Re: COMMIT NOWAIT Performance Option

"Bruce Momjian" <bruce(at)momjian(dot)us> 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.

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.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com