memory errors are far more common than disk errors and I it > would be much harder to protect against them. You can't check it when someone > may be writing to the buffer, which limits you to checking it only when you > acquire some form of lock on the buffer. It also means you would have to write > it before you release a lock if you've made any changes. > > Worse, I'm not sure how to handle hint bits though. We currently don't require > any lock at all to set hint bits which means someone may think they can check > a checksum while or after you've fiddled some bits. Yep, a problem. -- Bruce Momjian <[EMAIL PROTECTED]> http://momjian.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
