checkpoint writeback via sync_file_range

Greg Smith muttered a while ago about wanting to do something with
sync_file_range to improve checkpoint behavior on Linux. I thought he
was talking about trying to sync only the range of blocks known to be
dirty, which didn't seem like a very exciting idea, but after looking
at the man page for sync_file_range, I think I understand what he was
really going for: sync_file_range allows you to hint the Linux kernel
that you'd like it to clean a certain set of pages. I further recall
from Greg's previous comments that in the scenarios he's seen,
checkpoint I/O spikes are caused not so much by the data written out
by the checkpoint itself but from the other dirty data in the kernel
buffer cache. Based on that, I whipped up the attached patch, which,
if sync_file_range is available, simply iterates through everything
that will eventually be fsync'd before beginning the write phase and
tells the Linux kernel to put them all under write-out.

I don't know that I have a suitable place to test this, and I'm not
quite sure what a good test setup would look like either, so while
I've tested that this appears to issue the right kernel calls, I am
not sure whether it actually fixes the problem case. But here's the
patch, anyway.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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