I wrote:
> I fooled around with this idea and came up with the attached patch.
> It seems to do what's intended but could do with more eyeballs and
> testing before committing.  Comments please?

Earlier I said that I didn't want to back-patch this change, but on
looking at the CVS history I'm reconsidering.  The performance problem
originates from the decision some time ago to do an AbsorbFsyncRequests
every so often during the mdsync loop; without that, and assuming no
actual failures, there isn't any absorption of new requests before
mdsync can complete.  Originally that code only existed in 8.2.x, but
very recently we back-patched it into 8.1.x as part of fixing the
file-deletion-on-Windows problem.  This means that 8.1.x users could
see a performance degradation upon updating to 8.1.8 from prior
subreleases, which wouldn't make them happy.

So I'm now thinking we ought to back-patch into 8.2.x and 8.1.x,
but of course that makes it even more urgent that we test the patch

                        regards, tom lane

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