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.

James