linux/Documentation/md-cluster.txt

The cluster MD is a shared-device RAID for a cluster.


1. On-disk format

Separate write-intent-bitmap are used for each cluster node.
The bitmaps record all writes that may have been started on that node,
and may not yet have finished. The on-disk layout is:

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]  |                     |                     |

During "normal" functioning we assume the filesystem ensures that only one
node writes to any given block at a time, so a write
request will
 - set the appropriate bit (if not already set)
 - commit the write to all mirrors
 - schedule the bit to be cleared after a timeout.

Reads are just handled normally.  It is up to the filesystem to
ensure one node doesn't read from a location where another node (or the same
node) is writing.


2. DLM Locks for management

There are two locks for managing the device:

2.1 Bitmap lock resource (bm_lockres)

 The bm_lockres protects individual node bitmaps. They are named in the
 form bitmap001 for node 1, bitmap002 for node and so on. When a node
 joins the cluster, it acquires the lock in PW mode and it stays so
 during the lifetime the node is part of the cluster. The lock resource
 number is based on the slot number returned by the DLM subsystem. Since
 DLM starts node count from one and bitmap slots start from zero, one is
 subtracted from the DLM slot number to arrive at the bitmap slot number.

3. Communication

Each node has to communicate with other nodes when starting or ending
resync, and metadata superblock updates.

3.1 Message Types

 There are 3 types, of messages which are passed

 3.1.1 METADATA_UPDATED: informs other nodes that the metadata has been
   updated, and the node must re-read the md superblock. This is performed
   synchronously.

 3.1.2 RESYNC: informs other nodes that a resync is initiated or ended
   so that each node may suspend or resume the region.

3.2 Communication mechanism

 The DLM LVB is used to communicate within nodes of the cluster. There
 are three resources used for the purpose:

  3.2.1 Token: The resource which protects the entire communication
   system. The node having the token resource is allowed to
   communicate.

  3.2.2 Message: The lock resource which carries the data to
   communicate.

  3.2.3 Ack: The resource, acquiring which means the message has been
   acknowledged by all nodes in the cluster. The BAST of the resource
   is used to inform the receive node that a node wants to communicate.

The algorithm is:

 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

    Sender checks that it still needs to send a message. Messages received
    or other events that happened while waiting for the TOKEN may have made
    this message inappropriate or redundant.

 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. triggered by grant of EX on ACK (indicating all receivers have processed
    message)
    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


4. Handling Failures

4.1 Node Failure
 When a node fails, the DLM informs the cluster with the slot. The node
 starts a cluster recovery thread. The cluster recovery thread:
	- acquires the bitmap<number> lock of the failed node
	- opens the bitmap
	- reads the bitmap of the failed node
	- copies the set bitmap to local node
	- cleans the bitmap of the failed node
	- releases bitmap<number> lock of the failed node
	- initiates resync of the bitmap on the current node

 The resync process, is the regular md resync. However, in a clustered
 environment when a resync is performed, it needs to tell other nodes
 of the areas which are suspended. Before a resync starts, the node
 send out RESYNC_START with the (lo,hi) range of the area which needs
 to be suspended. Each node maintains a suspend_list, which contains
 the list  of ranges which are currently suspended. On receiving
 RESYNC_START, the node adds the range to the suspend_list. Similarly,
 when the node performing resync finishes, it send RESYNC_FINISHED
 to other nodes and other nodes remove the corresponding entry from
 the suspend_list.

 A helper function, should_suspend() can be used to check if a particular
 I/O range should be suspended or not.

4.2 Device Failure
 Device failures are handled and communicated with the metadata update
 routine.

5. Adding a new Device
For adding a new device, it is necessary that all nodes "see" the new device
to be added. For this, the following algorithm is used:

    1. Node 1 issues mdadm --manage /dev/mdX --add /dev/sdYY which issues
       ioctl(ADD_NEW_DISC with disc.state set to MD_DISK_CLUSTER_ADD)
    2. Node 1 sends NEWDISK with uuid and slot number
    3. Other nodes issue kobject_uevent_env with uuid and slot number
       (Steps 4,5 could be a udev rule)
    4. In userspace, the node searches for the disk, perhaps
       using blkid -t SUB_UUID=""
    5. Other nodes issue either of the following depending on whether the disk
       was found:
       ioctl(ADD_NEW_DISK with disc.state set to MD_DISK_CANDIDATE and
                disc.number set to slot number)
       ioctl(CLUSTERED_DISK_NACK)
    6. Other nodes drop lock on no-new-devs (CR) if device is found
    7. Node 1 attempts EX lock on no-new-devs
    8. If node 1 gets the lock, it sends METADATA_UPDATED after unmarking the disk
       as SpareLocal
    9. If not (get no-new-dev lock), it fails the operation and sends METADATA_UPDATED
    10. Other nodes get the information whether a disk is added or not
	by the following METADATA_UPDATED.