Re: Dead Space Map version 2

"Jim C. Nasby" <jim(at)nasby(dot)net> writes:
> Yes, DSM would make FSM recovery more important, but I thought it was
> recoverable now? Or is that only on a clean shutdown?

Currently we throw away FSM during any non-clean restart. This is
probably overkill but I'm quite unclear what would be a safe
alternative.

> I suspect we don't need perfect recoverability...

The main problem with the levels proposed by Takahiro-san is that any
transition from FROZEN to not-FROZEN *must* be exactly recovered,
because vacuum will never visit an allegedly frozen page at all. This
appears to require WAL-logging DSM state changes, which is a pretty
serious performance hit. I'd be happier if the DSM content could be
treated as just a hint. I think that means not trusting it for whether
a page is frozen to the extent of not needing vacuum even for
wraparound. So I'm inclined to propose that there be only two states
(hence only one DSM bit per page): page needs vacuum for space recovery,
or not. Vacuum for XID wraparound would have to hit every page
regardless.

regards, tom lane