bug#6131: [PATCH]: fiemap support for efficient sparse file copy

[Paul Eggert]

I looked at the proposed fiemap patch for cp and see another issue
with it, which needs to be thought through more carefully.

The proposed fiemap code tries to copy the _physical_ holes in the
source file.  But currently cp copies the _logical_ holes: that is, it
skips over writing (and thus attempts to create physical holes)
whenever the source file contains a block of zeros, regardless of
whether that block of zeros is physically stored in the file system.
The current cp behavior is often the desired behavior (particularly
for cp --sparse=always) and the fiemap code shouldn't alter that.

For example, if a fiemap_extent has the FIEMAP_EXTENT_UNWRITTEN flag
set, cp should treat that as a hole, because the extent is all zeros.
(This will greatly help performance in some cases.)  Also, if an input
extent is read and a block of it is found to be zeros, cp should skip
over that block when writing.

If the proposed fiemap code was intended to copy the physical holes,
then the code wasn't successful in implementing the intent, as there's
no guarantee that the extents of the destination are the same as the
extents of the source.  If it is useful to have 'cp' copy the physical
holes, this could be supported via a new cp option or options, but
surely the default should continue to be to copy logical holes.