Re: POC: Cleaning up orphaned files using undo logs

On Wed, Jul 17, 2019 at 3:53 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On 2019-07-15 12:26:21 -0400, Robert Haas wrote:
> > Yeah. I didn't understand that explanation. It seems to me that one
> > of the fundamental design questions for this system is whether we
> > should allow there to be an unbounded number of transactions that are
> > pending undo application, or whether it's OK to enforce a hard limit.
> > Either way, there should certainly be pressure applied to try to keep
> > the number low, like forcing undo application into the foreground when
> > a backlog is accumulating, but the question is what to do when that's
> > insufficient. My original idea was that we should not have a hard
> > limit, in which case the shared memory data on what is pending might
> > be incomplete, in which case we would need the discard workers to
> > discover transactions needing undo and add them to the shared memory
> > data structures, and if those structures are full, then we'd just skip
> > adding those details and rediscover those transactions again at some
> > future point.
> >
> > But, my understanding of the current design being implemented is that
> > there is a hard limit on the number of transactions that can be
> > pending undo and the in-memory data structures are sized accordingly.
>
> My understanding is that that's really just an outcome of needing to
> maintain oldestXidHavingUndo accurately, right?
>

Yes.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com