1COarse-grained LOck-stepping Virtual Machines for Non-stop Service 2---------------------------------------- 3Copyright (c) 2016 Intel Corporation 4Copyright (c) 2016 HUAWEI TECHNOLOGIES CO., LTD. 5Copyright (c) 2016 Fujitsu, Corp. 6 7This work is licensed under the terms of the GNU GPL, version 2 or later. 8See the COPYING file in the top-level directory. 9 10This document gives an overview of COLO's design and how to use it. 11 12== Background == 13Virtual machine (VM) replication is a well known technique for providing 14application-agnostic software-implemented hardware fault tolerance, 15also known as "non-stop service". 16 17COLO (COarse-grained LOck-stepping) is a high availability solution. 18Both primary VM (PVM) and secondary VM (SVM) run in parallel. They receive the 19same request from client, and generate response in parallel too. 20If the response packets from PVM and SVM are identical, they are released 21immediately. Otherwise, a VM checkpoint (on demand) is conducted. 22 23== Architecture == 24 25The architecture of COLO is shown in the diagram below. 26It consists of a pair of networked physical nodes: 27The primary node running the PVM, and the secondary node running the SVM 28to maintain a valid replica of the PVM. 29PVM and SVM execute in parallel and generate output of response packets for 30client requests according to the application semantics. 31 32The incoming packets from the client or external network are received by the 33primary node, and then forwarded to the secondary node, so that both the PVM 34and the SVM are stimulated with the same requests. 35 36COLO receives the outbound packets from both the PVM and SVM and compares them 37before allowing the output to be sent to clients. 38 39The SVM is qualified as a valid replica of the PVM, as long as it generates 40identical responses to all client requests. Once the differences in the outputs 41are detected between the PVM and SVM, COLO withholds transmission of the 42outbound packets until it has successfully synchronized the PVM state to the SVM. 43 44 Primary Node Secondary Node 45+------------+ +-----------------------+ +------------------------+ +------------+ 46| | | HeartBeat +<----->+ HeartBeat | | | 47| Primary VM | +-----------+-----------+ +-----------+------------+ |Secondary VM| 48| | | | | | 49| | +-----------|-----------+ +-----------|------------+ | | 50| | |QEMU +---v----+ | |QEMU +----v---+ | | | 51| | | |Failover| | | |Failover| | | | 52| | | +--------+ | | +--------+ | | | 53| | | +---------------+ | | +---------------+ | | | 54| | | | VM Checkpoint +-------------->+ VM Checkpoint | | | | 55| | | +---------------+ | | +---------------+ | | | 56|Requests<--------------------------\ /-----------------\ /--------------------->Requests| 57| | | ^ ^ | | | | | | | 58|Responses+---------------------\ /-|-|------------\ /-------------------------+Responses| 59| | | | | | | | | | | | | | | | 60| | | +-----------+ | | | | | | | | | | +----------+ | | | 61| | | | COLO disk | | | | | | | | | | | | COLO disk| | | | 62| | | | Manager +---------------------------->| Manager | | | | 63| | | ++----------+ v v | | | | | v v | +---------++ | | | 64| | | |+-----------+-+-+-++| | ++-+--+-+---------+ | | | | 65| | | || COLO Proxy || | | COLO Proxy | | | | | 66| | | || (compare packet || | |(adjust sequence | | | | | 67| | | ||and mirror packet)|| | | and ACK) | | | | | 68| | | |+------------+---+-+| | +-----------------+ | | | | 69+------------+ +-----------------------+ +------------------------+ +------------+ 70+------------+ | | | | +------------+ 71| VM Monitor | | | | | | VM Monitor | 72+------------+ | | | | +------------+ 73+---------------------------------------+ +----------------------------------------+ 74| Kernel | | | | | Kernel | | 75+---------------------------------------+ +----------------------------------------+ 76 | | | | 77 +--------------v+ +---------v---+--+ +------------------+ +v-------------+ 78 | Storage | |External Network| | External Network | | Storage | 79 +---------------+ +----------------+ +------------------+ +--------------+ 80 81 82== Components introduction == 83 84You can see there are several components in COLO's diagram of architecture. 85Their functions are described below. 86 87HeartBeat: 88Runs on both the primary and secondary nodes, to periodically check platform 89availability. When the primary node suffers a hardware fail-stop failure, 90the heartbeat stops responding, the secondary node will trigger a failover 91as soon as it determines the absence. 92 93COLO disk Manager: 94When primary VM writes data into image, the colo disk manger captures this data 95and sends it to secondary VM's which makes sure the context of secondary VM's 96image is consistent with the context of primary VM 's image. 97For more details, please refer to docs/block-replication.txt. 98 99Checkpoint/Failover Controller: 100Modifications of save/restore flow to realize continuous migration, 101to make sure the state of VM in Secondary side is always consistent with VM in 102Primary side. 103 104COLO Proxy: 105Delivers packets to Primary and Secondary, and then compare the responses from 106both side. Then decide whether to start a checkpoint according to some rules. 107Please refer to docs/colo-proxy.txt for more information. 108 109Note: 110HeartBeat has not been implemented yet, so you need to trigger failover process 111by using 'x-colo-lost-heartbeat' command. 112 113== COLO operation status == 114 115+-----------------+ 116| | 117| Start COLO | 118| | 119+--------+--------+ 120 | 121 | Main qmp command: 122 | migrate-set-capabilities with x-colo 123 | migrate 124 | 125 v 126+--------+--------+ 127| | 128| COLO running | 129| | 130+--------+--------+ 131 | 132 | Main qmp command: 133 | x-colo-lost-heartbeat 134 | or 135 | some error happened 136 v 137+--------+--------+ 138| | send qmp event: 139| COLO failover | COLO_EXIT 140| | 141+-----------------+ 142 143COLO use the qmp command to switch and report operation status. 144The diagram just shows the main qmp command, you can get the detail 145in test procedure. 146 147== Test procedure == 1481. Startup qemu 149Primary: 150# qemu-system-x86_64 -accel kvm -m 2048 -smp 2 -qmp stdio -name primary \ 151 -device piix3-usb-uhci -vnc :7 \ 152 -device usb-tablet -netdev tap,id=hn0,vhost=off \ 153 -device virtio-net-pci,id=net-pci0,netdev=hn0 \ 154 -drive if=virtio,id=primary-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\ 155 children.0.file.filename=1.raw,\ 156 children.0.driver=raw -S 157Secondary: 158# qemu-system-x86_64 -accel kvm -m 2048 -smp 2 -qmp stdio -name secondary \ 159 -device piix3-usb-uhci -vnc :7 \ 160 -device usb-tablet -netdev tap,id=hn0,vhost=off \ 161 -device virtio-net-pci,id=net-pci0,netdev=hn0 \ 162 -drive if=none,id=secondary-disk0,file.filename=1.raw,driver=raw,node-name=node0 \ 163 -drive if=virtio,id=active-disk0,driver=replication,mode=secondary,\ 164 file.driver=qcow2,top-id=active-disk0,\ 165 file.file.filename=/mnt/ramfs/active_disk.img,\ 166 file.backing.driver=qcow2,\ 167 file.backing.file.filename=/mnt/ramfs/hidden_disk.img,\ 168 file.backing.backing=secondary-disk0 \ 169 -incoming tcp:0:8888 170 1712. On Secondary VM's QEMU monitor, issue command 172{'execute':'qmp_capabilities'} 173{ 'execute': 'nbd-server-start', 174 'arguments': {'addr': {'type': 'inet', 'data': {'host': 'xx.xx.xx.xx', 'port': '8889'} } } 175} 176{'execute': 'nbd-server-add', 'arguments': {'device': 'secondary-disk0', 'writable': true } } 177 178Note: 179 a. The qmp command nbd-server-start and nbd-server-add must be run 180 before running the qmp command migrate on primary QEMU 181 b. Active disk, hidden disk and nbd target's length should be the 182 same. 183 c. It is better to put active disk and hidden disk in ramdisk. 184 1853. On Primary VM's QEMU monitor, issue command: 186{'execute':'qmp_capabilities'} 187{ 'execute': 'human-monitor-command', 188 'arguments': {'command-line': 'drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=xx.xx.xx.xx,file.port=8889,file.export=secondary-disk0,node-name=nbd_client0'}} 189{ 'execute':'x-blockdev-change', 'arguments':{'parent': 'primary-disk0', 'node': 'nbd_client0' } } 190{ 'execute': 'migrate-set-capabilities', 191 'arguments': {'capabilities': [ {'capability': 'x-colo', 'state': true } ] } } 192{ 'execute': 'migrate', 'arguments': {'uri': 'tcp:xx.xx.xx.xx:8888' } } 193 194 Note: 195 a. There should be only one NBD Client for each primary disk. 196 b. xx.xx.xx.xx is the secondary physical machine's hostname or IP 197 c. The qmp command line must be run after running qmp command line in 198 secondary qemu. 199 2004. After the above steps, you will see, whenever you make changes to PVM, SVM will be synced. 201You can issue command '{ "execute": "migrate-set-parameters" , "arguments":{ "x-checkpoint-delay": 2000 } }' 202to change the checkpoint period time 203 2045. Failover test 205You can kill Primary VM and run 'x_colo_lost_heartbeat' in Secondary VM's 206monitor at the same time, then SVM will failover and client will not detect this 207change. 208 209Before issuing '{ "execute": "x-colo-lost-heartbeat" }' command, we have to 210issue block related command to stop block replication. 211Primary: 212 Remove the nbd child from the quorum: 213 { 'execute': 'x-blockdev-change', 'arguments': {'parent': 'colo-disk0', 'child': 'children.1'}} 214 { 'execute': 'human-monitor-command','arguments': {'command-line': 'drive_del blk-buddy0'}} 215 Note: there is no qmp command to remove the blockdev now 216 217Secondary: 218 The primary host is down, so we should do the following thing: 219 { 'execute': 'nbd-server-stop' } 220 221== TODO == 2221. Support continuous VM replication. 2232. Support shared storage. 2243. Develop the heartbeat part. 2254. Reduce checkpoint VM’s downtime while doing checkpoint. 226