mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-25 13:14:07 +08:00
177 lines
7.0 KiB
Plaintext
177 lines
7.0 KiB
Plaintext
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.
|