documentation: explain ceph osd reweight vs crush weight

Using the wording from Gregory Farnum at
http://lists.ceph.com/pipermail/ceph-users-ceph.com/2014-June/040961.html

Signed-off-by: Loic Dachary <loic-201408@dachary.org>

diff --git a/doc/rados/operations/control.rst b/doc/rados/operations/control.rst
index f3d7b7c886c883a78d4fc5dfbf4f2bff060adbda..0e7237e8fc7b86940edab6e9828f24288504bdc6 100644 (file)
--- a/doc/rados/operations/control.rst
+++ b/doc/rados/operations/control.rst
@@ -203,9 +203,16 @@ resending pending requests. ::
        ceph osd pause
        ceph osd unpause
 
-Set the weight of ``{osd-num}`` to ``{weight}``. Two OSDs with the same weight will receive
-roughly the same number of I/O requests and store approximately the
-same amount of data. ::
+Set the weight of ``{osd-num}`` to ``{weight}``. Two OSDs with the
+same weight will receive roughly the same number of I/O requests and
+store approximately the same amount of data. ``ceph osd reweight``
+sets an override weight on the OSD. This value is in the range 0 to 1,
+and forces CRUSH to re-place (1-weight) of the data that would
+otherwise live on this drive. It does not change the weights assigned
+to the buckets above the OSD in the crush map, and is a corrective
+measure in case the normal CRUSH distribution isn't working out quite
+right. For instance, if one of your OSDs is at 90% and the others are
+at 50%, you could reduce this weight to try and compensate for it. ::
 
        ceph osd reweight {osd-num} {weight}