quoted

Hmm.

If it's related to lvm snapshots, there's a workaround I just added to
the
bcache-3.2 branch - I'd give that a try, looks plausible from your log.

Is that the default writeback_percent = 10 in commit
113740372e2244951fb34235b30a2ac646a5d8c2?

Can I effect the same change by "echo 10 >writeback_percent"? If so then it
would save the hassle of a rebuild just to test. I assume it's sleep time in your
timezone at the moment so I'll test it anyway and let you know - it was failing
pretty consistently, within 30 seconds of starting my restore job.

Setting writeback_percent=10 manually resulted in no change - crash still occurred within a few minutes of the restore job starting.

Deleting the snapshot and running on the original LV resulted in no problems, so it's definitely snapshot related.

James