1. On-disk format
-Separate write-intent-bitmap are used for each cluster node.
+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:
| 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
+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
+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:
+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 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.
+ 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
-Each node has to communicate with other nodes when starting or ending
-resync, and metadata superblock updates.
+ 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 3 types, of messages which are passed
+ 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.
+ 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 RESYNC: informs other nodes that a resync is initiated or ended
- so that each node may suspend or resume the region.
+ 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 over-rides any pervious
+ 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
+ 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
+ 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
+ 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.
+ is used to inform the receiving node that a node wants to
+ communicate.
The algorithm is:
- 1. receive status
+ 1. receive status - all nodes have concurrent-reader lock on "ack".
- sender receiver receiver
- ACK:CR ACK:CR ACK:CR
+ sender receiver receiver
+ "ack":CR "ack":CR "ack":CR
- 2. sender get EX of TOKEN
- sender get EX of MESSAGE
+ 2. sender get EX on "token"
+ sender get EX on "message"
sender receiver receiver
- TOKEN:EX ACK:CR ACK:CR
- MESSAGE:EX
- ACK:CR
+ "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.
+ 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 ]
+ 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 of MESSAGE
+ [ triggered by bast of "ack" ]
+ receiver get CR on "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
+ 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
+ "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:
+
+ 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
- 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
+ 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.
+ 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, should_suspend() can be used to check if a particular
- I/O range should be suspended or not.
+ 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.
+ 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:
+
+ 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
+ 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:
+ 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)
+ 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.
+ 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.
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