1 files changed, 15 insertions, 28 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-bdi b/Documentation/ABI/testing/sysfs-class-bdi
index c55e811ca180..5ac1e01bbd48 100644
--- a/Documentation/ABI/testing/sysfs-class-bdi
+++ b/Documentation/ABI/testing/sysfs-class-bdi
@@ -3,8 +3,8 @@ 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.
+Provide a place in sysfs for the backing_dev_info object.  This allows
+setting and retrieving various BDI specific variables.
 
 The <bdi> identifier can be either of the following:
 
@@ -26,34 +26,21 @@ 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
+        Under normal circumstances each device is given a part of the
+        total write-back cache that relates to its current average
+        writeout speed in relation to the other devices.
+
+        The 'min_ratio' parameter allows assigning a minimum
+        percentage of the write-back cache to a particular device.
+        For example, this is useful for providing a minimum QoS.
 
 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
+        Allows limiting a particular device to use not more than the
+        given percentage of the write-back cache.  This is useful in
+        situations where we want to avoid one device taking all or
+        most of the write-back cache.  For example in case of an NFS
+        mount that is prone to get stuck, or a FUSE mount which cannot
+        be trusted to play fair.