Re: BTRFS file clone support for cp

[Pádraig Brady]

Chris Mason wrote:
> On Tue, Jul 28, 2009 at 10:06:35PM +0200, Giuseppe Scrivano wrote:
>>
>> I can't replicate it now, all tests I am doing report that blocks used
>> before and after the clone are the same.  Probably yesterday the
>> difference I noticed was in reality the original file flushed to the
>> disk.
> 
> The clone will use some additional space for the metadata required to
> point to the cloned blocks.  It isn't exactly O(1) it is O(metadata for
> the file).

Thanks for the clarification Chris.
So the just committed change in cp will
link the destination file to the extents of the source.

We may need to play around with fallocate()
if we want to get back to the original
cp semantics of actually allocating space
on the file system for the new file.

I'll test this when I get an up to date btrfs
and when the fallocate interface in glibc settles down.

cheers,
Pádraig.