Re: Block-level CRC checks

* Alvaro Herrera <alvherre(at)commandprompt(dot)com> [081118 12:25]:

> I don't think it's a matter of hoy many writes or how much IO. The
> question is locks. Right now we flip hint bits without taking any kind
> of lock on the page. If we're going to WAL-log each hint bit change,
> then we will need to lock the page to update the LSN. This will make
> changing a hint bit a very expensive operation, and maybe a possible
> cause for deadlocks.

Ya, that's obviously the worst option.

> What my patch did was log hint bits in bulk. The problem of that
> approach was precisely that it was not locking the logged page enough
> (locking before setting the "this page needs hint bits logged" bit). Of
> course, the trivial solution is just to lock the page before flipping
> hint bits, but I don't know (and I doubt) whether it would really work
> at all.

But why can't you wal-log the hint bits from the "buffered" page. then your
consitent. At least as consistent as the original write was.

So you're CRC ends up being:
Buffer the page
Calculate CRC on the buffered page
WAL (in bulk) the hint bits (and maybe CRC?)
write buffered page

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