Re: crash-safe visibility map, take three

Heikki Linnakangas wrote:
> On 01.12.2010 15:39, Bruce Momjian wrote:
> > Heikki Linnakangas wrote:
> >> On 01.12.2010 03:35, Bruce Momjian wrote:
> >>> Heikki Linnakangas wrote:
> >>>> Let's recap what happens when a VM bit is set: You set the
> >>>> PD_ALL_VISIBLE flag on the heap page (assuming it's not set already, it
> >>>> usually isn't), and then set the bit in the VM while keeping the heap
> >>>> page locked.
> >>>
> >>> What if we set PD_ALL_VISIBLE on the heap page, wait for a checkpoint to
> >>> happen so the heap page is guaranteed to be on disk, then on next read,
> >>> if PD_ALL_VISIBLE is set and the VM all-visible bit is not set, set the
> >>> VM bit.
> >>
> >> Hmm, you'd somehow have to know if a checkpoint has happened since the
> >> flag was set. And it might be a long wait, which makes it less
> >
> > Well, doesn't the page LSN tell you that already?
> 
> If we update the LSN when we set the flag, then we have to write the 
> full-page-image. That's very expensive. If we don't update the LSN, then 
> the LSN says nothing about when the flag was set.

Oh, we don't update the LSN when we set the PD_ALL_VISIBLE flag?  OK,
please let me think some more.  Thanks.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +