VACUUM delay (was Re: What's planned for 7.5?)

Christopher Browne <cbbrowne@libertyrms.info> writes:
> "Stephen" <jleelim@xxxxxxx.com> writes:
>> Any chance we'll see the VACUUM delay patch (throttle) get into 7.5?

> The hope, in 7.5, is to have ARC, which is the "super-duper-duper"
> version, working.

Actually, I'm not sure that ARC should be considered to supersede the
usefulness of a per-page delay in VACUUM.  ARC should prevent VACUUM
from trashing the contents of Postgres' shared buffer arena, but it
won't do much of anything to prevent VACUUM from trashing the kernel
buffer contents.  And it definitely won't do anything to help if the
real problem is that you're short of disk bandwidth and VACUUM's extra
I/O demand pushes your total load over the knee of the response-time
curve.  What you need then is a throttle.

The original patch I posted was incomplete for a number of reasons,
but I think it may still be worth working on.  Jan, any comments?
        regards, tom lane