block-replication.txt (10875B)
1 Block replication 2 ---------------------------------------- 3 Copyright Fujitsu, Corp. 2016 4 Copyright (c) 2016 Intel Corporation 5 Copyright (c) 2016 HUAWEI TECHNOLOGIES CO., LTD. 6 7 This work is licensed under the terms of the GNU GPL, version 2 or later. 8 See the COPYING file in the top-level directory. 9 10 Block replication is used for continuous checkpoints. It is designed 11 for COLO (COarse-grain LOck-stepping) where the Secondary VM is running. 12 It can also be applied for FT/HA (Fault-tolerance/High Assurance) scenario, 13 where the Secondary VM is not running. 14 15 This document gives an overview of block replication's design. 16 17 == Background == 18 High availability solutions such as micro checkpoint and COLO will do 19 consecutive checkpoints. The VM state of the Primary and Secondary VM is 20 identical right after a VM checkpoint, but becomes different as the VM 21 executes till the next checkpoint. To support disk contents checkpoint, 22 the modified disk contents in the Secondary VM must be buffered, and are 23 only dropped at next checkpoint time. To reduce the network transportation 24 effort during a vmstate checkpoint, the disk modification operations of 25 the Primary disk are asynchronously forwarded to the Secondary node. 26 27 == Workflow == 28 The following is the image of block replication workflow: 29 30 +----------------------+ +------------------------+ 31 |Primary Write Requests| |Secondary Write Requests| 32 +----------------------+ +------------------------+ 33 | | 34 | (4) 35 | V 36 | /-------------\ 37 | Copy and Forward | | 38 |---------(1)----------+ | Disk Buffer | 39 | | | | 40 | (3) \-------------/ 41 | speculative ^ 42 | write through (2) 43 | | | 44 V V | 45 +--------------+ +----------------+ 46 | Primary Disk | | Secondary Disk | 47 +--------------+ +----------------+ 48 49 1) Primary write requests will be copied and forwarded to Secondary 50 QEMU. 51 2) Before Primary write requests are written to Secondary disk, the 52 original sector content will be read from Secondary disk and 53 buffered in the Disk buffer, but it will not overwrite the existing 54 sector content (it could be from either "Secondary Write Requests" or 55 previous COW of "Primary Write Requests") in the Disk buffer. 56 3) Primary write requests will be written to Secondary disk. 57 4) Secondary write requests will be buffered in the Disk buffer and it 58 will overwrite the existing sector content in the buffer. 59 60 == Architecture == 61 We are going to implement block replication from many basic 62 blocks that are already in QEMU. 63 64 virtio-blk || 65 ^ || .---------- 66 | || | Secondary 67 1 Quorum || '---------- 68 / \ || virtio-blk 69 / \ || ^ 70 Primary 2 filter | 71 disk ^ 7 Quorum 72 | / 73 3 NBD -------> 3 NBD / 74 client || server 2 filter 75 || ^ ^ 76 --------. || | | 77 Primary | || Secondary disk <--------- hidden-disk 5 <--------- active-disk 4 78 --------' || | backing ^ backing 79 || | | 80 || | | 81 || '-------------------------' 82 || blockdev-backup sync=none 6 83 84 1) The disk on the primary is represented by a block device with two 85 children, providing replication between a primary disk and the host that 86 runs the secondary VM. The read pattern (fifo) for quorum can be extended 87 to make the primary always read from the local disk instead of going through 88 NBD. 89 90 2) The new block filter (the name is replication) will control the block 91 replication. 92 93 3) The secondary disk receives writes from the primary VM through QEMU's 94 embedded NBD server (speculative write-through). 95 96 4) The disk on the secondary is represented by a custom block device 97 (called active-disk). It should start as an empty disk, and the format 98 should support bdrv_make_empty() and backing file. 99 100 5) The hidden-disk is created automatically. It buffers the original content 101 that is modified by the primary VM. It should also start as an empty disk, 102 and the driver supports bdrv_make_empty() and backing file. 103 104 6) The blockdev-backup job (sync=none) is run to allow hidden-disk to buffer 105 any state that would otherwise be lost by the speculative write-through 106 of the NBD server into the secondary disk. So before block replication, 107 the primary disk and secondary disk should contain the same data. 108 109 7) The secondary also has a quorum node, so after secondary failover it 110 can become the new primary and continue replication. 111 112 113 == Failure Handling == 114 There are 7 internal errors when block replication is running: 115 1. I/O error on primary disk 116 2. Forwarding primary write requests failed 117 3. Backup failed 118 4. I/O error on secondary disk 119 5. I/O error on active disk 120 6. Making active disk or hidden disk empty failed 121 7. Doing failover failed 122 In case 1 and 5, we just report the error to the disk layer. In case 2, 3, 123 4 and 6, we just report block replication's error to FT/HA manager (which 124 decides when to do a new checkpoint, when to do failover). 125 In case 7, if active commit failed, we use replication failover failed state 126 in Secondary's write operation (what decides which target to write). 127 128 == New block driver interface == 129 We add four block driver interfaces to control block replication: 130 a. replication_start_all() 131 Start block replication, called in migration/checkpoint thread. 132 We must call block_replication_start_all() in secondary QEMU before 133 calling block_replication_start_all() in primary QEMU. The caller 134 must hold the I/O mutex lock if it is in migration/checkpoint 135 thread. 136 b. replication_do_checkpoint_all() 137 This interface is called after all VM state is transferred to 138 Secondary QEMU. The Disk buffer will be dropped in this interface. 139 The caller must hold the I/O mutex lock if it is in migration/checkpoint 140 thread. 141 c. replication_get_error_all() 142 This interface is called to check if error happened in replication. 143 The caller must hold the I/O mutex lock if it is in migration/checkpoint 144 thread. 145 d. replication_stop_all() 146 It is called on failover. We will flush the Disk buffer into 147 Secondary Disk and stop block replication. The vm should be stopped 148 before calling it if you use this API to shutdown the guest, or other 149 things except failover. The caller must hold the I/O mutex lock if it is 150 in migration/checkpoint thread. 151 152 == Usage == 153 Primary: 154 -drive if=xxx,driver=quorum,read-pattern=fifo,id=colo1,vote-threshold=1,\ 155 children.0.file.filename=1.raw,\ 156 children.0.driver=raw 157 158 Run qmp command in primary qemu: 159 { "execute": "human-monitor-command", 160 "arguments": { 161 "command-line": "drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=xxxx,file.port=xxxx,file.export=colo1,node-name=nbd_client1" 162 } 163 } 164 { "execute": "x-blockdev-change", 165 "arguments": { 166 "parent": "colo1", 167 "node": "nbd_client1" 168 } 169 } 170 Note: 171 1. There should be only one NBD Client for each primary disk. 172 2. host is the secondary physical machine's hostname or IP 173 3. Each disk must have its own export name. 174 4. It is all a single argument to -drive and you should ignore the 175 leading whitespace. 176 5. The qmp command line must be run after running qmp command line in 177 secondary qemu. 178 6. After primary failover we need remove children.1 (replication driver). 179 180 Secondary: 181 -drive if=none,driver=raw,file.filename=1.raw,id=colo1 \ 182 -drive if=none,id=childs1,driver=replication,mode=secondary,top-id=top-disk1 183 file.file.filename=active_disk.qcow2,\ 184 file.driver=qcow2,\ 185 file.backing.file.filename=hidden_disk.qcow2,\ 186 file.backing.driver=qcow2,\ 187 file.backing.backing=colo1 188 -drive if=xxx,driver=quorum,read-pattern=fifo,id=top-disk1,\ 189 vote-threshold=1,children.0=childs1 190 191 Then run qmp command in secondary qemu: 192 { "execute": "nbd-server-start", 193 "arguments": { 194 "addr": { 195 "type": "inet", 196 "data": { 197 "host": "xxx", 198 "port": "xxx" 199 } 200 } 201 } 202 } 203 { "execute": "nbd-server-add", 204 "arguments": { 205 "device": "colo1", 206 "writable": true 207 } 208 } 209 210 Note: 211 1. The export name in secondary QEMU command line is the secondary 212 disk's id. 213 2. The export name for the same disk must be the same 214 3. The qmp command nbd-server-start and nbd-server-add must be run 215 before running the qmp command migrate on primary QEMU 216 4. Active disk, hidden disk and nbd target's length should be the 217 same. 218 5. It is better to put active disk and hidden disk in ramdisk. 219 6. It is all a single argument to -drive, and you should ignore 220 the leading whitespace. 221 222 After Failover: 223 Primary: 224 The secondary host is down, so we should run the following qmp command 225 to remove the nbd child from the quorum: 226 { "execute": "x-blockdev-change", 227 "arguments": { 228 "parent": "colo1", 229 "child": "children.1" 230 } 231 } 232 { "execute": "human-monitor-command", 233 "arguments": { 234 "command-line": "drive_del xxxx" 235 } 236 } 237 Note: there is no qmp command to remove the blockdev now 238 239 Secondary: 240 The primary host is down, so we should do the following thing: 241 { "execute": "nbd-server-stop" } 242 243 Promote Secondary to Primary: 244 see COLO-FT.txt 245 246 TODO: 247 1. Shared disk