Re: Block-level CRC checks

* Alvaro Herrera <alvherre@commandprompt.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
maybeCRC?)  write buffered page
 

-- 
aidan van dyk                                             create like a god,
aidan@highrise.ca                                       command like a king,
http://www.highrise.ca/                                   work like a slave.