Re: Resumable vacuum proposal and design overview

Tom Lane wrote:
> "Simon Riggs" <simon@2ndquadrant.com> writes:
>> On Tue, 2007-02-27 at 10:37 -0600, Jim C. Nasby wrote:
>>> ... The idea would be to give vacuum a target run time, and it
>>> would monitor how much time it had remaining, taking into account how
>>> long it should take to scan the indexes based on how long it's been
>>> taking to scan the heap. When the amount of time left becomes less than
>>> the estimate of the amount of time required to scan the indexes (and
>>> clean the heap), you stop the heap scan and start scanning indexes.
> 
>> I do like this idea, but it also seems easy to calculate that bit
>> yourself. Run VACUUM, after X minutes issue stop_vacuum() and see how
>> long it takes to finish. Adjust X until you have it right.
> 
> One problem with it is that a too-small target would result in vacuum
> proceeding to scan indexes after having accumulated only a few dead
> tuples, resulting in increases (potentially enormous ones) in the total
> work needed to vacuum the table completely.
> 
> I think it's sufficient to have two cases: abort now, and restart from
> the last cycle-completion point next time (this would basically just be
> SIGINT); or set a flag to stop at the next cycle-completion point.
> 
> 
> It occurs to me that we may be thinking about this the wrong way
> entirely.  Perhaps a more useful answer to the problem of using a
> defined maintenance window is to allow VACUUM to respond to changes in
> the vacuum cost delay settings on-the-fly.  So when your window closes,
> you don't abandon your work so far, you just throttle your I/O rate back
> to whatever's considered acceptable for daytime vacuuming.

I thought we already did that?  Which BTW was part of my plan on how to 
deal with a vacuum that is still running after it's maintenance window 
has expired.