Re: Logical to physical page mapping

On Mon, Oct 29, 2012 at 07:05:39AM -0400, Robert Haas wrote:
> Yet another idea we've tossed around is to make only vacuum records
> include FPWs, and have the more common heap insert/update/delete
> operations include enough information that they can still be applied
> correctly even if the page has been "torn" by the previous replay of
> such a record.  This would involve modifying the recovery algorithm so
> that, until consistency is reached, we replay all records, regardless
> of LSN, which would cost some extra I/O, but maybe not too much to
> live with?  It would also require that, for example, a heap-insert
> record mention the line pointer index used for the insertion;
> currently, we count on the previous state of the page to tell us that.
>  For checkpoint cycles of reasonable length, the cost of storing the
> line pointer in every WAL record seems like it'll be less than the
> cost needing to write an FPI for the page once per checkpoint cycle,
> but this isn't certain to be the case for all workloads.

This last idea has the most promise for me.  Vacuum is far less common
than row modification writes.

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