Stephen R. van den Berg wrote:

d. Came back in a patched kernel, 100% CPU usage is gone.

I spoke too soon.  After some more copying, the 100% CPU is back, but
different:
Before I had four or five HDDs, but in both cases I had four bcache_writebac
processes running at 100%.
Now I have four HDDs, and also four bcache_writebac processes, but
only three of those run at 100% CPU.  The fourth one is idle (mostly).
-- 
Stephen.