[Btrfs-devel] cloning file data

[Sage Weil]

On Fri, 25 Apr 2008, Zach Brown wrote:
> We still have the same inode, and it has the same size, but its extent
> items look very different.  The extent for the first 64k looks much the
> same.  It references the old 64m extent on disk.  But see the 'nr
> 65536', it only maps 64k of that 64m into the file.  Then we have the 4k
> extent that we just wrote.  Then we have another reference to that 64m
> extent but for the remaining data after the new 4k.

Is there anything in the defragger (or whatever) that looks for minimally 
referenced extents?  Once can imagine a situation where only a small piece 
of a large extent is remains referenced, but that information is buried in 
the forward reference(s).

> debug-tree is fantastic, but it can be kind of intimidating if you don't
> already know what all the numbers mean :).  Reducing the barrier to

Yep.  Although in my case, the biggest stumbling block was realizing that 
the key type

> 	item 3 key (258 12 0) itemoff 2652 itemsize 41
                        ^^
is in printed in hex for some reason.  Der.

sage