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Re: crash-safe visibility map, take three

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: crash-safe visibility map, take three
Date: 2010-12-01 01:35:56
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Heikki Linnakangas wrote:
> On 30.11.2010 18:33, Tom Lane wrote:
> > Robert Haas<robertmhaas(at)gmail(dot)com>  writes:
> >> Oh, but it's worse than that.  When you XLOG a WAL record for each of
> >> those pages, you're going to trigger full-page writes for all of them.
> >>   So now you've turned 1GB of data to write into 2+ GB of data to
> >> write.
> >
> > No, because only the first mod of each VM page would trigger a full page
> > write, at least assuming a reasonable ordering of the operations.
> If you change the LSN on the heap pages, you have to write full page 
> images of those as well.
> 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.

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

  + It's impossible for everything to be true. +

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