Snapshots: talk a bit more about space requirements

Signed-off-by: David Scott <[email protected]>

Author: David Scott

features/snapshots/snapshots.md

@@ -121,23 +121,26 @@ run a garbage collector process.
 
 The garbage collector will first determine whether a "coalesce" should happen i.e.
 whether any parent nodes have only one child i.e. the "shared" blocks are only
-shared with one other node. In the following example the snapshot delete leaves
+"shared" with one other node. In the following example the snapshot delete leaves
 such a parent node and the coalesce process copies blocks from the redundant
 parent's only child into the parent:
 
 ![We coalesce parent blocks into grand parent nodes](coalesce1.png)
 
+Note that if the vhd data is being stored in LVM, then the parent node will
+have had to be expanded to full size to accommodate the writes. Unfortunately
+this means the act of reclaiming space actually consumes space itself, which
+means it is important to never completely run out of space in such an SR.
+
 Once the blocks have been copied, we can now cut one of the parents out of the
 tree by relinking its children into their grandparent:
 
 ![Relink children into grand parent](coalesce2.png)
 
-Finally the garbage collector can remove unused leftovers:
+Finally the garbage collector can remove unused vhd files / LVM LVs:
 
 ![Clean up](coalesce3.png)
 
-TODO: space requirements
-
 Reverting VM snapshots
 ======================