doc/rados: Add explanation of straw2

Signed-off-by: Shinobu Kinjo <shinobu@redhat.com>

diff --git a/doc/rados/operations/crush-map-edits.rst b/doc/rados/operations/crush-map-edits.rst
index 6a2094277fb653dffaad6e223cb1fa5897cbb30d..2addc59289a90b736407767b44546e782d7168b4 100644 (file)
--- a/doc/rados/operations/crush-map-edits.rst
+++ b/doc/rados/operations/crush-map-edits.rst
@@ -279,7 +279,7 @@ and one rack bucket. The OSDs are declared as items within the host buckets::
    `CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data`_,
    and more specifically to **Section 3.4**. The bucket types are:
 
-       #. **Uniform:** Uniform buckets aggregate devices with **exactly** the same
+       #. **Uniform**: Uniform buckets aggregate devices with **exactly** the same
           weight. For example, when firms commission or decommission hardware, they
           typically do so with many machines that have exactly the same physical
           configuration (e.g., bulk purchases). When storage devices have exactly
@@ -303,7 +303,7 @@ and one rack bucket. The OSDs are declared as items within the host buckets::
           tree buckets reduce the placement time to O(log :sub:`n`), making them
           suitable for managing much larger sets of devices or nested buckets.
 
-       #. **Straw:** List and Tree buckets use a divide and conquer strategy
+       #. **Straw**: List and Tree buckets use a divide and conquer strategy
           in a way that either gives certain items precedence (e.g., those
           at the beginning of a list) or obviates the need to consider entire
           subtrees of items at all. That improves the performance of the replica
@@ -313,6 +313,12 @@ and one rack bucket. The OSDs are declared as items within the host buckets::
           fairly “compete” against each other for replica placement through a
           process analogous to a draw of straws.
 
+        #. **Straw2**: Straw2 buckets improve Straw to correctly avoid any data 
+           movement between items when neighbor weights change.
+
+           For example the weight of item A including adding it anew or removing
+           it completely, there will be data movement only to or from item A.
+
 .. topic:: Hash
 
    Each bucket uses a hash algorithm. Currently, Ceph supports ``rjenkins1``.