What:		/sys/class/bdi/<bdi>/
Date:		January 2008
Contact:	Peter Zijlstra <a.p.zijlstra@chello.nl>
Description:

Provide a place in sysfs for the backing_dev_info object.
This allows us to see and set the various BDI specific variables.

The <bdi> identifier can be either of the following:

MAJOR:MINOR

	Device number for block devices, or value of st_dev on
	non-block filesystems which provide their own BDI, such as NFS
	and FUSE.

default

	The default backing dev, used for non-block device backed
	filesystems which do not provide their own BDI.

Files under /sys/class/bdi/<bdi>/
---------------------------------

read_ahead_kb (read-write)

	Size of the read-ahead window in kilobytes

reclaimable_kb (read-only)

	Reclaimable (dirty or unstable) memory destined for writeback
	to this device

writeback_kb (read-only)

	Memory currently under writeback to this device

dirty_kb (read-only)

	Global threshold for reclaimable + writeback memory

bdi_dirty_kb (read-only)

	Current threshold on this BDI for reclaimable + writeback
	memory

min_ratio (read-write)

	Minimal percentage of global dirty threshold allocated to this
	bdi.  If the value written to this file would make the the sum
	of all min_ratio values exceed 100, then EINVAL is returned.
	If min_ratio would become larger than the current max_ratio,
	then also EINVAL is returned.  The default is zero

max_ratio (read-write)

	Maximal percentage of global dirty threshold allocated to this
	bdi.  If max_ratio would become smaller than the current
	min_ratio, then EINVAL is returned.  The default is 100