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 CW 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 PR 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 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 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.