Re: Remove vacuum_defer_cleanup_age

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 <bruce@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)