[PATCH] adding two new options to 'cp'

[Bakul Shah]

> Eric Anderson wrote:
> 
> > It could possibly be bad if you have a real file (say a 10GB file, 
> > partially filled with zeros - a disk image created with dd for 
> > instance), and you use cp with something like -spR to recursively copy 
> > all files.  Your destination disk image would then be a sparse file, so 
> 
> Incidentally, this is exactly why I've needed it - I like to create disk 
> images for virtual machines as sparse files, when I know they won't be 
> much filled, but need the "virtual" space :)

[Basic idea from the plan 9 guys]
Rather than modify every tool for this may be the OS should
avoid writing a block of zeroes?  The idea is to check if the
first and last word are 0.  If so, check the whole block (you
can avoid the necessary bounds check by temporarily unzeroing
the last word).  If all zeroes and there is no existing
allocation for the range being written, just advance the
current offset (essentially lseek).  Else proceed as normal
write.

This test is very cheap in practice and if you can avoid one
write in ten thousand this way you will likely see overall
savings.  Of course, you still need rsync but it help all
local copying, the common case.  This being an optimization
you don't need to implement a complete solution.  For
instance writing 1 zero byte in one call and then 4095 zero
bytes in another may defeat the optimization.