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1 The cluster MD is a shared-device RAID for a cluster.
4 1. On-disk format
6 Separate write-intent-bitmaps are used for each cluster node.
7 The bitmaps record all writes that may have been started on that node,
8 and may not yet have finished. The on-disk layout is:
10 0 4k 8k 12k
11 -------------------------------------------------------------------
12 | idle | md super | bm super [0] + bits |
13 | bm bits[0, contd] | bm super[1] + bits | bm bits[1, contd] |
14 | bm super[2] + bits | bm bits [2, contd] | bm super[3] + bits |
15 | bm bits [3, contd] | | |
17 During "normal" functioning we assume the filesystem ensures that only
18 one node writes to any given block at a time, so a write request will
20 - set the appropriate bit (if not already set)
21 - commit the write to all mirrors
22 - schedule the bit to be cleared after a timeout.
24 Reads are just handled normally. It is up to the filesystem to ensure
25 one node doesn't read from a location where another node (or the same
26 node) is writing.
29 2. DLM Locks for management
31 There are three groups of locks for managing the device:
33 2.1 Bitmap lock resource (bm_lockres)
35 The bm_lockres protects individual node bitmaps. They are named in
36 the form bitmap000 for node 1, bitmap001 for node 2 and so on. When a
37 node joins the cluster, it acquires the lock in PW mode and it stays
38 so during the lifetime the node is part of the cluster. The lock
39 resource number is based on the slot number returned by the DLM
40 subsystem. Since DLM starts node count from one and bitmap slots
41 start from zero, one is subtracted from the DLM slot number to arrive
42 at the bitmap slot number.
44 The LVB of the bitmap lock for a particular node records the range
45 of sectors that are being re-synced by that node. No other
46 node may write to those sectors. This is used when a new nodes
47 joins the cluster.
49 2.2 Message passing locks
51 Each node has to communicate with other nodes when starting or ending
52 resync, and for metadata superblock updates. This communication is
53 managed through three locks: "token", "message", and "ack", together
54 with the Lock Value Block (LVB) of one of the "message" lock.
56 2.3 new-device management
58 A single lock: "no-new-dev" is used to co-ordinate the addition of
59 new devices - this must be synchronized across the array.
60 Normally all nodes hold a concurrent-read lock on this device.
62 3. Communication
64 Messages can be broadcast to all nodes, and the sender waits for all
65 other nodes to acknowledge the message before proceeding. Only one
66 message can be processed at a time.
68 3.1 Message Types
70 There are six types of messages which are passed:
72 3.1.1 METADATA_UPDATED: informs other nodes that the metadata has
73 been updated, and the node must re-read the md superblock. This is
74 performed synchronously. It is primarily used to signal device
75 failure.
77 3.1.2 RESYNCING: informs other nodes that a resync is initiated or
78 ended so that each node may suspend or resume the region. Each
79 RESYNCING message identifies a range of the devices that the
80 sending node is about to resync. This over-rides any pervious
81 notification from that node: only one ranged can be resynced at a
82 time per-node.
84 3.1.3 NEWDISK: informs other nodes that a device is being added to
85 the array. Message contains an identifier for that device. See
86 below for further details.
88 3.1.4 REMOVE: A failed or spare device is being removed from the
89 array. The slot-number of the device is included in the message.
91 3.1.5 RE_ADD: A failed device is being re-activated - the assumption
92 is that it has been determined to be working again.
94 3.1.6 BITMAP_NEEDS_SYNC: if a node is stopped locally but the bitmap
95 isn't clean, then another node is informed to take the ownership of
96 resync.
98 3.2 Communication mechanism
100 The DLM LVB is used to communicate within nodes of the cluster. There
101 are three resources used for the purpose:
103 3.2.1 token: The resource which protects the entire communication
104 system. The node having the token resource is allowed to
105 communicate.
107 3.2.2 message: The lock resource which carries the data to
108 communicate.
110 3.2.3 ack: The resource, acquiring which means the message has been
111 acknowledged by all nodes in the cluster. The BAST of the resource
112 is used to inform the receiving node that a node wants to
113 communicate.
115 The algorithm is:
117 1. receive status - all nodes have concurrent-reader lock on "ack".
119 sender receiver receiver
120 "ack":CR "ack":CR "ack":CR
122 2. sender get EX on "token"
123 sender get EX on "message"
124 sender receiver receiver
125 "token":EX "ack":CR "ack":CR
126 "message":EX
127 "ack":CR
129 Sender checks that it still needs to send a message. Messages
130 received or other events that happened while waiting for the
131 "token" may have made this message inappropriate or redundant.
133 3. sender writes LVB.
134 sender down-convert "message" from EX to CW
135 sender try to get EX of "ack"
136 [ wait until all receivers have *processed* the "message" ]
138 [ triggered by bast of "ack" ]
139 receiver get CR on "message"
140 receiver read LVB
141 receiver processes the message
142 [ wait finish ]
143 receiver releases "ack"
144 receiver tries to get PR on "message"
146 sender receiver receiver
147 "token":EX "message":CR "message":CR
148 "message":CW
149 "ack":EX
151 4. triggered by grant of EX on "ack" (indicating all receivers
152 have processed message)
153 sender down-converts "ack" from EX to CR
154 sender releases "message"
155 sender releases "token"
156 receiver upconvert to PR on "message"
157 receiver get CR of "ack"
158 receiver release "message"
160 sender receiver receiver
161 "ack":CR "ack":CR "ack":CR
164 4. Handling Failures
166 4.1 Node Failure
168 When a node fails, the DLM informs the cluster with the slot
169 number. The node starts a cluster recovery thread. The cluster
170 recovery thread:
172 - acquires the bitmap<number> lock of the failed node
173 - opens the bitmap
174 - reads the bitmap of the failed node
175 - copies the set bitmap to local node
176 - cleans the bitmap of the failed node
177 - releases bitmap<number> lock of the failed node
178 - initiates resync of the bitmap on the current node
179 md_check_recovery is invoked within recover_bitmaps,
180 then md_check_recovery -> metadata_update_start/finish,
181 it will lock the communication by lock_comm.
182 Which means when one node is resyncing it blocks all
183 other nodes from writing anywhere on the array.
185 The resync process is the regular md resync. However, in a clustered
186 environment when a resync is performed, it needs to tell other nodes
187 of the areas which are suspended. Before a resync starts, the node
188 send out RESYNCING with the (lo,hi) range of the area which needs to
189 be suspended. Each node maintains a suspend_list, which contains the
190 list of ranges which are currently suspended. On receiving RESYNCING,
191 the node adds the range to the suspend_list. Similarly, when the node
192 performing resync finishes, it sends RESYNCING with an empty range to
193 other nodes and other nodes remove the corresponding entry from the
194 suspend_list.
196 A helper function, ->area_resyncing() can be used to check if a
197 particular I/O range should be suspended or not.
199 4.2 Device Failure
201 Device failures are handled and communicated with the metadata update
202 routine. When a node detects a device failure it does not allow
203 any further writes to that device until the failure has been
204 acknowledged by all other nodes.
206 5. Adding a new Device
208 For adding a new device, it is necessary that all nodes "see" the new
209 device to be added. For this, the following algorithm is used:
211 1. Node 1 issues mdadm --manage /dev/mdX --add /dev/sdYY which issues
212 ioctl(ADD_NEW_DISK with disc.state set to MD_DISK_CLUSTER_ADD)
213 2. Node 1 sends a NEWDISK message with uuid and slot number
214 3. Other nodes issue kobject_uevent_env with uuid and slot number
215 (Steps 4,5 could be a udev rule)
216 4. In userspace, the node searches for the disk, perhaps
217 using blkid -t SUB_UUID=""
218 5. Other nodes issue either of the following depending on whether
219 the disk was found:
220 ioctl(ADD_NEW_DISK with disc.state set to MD_DISK_CANDIDATE and
221 disc.number set to slot number)
222 ioctl(CLUSTERED_DISK_NACK)
223 6. Other nodes drop lock on "no-new-devs" (CR) if device is found
224 7. Node 1 attempts EX lock on "no-new-dev"
225 8. If node 1 gets the lock, it sends METADATA_UPDATED after
226 unmarking the disk as SpareLocal
227 9. If not (get "no-new-dev" lock), it fails the operation and sends
228 METADATA_UPDATED.
229 10. Other nodes get the information whether a disk is added or not
230 by the following METADATA_UPDATED.
232 6. Module interface.
234 There are 17 call-backs which the md core can make to the cluster
235 module. Understanding these can give a good overview of the whole
236 process.
238 6.1 join(nodes) and leave()
240 These are called when an array is started with a clustered bitmap,
241 and when the array is stopped. join() ensures the cluster is
242 available and initializes the various resources.
243 Only the first 'nodes' nodes in the cluster can use the array.
245 6.2 slot_number()
247 Reports the slot number advised by the cluster infrastructure.
248 Range is from 0 to nodes-1.
250 6.3 resync_info_update()
252 This updates the resync range that is stored in the bitmap lock.
253 The starting point is updated as the resync progresses. The
254 end point is always the end of the array.
255 It does *not* send a RESYNCING message.
257 6.4 resync_start(), resync_finish()
259 These are called when resync/recovery/reshape starts or stops.
260 They update the resyncing range in the bitmap lock and also
261 send a RESYNCING message. resync_start reports the whole
262 array as resyncing, resync_finish reports none of it.
264 resync_finish() also sends a BITMAP_NEEDS_SYNC message which
265 allows some other node to take over.
267 6.5 metadata_update_start(), metadata_update_finish(),
268 metadata_update_cancel().
270 metadata_update_start is used to get exclusive access to
271 the metadata. If a change is still needed once that access is
272 gained, metadata_update_finish() will send a METADATA_UPDATE
273 message to all other nodes, otherwise metadata_update_cancel()
274 can be used to release the lock.
276 6.6 area_resyncing()
278 This combines two elements of functionality.
280 Firstly, it will check if any node is currently resyncing
281 anything in a given range of sectors. If any resync is found,
282 then the caller will avoid writing or read-balancing in that
283 range.
285 Secondly, while node recovery is happening it reports that
286 all areas are resyncing for READ requests. This avoids races
287 between the cluster-filesystem and the cluster-RAID handling
288 a node failure.
290 6.7 add_new_disk_start(), add_new_disk_finish(), new_disk_ack()
292 These are used to manage the new-disk protocol described above.
293 When a new device is added, add_new_disk_start() is called before
294 it is bound to the array and, if that succeeds, add_new_disk_finish()
295 is called the device is fully added.
297 When a device is added in acknowledgement to a previous
298 request, or when the device is declared "unavailable",
299 new_disk_ack() is called.
301 6.8 remove_disk()
303 This is called when a spare or failed device is removed from
304 the array. It causes a REMOVE message to be send to other nodes.
306 6.9 gather_bitmaps()
308 This sends a RE_ADD message to all other nodes and then
309 gathers bitmap information from all bitmaps. This combined
310 bitmap is then used to recovery the re-added device.
312 6.10 lock_all_bitmaps() and unlock_all_bitmaps()
314 These are called when change bitmap to none. If a node plans
315 to clear the cluster raid's bitmap, it need to make sure no other
316 nodes are using the raid which is achieved by lock all bitmap
317 locks within the cluster, and also those locks are unlocked
318 accordingly.