Re: autovacuum not prioritising for-wraparound tables

Christopher Browne <cbbrowne@gmail.com> writes:
> On Thu, Jan 24, 2013 at 5:22 PM, Heikki Linnakangas
> <hlinnakangas@vmware.com> wrote:
>> Backpatching sounds a bit scary. It's not a clear-cut bug, it's just that
>> autovacuum could be smarter about its priorities. There are other ways you
>> can still bump into the xid-wraparound issue, even with this patch.

> I don't think this is a single-priority issue.  It's *also* crucial
> that small tables with high "tuple attrition rates" get vacuumed
> extremely frequently; your system will bog down, albeit in a different
> way, if the small tables don't get vacuumed enough.

Yeah.  Another problem with a simple-minded priority arrangement is that
it might cause some tables to get starved for service because workers
keep on choosing other ones; we have to be sure the sorting rule is
designed to prevent that.

As posted, what we've got here is sorting on a boolean condition, with
the behavior within each group totally up to the whims of qsort().  That
seems especially dangerous since the priority order is mostly undefined.

I was a bit surprised that Alvaro didn't propose sorting by the age of
relfrozenxid, at least for the subset of tables that are considered
wraparound hazards.  Not sure what a good criterion is for the rest.
        regards, tom lane