Re: [RFC][PATCH] btrfs-progs: inspect: new subcommand to dump chunks

On Thu, Jun 23, 2016 at 12:20:38AM +0200, Hans van Kranenburg wrote:

quoted

Printing 'usage' is not default as it's quite slow, it uses the search ioctl
and probably not in the best way, or there's some other issue in the
implementation.

Interesting.

So after reading this, I wrote a little test to test some scenarios:

https://github.com/knorrie/python-btrfs/commit/1ca99880dfa0e14b148f3d9e2b6b381b781eb52d

It's very clear that the most optimal way of doing this search is to 
have nr_items=1 and if possible, specify the length in offset.

And that solved it.

[...]

It seems that searching in the empty space between
  (vaddr BLOCK_GROUP_ITEM length+1) and
  (vaddr BLOCK_GROUP_ITEM ULLONG_MAX)
is really expensive, while there's absolutely nothing to find.

Yeah, the few BLOCK_GROUP_ITEMs are scattered among tons of EXTENT_ITEMs