Find primary storage

Adding a section describing how CloudStack finds suitable primary storage for volume migration.

source/adminguide/storage.rst

@@ -636,6 +636,15 @@ be restarted.
       Instance's root disk is allowed from one PowerFlex/ScaleIO storage pool
       to another, without stopping the Instance.
 
+Finding Primary Storage for Migration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When you click on migrate volume, CloudStack lists the available primary storage.  CloudStack uses its storage pool allocators to identify the primary storages that are available and returns a list that is suitable for the selected volume's migration. 
+The list also could include primary storages that are mentioned as 'Not suitable'. The criteria for which the primary storages are not suitable are: 
+	- Storage tag mismatch with the volume.
+	- Doesn't have enough capacity. 
+	- Reached its disable threshold
+	- Disabled. 
+Mismatch in the type of storage such as shared /Local. 
 
 Resizing Volumes
 ~~~~~~~~~~~~~~~~