suggest increaseing writeback_rate_minimum by default

[Steven Parker]

If the sys/fs/bcache/$BCACHE_UUID/cache_available_percent falls under 30% the bcache is no longer used to cache disk writes (writeback mode)

This has a significant impact on random write performance. For context I have attached the bug.

https://bugzilla.kernel.org/show_bug.cgi?id=206767

The current minimum is set to 8 I suggest avoiding cache exhaustion by increasing the writeback rate.

sudo 'echo 3840 | sudo tee /sys/block/$bcache/bcache/writeback_rate_minimum'"

This has impacted some of our operational clouds. For systems that have a large amount of write traffic to disk.