md-cluster: Design Documentation
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
diff --git a/Documentation/md-cluster.txt b/Documentation/md-cluster.txt
new file mode 100644
index 0000000..de1af7d
--- /dev/null
+++ b/Documentation/md-cluster.txt
@@ -0,0 +1,176 @@
+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.