Slow writing of dirty data (bcache)

[Larkin Lowrey <llowrey@nuclearwinter.com>]

I'm seeing an odd behavior after updating to kernel
3.13.5-202.fc20.x86_64 from 3.12.7-300.fc20.x86_64.

I have a bcache device (bcache2) which backs a raid5 array (md2) that
has 160+MB of dirty data. The dirty data is being written at a rate of 1
page per second (8 sectors). At this rate it'll take 11 hrs to finish. I
like to keep this array spun down when inactive but the drives can't
spin down since they're being written to once a second.

# cat /sys/block/bcache2/bcache/writeback_rate
512

# cat /sys/block/bcache2/bcache/writeback_rate_debug
rate:           512/sec
dirty:          161M
target:         40.8G
proportional:   -35.2M
derivative:     0
change:         -35.2M/sec
next io:        624ms

This never happened when running 3.12.7. Is there anything I can do
about this?

--Larkin