[Doc]Adding doc for volds (gluster#294)

The volume data structure required some special
treatment, given its importance across the test runs.
Hence a detailed explanation for what, why and how it is.

Signed-off-by: srijan-sivakumar <[email protected]>

docs/RP/Common/volds.md

@@ -0,0 +1,86 @@
+# Volume data structure (volds)
+
+What prompted the creation of a data structure to store volume related
+data in a test case run ?
+To answer this in simpler terms, we needed a way to handle cleanups, addition,
+removal, replacement of bricks, accessing mountpoints, volume options, etc.
+Now, we can either query glusterfs everytime we require such operation but
+wait...a test case should have a clearly defined operations which it is
+supposed to perform and if we were to bring the collection of these data
+whenever we need to perform any of the aforementioned actions, that'd mean
+we are performing more gluster related operations.
+Even if we were to ignore this, we still have to address the part wherein the
+gluster related operations are performed remotely. This implies a some delay,
+infinitesimal ( Not really..certain operations take time to be performed by 
+gluster and then the result to be handed back to the shell env created by Rexe)
+which we wouldn't want to add to out cases.
+
+Also, we would have to create separate methods for handling the parsing to get
+exactly what we require, etc...etc.
+
+The clean way out of this was to have one single data structure. This data
+structure will be instantiated whenever a new test case run starts. Hence
+every test case run has its own volds ( from hereon, the data structure will be
+referred to as volds ).
+
+## Where is volds created.
+
+volds is instantiated when a mixin mixn object is created by a test case run.
+If anyone wants to take a peek at how it looks like -> [Mixin](../../../common/mixin.py)
+
+## Structure of volds.
+
+Let's look at the set of requirements to better understand why volds is the way
+it is.
+
+1. Every volume created by a test case needs to be tracked, hence information
+about that volume must be part of the volds. Let's say the name of the volume.
+2. What is the type of volume ? This can be discerned by certain parameters
+such as replication count, arbiter count, etc. 
+3. Building on pt. 1, the second level of information we'd require is which
+all servers were used to create this volume and also the bricks coming under
+this volume.
+4. The next level information is about the mountpoints. Now, there might be
+multiple clients, and even in a single client, one can mount at multiple paths,
+ hence clients and mountpoints under them.
+5. We would also require information regarding the volume options which are
+set by the test case ( do we ? If you feel there is a better way, let's discuss
+in github discussions :D )
+
+With all these information, one can derive at a guess that volds would be a
+dictionary and the approximate form similar to what the actual format is..
+
+```javascript
+{
+  "volname" : {
+                "voltype" : {
+                              "replica_count" : q,
+                              "dist_count" : r,
+                              "arbiter_count" : x,
+                              "disperse_count" : y,
+                              "redundancy_count" : z
+                            },
+                "options" : {
+                              "option-1" : "value",
+                              "option-2" : "value",
+                              ...
+                              "option-n" : "value"
+                            },
+                "brickdata" : {
+                                "node-1" : ["brickpath1", "brickpath2"],
+                                "node-2" : ["brickpath3", "brickpath4"],
+                                ...
+                                "node-n" : ["brickpathn"]
+                              },
+                "mountpath" : {
+                                "node-1" : ["mnt-path1", "mnt-path2"],
+                                ...
+                                "node-n" : ["mnt-pathn"]
+                              },
+                "started" : True/False
+              }
+}
+```
+
+## Methods to get and set data related to volds
+TODO: Will be updated once the methods are merged.