On 02/13/18 12:40, Qu Wenruo wrote:
>>> The problem is not about how much space it takes, but how many extents
>>> are here in the filesystem.

I have no idea why btrfs' mount even needs to touch all block groups to
get going (which seems to be the root of the problem), but here's a
not so crazy idea for more "mechanical sympathy". Feel free to mock
me if this is terribly wrong or not possible. ;)

Mounting of even large filesystems (with many extents) seems to be fine
on SSDS, but not so fine on rotational storage. We've heard that from
several people with large (multi-TB) filesystems, and obviously it's
even more terrible on 5400RPM drives because their seeks are sooo sloow.

If the problem is that the bgs are touched/iterated in "tree order",
would it then not be possible to sort the block groups in physical order
before trying to load whatever mount needs to load? That way the entire
process would involve less seeking (no backward seeks for one) and the
drive could very likely get more done during a rotation before stepping
further.

cheers,
Holger