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diff --git a/Documentation/md/md-cluster.txt b/Documentation/md/md-cluster.txt deleted file mode 100644 index e1055f105cf5..000000000000 --- a/Documentation/md/md-cluster.txt +++ /dev/null @@ -1,325 +0,0 @@ -The cluster MD is a shared-device RAID for a cluster, it supports -two levels: raid1 and raid10 (limited support). - - -1. On-disk format - -Separate write-intent-bitmaps 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 three groups of 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 bitmap000 for node 1, bitmap001 for node 2 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. - - The LVB of the bitmap lock for a particular node records the range - of sectors that are being re-synced by that node. No other - node may write to those sectors. This is used when a new nodes - joins the cluster. - -2.2 Message passing locks - - Each node has to communicate with other nodes when starting or ending - resync, and for metadata superblock updates. This communication is - managed through three locks: "token", "message", and "ack", together - with the Lock Value Block (LVB) of one of the "message" lock. - -2.3 new-device management - - A single lock: "no-new-dev" is used to co-ordinate the addition of - new devices - this must be synchronized across the array. - Normally all nodes hold a concurrent-read lock on this device. - -3. Communication - - Messages can be broadcast to all nodes, and the sender waits for all - other nodes to acknowledge the message before proceeding. Only one - message can be processed at a time. - -3.1 Message Types - - There are six 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. It is primarily used to signal device - failure. - - 3.1.2 RESYNCING: informs other nodes that a resync is initiated or - ended so that each node may suspend or resume the region. Each - RESYNCING message identifies a range of the devices that the - sending node is about to resync. This overrides any previous - notification from that node: only one ranged can be resynced at a - time per-node. - - 3.1.3 NEWDISK: informs other nodes that a device is being added to - the array. Message contains an identifier for that device. See - below for further details. - - 3.1.4 REMOVE: A failed or spare device is being removed from the - array. The slot-number of the device is included in the message. - - 3.1.5 RE_ADD: A failed device is being re-activated - the assumption - is that it has been determined to be working again. - - 3.1.6 BITMAP_NEEDS_SYNC: if a node is stopped locally but the bitmap - isn't clean, then another node is informed to take the ownership of - resync. - -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 receiving node that a node wants to - communicate. - -The algorithm is: - - 1. receive status - all nodes have concurrent-reader lock on "ack". - - sender receiver receiver - "ack":CR "ack":CR "ack":CR - - 2. sender get EX on "token" - sender get EX on "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 writes LVB. - sender down-convert "message" from EX to CW - sender try to get EX of "ack" - [ wait until all receivers have *processed* the "message" ] - - [ triggered by bast of "ack" ] - receiver get CR on "message" - receiver read LVB - receiver processes the message - [ wait finish ] - receiver releases "ack" - receiver tries to get PR on "message" - - sender receiver receiver - "token":EX "message":CR "message":CR - "message":CW - "ack":EX - - 4. triggered by grant of EX on "ack" (indicating all receivers - have processed message) - sender down-converts "ack" from EX to CR - sender releases "message" - sender releases "token" - receiver upconvert to PR on "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 - number. 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 - md_check_recovery is invoked within recover_bitmaps, - then md_check_recovery -> metadata_update_start/finish, - it will lock the communication by lock_comm. - Which means when one node is resyncing it blocks all - other nodes from writing anywhere on the array. - - 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 RESYNCING 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 RESYNCING, - the node adds the range to the suspend_list. Similarly, when the node - performing resync finishes, it sends RESYNCING with an empty range to - other nodes and other nodes remove the corresponding entry from the - suspend_list. - - A helper function, ->area_resyncing() 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. When a node detects a device failure it does not allow - any further writes to that device until the failure has been - acknowledged by all other nodes. - -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_DISK with disc.state set to MD_DISK_CLUSTER_ADD) - 2. Node 1 sends a NEWDISK message 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-dev" - 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. - -6. Module interface. - - There are 17 call-backs which the md core can make to the cluster - module. Understanding these can give a good overview of the whole - process. - -6.1 join(nodes) and leave() - - These are called when an array is started with a clustered bitmap, - and when the array is stopped. join() ensures the cluster is - available and initializes the various resources. - Only the first 'nodes' nodes in the cluster can use the array. - -6.2 slot_number() - - Reports the slot number advised by the cluster infrastructure. - Range is from 0 to nodes-1. - -6.3 resync_info_update() - - This updates the resync range that is stored in the bitmap lock. - The starting point is updated as the resync progresses. The - end point is always the end of the array. - It does *not* send a RESYNCING message. - -6.4 resync_start(), resync_finish() - - These are called when resync/recovery/reshape starts or stops. - They update the resyncing range in the bitmap lock and also - send a RESYNCING message. resync_start reports the whole - array as resyncing, resync_finish reports none of it. - - resync_finish() also sends a BITMAP_NEEDS_SYNC message which - allows some other node to take over. - -6.5 metadata_update_start(), metadata_update_finish(), - metadata_update_cancel(). - - metadata_update_start is used to get exclusive access to - the metadata. If a change is still needed once that access is - gained, metadata_update_finish() will send a METADATA_UPDATE - message to all other nodes, otherwise metadata_update_cancel() - can be used to release the lock. - -6.6 area_resyncing() - - This combines two elements of functionality. - - Firstly, it will check if any node is currently resyncing - anything in a given range of sectors. If any resync is found, - then the caller will avoid writing or read-balancing in that - range. - - Secondly, while node recovery is happening it reports that - all areas are resyncing for READ requests. This avoids races - between the cluster-filesystem and the cluster-RAID handling - a node failure. - -6.7 add_new_disk_start(), add_new_disk_finish(), new_disk_ack() - - These are used to manage the new-disk protocol described above. - When a new device is added, add_new_disk_start() is called before - it is bound to the array and, if that succeeds, add_new_disk_finish() - is called the device is fully added. - - When a device is added in acknowledgement to a previous - request, or when the device is declared "unavailable", - new_disk_ack() is called. - -6.8 remove_disk() - - This is called when a spare or failed device is removed from - the array. It causes a REMOVE message to be send to other nodes. - -6.9 gather_bitmaps() - - This sends a RE_ADD message to all other nodes and then - gathers bitmap information from all bitmaps. This combined - bitmap is then used to recovery the re-added device. - -6.10 lock_all_bitmaps() and unlock_all_bitmaps() - - These are called when change bitmap to none. If a node plans - to clear the cluster raid's bitmap, it need to make sure no other - nodes are using the raid which is achieved by lock all bitmap - locks within the cluster, and also those locks are unlocked - accordingly. - -7. Unsupported features - -There are somethings which are not supported by cluster MD yet. - -- change array_sectors. |