doc/cephfs: edit front matter in mantle.rst

Improve the structure and grammar of the front matter in the
doc/cephfs/mantle.rst file.

Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

doc/cephfs/mantle.rst

@@ -6,20 +6,25 @@ Mantle
     Mantle is for research and development of metadata balancer algorithms,
     not for use on production CephFS clusters.
 
-Multiple, active MDSs can migrate directories to balance metadata load. The
-policies for when, where, and how much to migrate are hard-coded into the
-metadata balancing module. Mantle is a programmable metadata balancer built
-into the MDS. The idea is to protect the mechanisms for balancing load
-(migration, replication, fragmentation) but stub out the balancing policies
-using Lua. Mantle is based on [1] but the current implementation does *NOT*
-have the following features from that paper:
+Mantle is a programmable metadata balancer that is built into the MDS.
+
+By default (without Mantle), multiple, active MDSs can migrate directories to
+balance metadata load. The policies for when, where, and how much to migrate
+are hard-coded into the metadata balancing module. 
+
+Mantle works by protecting the mechanisms for balancing load (migration,
+replication, fragmentation) while suppressing the balancing policies using Lua.
+Mantle is based on [1] but the current implementation does *NOT* have the
+following features from that paper:
 
 1. Balancing API: in the paper, the user fills in when, where, how much, and
-   load calculation policies; currently, Mantle only requires that Lua policies
-   return a table of target loads (e.g., how much load to send to each MDS)
-2. "How much" hook: in the paper, there was a hook that let the user control
-   the fragment selector policy; currently, Mantle does not have this hook
-3. Instantaneous CPU utilization as a metric
+   load calculation policies. Currently, Mantle requires only that Lua policies
+   return a table of target loads (for example, how much load to send to each
+   MDS)
+2. The "how much" hook: in the paper, there was a hook that allowed the user to
+   control the "fragment selector policy". Currently, Mantle does not have this
+   hook.
+3. "Instantaneous CPU utilization" as a metric.
 
 [1] Supercomputing '15 Paper:
 http://sc15.supercomputing.org/schedule/event_detail-evid=pap168.html