On Sat, Oct 27, 2012 at 06:28:41PM +0800, Liu Bo wrote:

This feature works on our crucial write endio path, so if we've got
lots of fragments to process, it will be kind of a disaster to the
performance, so I make such a change.

One can benifit from it while mounting with '-o snap_aware_defrag'.

I vote for a more fine grained control over this feature, ie.  via
'btrfs fi defrag', off by default (current behaviour). The defrag ioctl
is the only place that actually calls set_extent_defrag, so this will
not affect normal operation and is fully in hands of the user who runs
defrag.

Do you have a usecase for setting it through the mount option?

thanks,
david