On Tue, Aug 1, 2017 at 2:43 AM, Leonidas Spyropoulos
[off-list ref] wrote:

Hi Duncan,

Thanks for your answer

In general I think btrfs takes time proportional to the size of your
metadata to mount. Bigger and/or fragmented metadata leads to longer
mount times. My big backup fs with >300GB of metadata takes over
20minutes to mount, and that's with the space tree which is
significantly faster then space cache v1.

quoted

If a device takes too long and times out you'll see resets and the like
in dmesg, but that normally starts at ~30 seconds, not the 5 seconds you
mention.  Still, doesn't hurt to check.