md-cluster: Design Documentation
Add number of nodes to bitmap structure for clustering
Create a separate module for clustering support
DLM lock and unlock functions

A dlm_lock_resource is a structure which contains all information
required for locking using DLM. The init function allocates the
lock and acquires the lock in NL mode. The unlock function
converts the lock resource to NL mode. This is done to preserve
LVB and for faster processing of locks. The lock resource is
DLM unlocked only in the lockres_free function, which is the end
of life of the lock resource.
Introduce md_cluster_operations to handle cluster functions

This allows dynamic registering of cluster hooks.
Introduce md_cluster_info

md_cluster_info stores the cluster information in the MD device.

The join() is called when mddev detects it is a clustered device.
The main responsibilities are:
	1. Setup a DLM lockspace
	2. Setup all initial locks such as super block locks and bitmap lock (will come later)

The leave() clears up the lockspace and all the locks held.
Return MD_SB_CLUSTERED if mddev is clustered
Add node recovery callbacks

DLM offers callbacks when a node fails and the lock remastery
is performed:

1. recover_prep: called when DLM discovers a node is down
2. recover_slot: called when DLM identifies the node and recovery
		can start
3. recover_done: called when all nodes have completed recover_slot

recover_slot() and recover_done() are also called when the node joins
initially in order to inform the node with its slot number. These slot
numbers start from one, so we deduct one to make it start with zero
which the cluster-md code uses.
Use separate bitmaps for each nodes in the cluster

On-disk format:

0                    4k                     8k                    12k
-------------------------------------------------------------------
| idle                | md super            | bm super [0] + bits |
| bm bits[0, contd]   | bm super[1] + bits  | bm bits[1, contd]   |
| bm super[2] + bits  | bm bits [2, contd]  | bm super[3] + bits  |
| bm bits [3, contd]  |                     |                     |

Bitmap super has a field nodes, which defines the maximum number
of nodes the device can use. While reading the bitmap super, if
the cluster finds out that the number of nodes is > 0:
1. Requests the md-cluster module.
2. Calls md_cluster_ops->join(), which sets up clustering such as
   joining DLM lockspace.

Since the first time, the first bitmap is read. After the call
to the cluster_setup, the bitmap offset is adjusted and the
superblock is re-read. This also ensures the bitmap is read
the bitmap lock (when bitmap lock is introduced in later patches)

Questions:
1. cluster name is repeated in all bitmap supers. Is that okay?
Lock bitmap while joining the cluster
Gather on-going resync information of other nodes

When a node joins, it does not know of other nodes performing resync.
So, each node keeps the resync information in it's LVB. When a new
node joins, it reads the LVB of each "online" bitmap.

[TODO] The new node attempts to get the PW lock on other bitmap, if
it is successful, it reads the bitmap and performs the resync (if
required) on it's behalf.

If the node does not get the PW, it requests CR and reads the LVB
for the resync information.
bitmap_create returns bitmap pointer

This is done to have multiple bitmaps open at the same time.
Copy set bits from another slot

bitmap_copy_from_slot reads the bitmap from the slot mentioned.
It then copies the set bits to the node local bitmap.

This is helper function for the resync operation on node failure.

bitmap_set_memory_bits() currently assumes it is only run at startup and that
they bitmap is currently empty.  So if it finds that a region is already
marked as dirty, it won't mark it dirty again. Change bitmap_set_memory_bits()
to always set the NEEDED_MASK bit if 'needed' is set.
Initiate recovery on node failure

The DLM informs us in case of node failure with the DLM slot number.
cluster_info->recovery_map sets the bit corresponding to the slot number
and wakes up the recovery thread.

The recovery thread:
1. Derives the slot number from the recovery_map
2. Locks the bitmap corresponding to the slot
3. Copies the set bits to the node-local bitmap
Perform resync for cluster node failure

If bitmap_copy_slot returns hi>0, we need to perform resync.
Communication Framework: Receiving

1. receive status

   sender                         receiver                   receiver
   ACK:CR                          ACK:CR                     ACK:CR

2. sender get EX of TOKEN
   sender get EX of MESSAGE
   sender                          receiver                   receiver
   TOKEN:EX                         ACK:CR                     ACK:CR
   MESSAGE:EX
   ACK:CR

3. sender write LVB.
   sender down-convert MESSAGE from EX to CR
   sender try to get EX of ACK
   [ wait until all receiver has *processed* the MESSAGE ]

                                     [ triggered by bast of ACK ]
                                     receiver get CR of MESSAGE
                                     receiver read LVB
                                     receiver processes the message
				     [ wait finish ]
                                     receiver release ACK

   sender                         receiver                   receiver
   TOKEN:EX                       MESSAGE:CR                 MESSAGE:CR
   MESSAGE:CR
   ACK:EX

4. sender down-convert ACK from EX to CR
   sender release MESSAGE
   sender release TOKEN
				  receiver upconvert to EX of MESSAGE
                                  receiver get CR of ACK
				  receiver release MESSAGE

   sender                        receiver                   receiver
   ACK:CR                         ACK:CR                     ACK:CR
Communication Framework: Sending functions

The sending part is split in two functions to make sure
atomicity of the operations, such as the MD superblock update.
metadata_update sends message to other nodes

   - request to send a message
   - make changes to superblock
   - send messages telling everyone that the superblock has changed
   - other nodes all read the superblock
   - other nodes all ack the messages
   - updating node release the "I'm sending a message" resource.
Reload superblock if METADATA_UPDATED is received
Send RESYNCING while performing resync start/stop

When a resync is initiated, RESYNCING message is sent to all active
nodes with the range (lo,hi). When the resync is over, a RESYNCING
message is sent with (0,0). A high sector value of zero indicates
that the resync is over.