On 2019-09-09 11:29 a.m., Graham Cobb wrote:

>  and does anyone really care about
> defrag any more?).
> 


Err, yes, yes absolutely.

I don't have any issues with the current btrfs defrag implementions, but
it's *vital* for btrfs. (which works just as the OP requested, as far as
I can tell, recursively for a subvolume)

Just booting Windows on a BTRFS virtual image, for example, will create
almost 20,000 file fragments.  Even on SSD's, you get into problems
trying to work with files that are over 200,000 fragments.

Another huge problem is rsync --inplace.  which is perfect backup
solution to take advantage of BTRFS snapshots, but fragments larges
files into tiny pieces (and subsequently creates files that are very
slow to read.).. for some reason, autodefrag doesn't catch that one either.

But the wiki could do a beter job of trying to explain that the snapshot
duplication of defrag only affects the fragmented portions.  As I
understand, it's really only a problem when using defrag to change
compression.