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<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.