Re: Reviewing freeze map code

On Tue, Jun 21, 2016 at 9:08 AM, Thomas Munro <thomas.munro@enterprisedb.com> wrote:
>
> On Tue, Jun 21, 2016 at 3:29 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> > On Tue, Jun 21, 2016 at 1:03 AM, Andres Freund <andres@anarazel.de> wrote:
> >> Well, I think generally nobody seriously looked at actually refactoring
> >> heap_update(), even though that'd be a good idea.  But in this instance,
> >> the problem seems relatively fundamental:
> >>
> >> We need to lock the origin page, to do visibility checks, etc. Then we
> >> need to figure out the target page. Even disregarding toasting - which
> >> we could be doing earlier with some refactoring - we're going to have to
> >> release the page level lock, to lock them in ascending order. Otherwise
> >> we'll risk kinda likely deadlocks.
> >
> > Can we consider to use some strategy to avoid deadlocks without releasing
> > the lock on old page?  Consider if we could have a mechanism such that
> > RelationGetBufferForTuple() will ensure that it always returns a new buffer
> > which has targetblock greater than the old block (on which we already held a
> > lock).  I think here tricky part is whether we can get anything like that
> > from FSM. Also, there could be cases where we need to extend the heap when
> > there were pages in heap with space available, but we have ignored them
> > because there block number is smaller than the block number on which we have
> > lock.
>
> Doesn't that mean that over time, given a workload that does only or
> mostly updates, your records tend to migrate further and further away
> from the start of the file, leaving a growing unusable space at the
> beginning, until you eventually need to CLUSTER/VACUUM FULL?