Re: file cloning in pg_upgrade and CREATE DATABASE

I think we have raised a number of interesting issues here which require
more deeper consideration.  So I suggest to set this patch to Returned
with feedback.

Btw., I just learned that copy_file_range() only works on files on the
same device.  So more arrangements will need to be made for that.

> I have run some more tests on both macOS and Linux with ext4, and my> results are that the bigger the flush distance,
thebetter.  Before
 
we> made the adjustments for APFS, we had a flush size of 64kB, now it's
1MB> and 32MB on macOS.  In my tests, I see 256MB as the best across
both> platforms, and not flushing early at all is only minimally worse.
Based on this, I suggest that we set the flush distance to 32MB on all
platforms.  Not only is it faster, it avoids having different settings
on some platforms.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services