On 10/19/2016 09:59 AM, Bruce Momjian wrote:
> On Wed, Oct 19, 2016 at 09:00:06AM -0400, Robert Haas wrote:
>> On Wed, Oct 19, 2016 at 8:47 AM, Bruce Momjian <br...@momjian.us> wrote:
>>> On Wed, Oct 19, 2016 at 08:33:20AM -0400, Robert Haas wrote:

>>>> Actually, I think vacuum_defer_cleanup_age is, and always has been, an
>>>> ugly hack.  But for some people it may be the ugly hack that is
>>>> letting them continue to use PostgreSQL.
>>> I see vacuum_defer_cleanup_age as old_snapshot_threshold for standby
>>> servers --- it cancels transactions rather than delaying cleanup.
>> I think it's the opposite, isn't it?  vacuum_defer_cleanup_age
>> prevents cancellations.
> Uh, vacuum_defer_cleanup_age sets an upper limit on how long, in terms
> of xids, that a standby query can run before cancel, like
> old_snapshot_threshold, no?  After that, we can cancel standby queries. 
> I see hot_standby_feedback as our current behavior on the master where
> we never cancel standby queries.
> To me, hot_standby_feedback extends no-cleanup-no-cancel from the
> standby to the master, while vacuum_defer_cleanup_age behaves like
> old_snapshot_threshold in that it causes cancel for long-running
> queries.

See Andres' response on this thread.  He's already covered why the
setting is still useful, but why we might want to remove it anyway.

Josh Berkus
Red Hat OSAS
(any opinions are my own)

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