[qemu.git] / docs / block-replication.txt

Block replication
----------------------------------------
Copyright Fujitsu, Corp. 2016
Copyright (c) 2016 Intel Corporation
Copyright (c) 2016 HUAWEI TECHNOLOGIES CO., LTD.

This work is licensed under the terms of the GNU GPL, version 2 or later.
See the COPYING file in the top-level directory.

Block replication is used for continuous checkpoints. It is designed
for COLO (COarse-grain LOck-stepping) where the Secondary VM is running.
It can also be applied for FT/HA (Fault-tolerance/High Assurance) scenario,
where the Secondary VM is not running.

This document gives an overview of block replication's design.

== Background ==
High availability solutions such as micro checkpoint and COLO will do
consecutive checkpoints. The VM state of the Primary and Secondary VM is
identical right after a VM checkpoint, but becomes different as the VM
executes till the next checkpoint. To support disk contents checkpoint,
the modified disk contents in the Secondary VM must be buffered, and are
only dropped at next checkpoint time. To reduce the network transportation
effort during a vmstate checkpoint, the disk modification operations of
the Primary disk are asynchronously forwarded to the Secondary node.

== Workflow ==
The following is the image of block replication workflow:

        +----------------------+            +------------------------+
        |Primary Write Requests|            |Secondary Write Requests|
        +----------------------+            +------------------------+
                  |                                       |
                  |                                      (4)
                  |                                       V
                  |                              /-------------\
                  |      Copy and Forward        |             |
                  |---------(1)----------+       | Disk Buffer |
                  |                      |       |             |
                  |                     (3)      \-------------/
                  |                 speculative      ^
                  |                write through    (2)
                  |                      |           |
                  V                      V           |
           +--------------+           +----------------+
           | Primary Disk |           | Secondary Disk |
           +--------------+           +----------------+

    1) Primary write requests will be copied and forwarded to Secondary
       QEMU.
    2) Before Primary write requests are written to Secondary disk, the
       original sector content will be read from Secondary disk and
       buffered in the Disk buffer, but it will not overwrite the existing
       sector content (it could be from either "Secondary Write Requests" or
       previous COW of "Primary Write Requests") in the Disk buffer.
    3) Primary write requests will be written to Secondary disk.
    4) Secondary write requests will be buffered in the Disk buffer and it
       will overwrite the existing sector content in the buffer.

== Architecture ==
We are going to implement block replication from many basic
blocks that are already in QEMU.

         virtio-blk       ||
             ^            ||                            .----------
             |            ||                            | Secondary
        1 Quorum          ||                            '----------
         /      \         ||
        /        \        ||
   Primary    2 filter
     disk         ^                                                             virtio-blk
                  |                                                                  ^
                3 NBD  ------->  3 NBD                                               |
                client    ||     server                                          2 filter
                          ||        ^                                                ^
--------.                 ||        |                                                |
Primary |                 ||  Secondary disk <--------- hidden-disk 5 <--------- active-disk 4
--------'                 ||        |          backing        ^       backing
                          ||        |                         |
                          ||        |                         |
                          ||        '-------------------------'
                          ||           drive-backup sync=none 6

1) The disk on the primary is represented by a block device with two
children, providing replication between a primary disk and the host that
runs the secondary VM. The read pattern (fifo) for quorum can be extended
to make the primary always read from the local disk instead of going through
NBD.

2) The new block filter (the name is replication) will control the block
replication.

3) The secondary disk receives writes from the primary VM through QEMU's
embedded NBD server (speculative write-through).

4) The disk on the secondary is represented by a custom block device
(called active-disk). It should start as an empty disk, and the format
should support bdrv_make_empty() and backing file.

5) The hidden-disk is created automatically. It buffers the original content
that is modified by the primary VM. It should also start as an empty disk,
and the driver supports bdrv_make_empty() and backing file.

6) The drive-backup job (sync=none) is run to allow hidden-disk to buffer
any state that would otherwise be lost by the speculative write-through
of the NBD server into the secondary disk. So before block replication,
the primary disk and secondary disk should contain the same data.

== Failure Handling ==
There are 7 internal errors when block replication is running:
1. I/O error on primary disk
2. Forwarding primary write requests failed
3. Backup failed
4. I/O error on secondary disk
5. I/O error on active disk
6. Making active disk or hidden disk empty failed
7. Doing failover failed
In case 1 and 5, we just report the error to the disk layer. In case 2, 3,
4 and 6, we just report block replication's error to FT/HA manager (which
decides when to do a new checkpoint, when to do failover).
In case 7, if active commit failed, we use replication failover failed state
in Secondary's write operation (what decides which target to write).

== New block driver interface ==
We add four block driver interfaces to control block replication:
a. replication_start_all()
   Start block replication, called in migration/checkpoint thread.
   We must call block_replication_start_all() in secondary QEMU before
   calling block_replication_start_all() in primary QEMU. The caller
   must hold the I/O mutex lock if it is in migration/checkpoint
   thread.
b. replication_do_checkpoint_all()
   This interface is called after all VM state is transferred to
   Secondary QEMU. The Disk buffer will be dropped in this interface.
   The caller must hold the I/O mutex lock if it is in migration/checkpoint
   thread.
c. replication_get_error_all()
   This interface is called to check if error happened in replication.
   The caller must hold the I/O mutex lock if it is in migration/checkpoint
   thread.
d. replication_stop_all()
   It is called on failover. We will flush the Disk buffer into
   Secondary Disk and stop block replication. The vm should be stopped
   before calling it if you use this API to shutdown the guest, or other
   things except failover. The caller must hold the I/O mutex lock if it is
   in migration/checkpoint thread.

== Usage ==
Primary:
  -drive if=xxx,driver=quorum,read-pattern=fifo,id=colo1,vote-threshold=1,\
         children.0.file.filename=1.raw,\
         children.0.driver=raw

  Run qmp command in primary qemu:
    { 'execute': 'human-monitor-command',
      'arguments': {
          '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'
      }
    }
    { 'execute': 'x-blockdev-change',
      'arguments': {
          'parent': 'colo1',
          'node': 'nbd_client1'
      }
    }
  Note:
  1. There should be only one NBD Client for each primary disk.
  2. host is the secondary physical machine's hostname or IP
  3. Each disk must have its own export name.
  4. It is all a single argument to -drive and you should ignore the
     leading whitespace.
  5. The qmp command line must be run after running qmp command line in
     secondary qemu.
  6. After failover we need remove children.1 (replication driver).

Secondary:
  -drive if=none,driver=raw,file.filename=1.raw,id=colo1 \
  -drive if=xxx,id=topxxx,driver=replication,mode=secondary,top-id=topxxx\
         file.file.filename=active_disk.qcow2,\
         file.driver=qcow2,\
         file.backing.file.filename=hidden_disk.qcow2,\
         file.backing.driver=qcow2,\
         file.backing.backing=colo1

  Then run qmp command in secondary qemu:
    { 'execute': 'nbd-server-start',
      'arguments': {
          'addr': {
              'type': 'inet',
              'data': {
                  'host': 'xxx',
                  'port': 'xxx'
              }
          }
      }
    }
    { 'execute': 'nbd-server-add',
      'arguments': {
          'device': 'colo1',
          'writable': true
      }
    }

  Note:
  1. The export name in secondary QEMU command line is the secondary
     disk's id.
  2. The export name for the same disk must be the same
  3. The qmp command nbd-server-start and nbd-server-add must be run
     before running the qmp command migrate on primary QEMU
  4. Active disk, hidden disk and nbd target's length should be the
     same.
  5. It is better to put active disk and hidden disk in ramdisk.
  6. It is all a single argument to -drive, and you should ignore
     the leading whitespace.

After Failover:
Primary:
  The secondary host is down, so we should run the following qmp command
  to remove the nbd child from the quorum:
  { 'execute': 'x-blockdev-change',
    'arguments': {
        'parent': 'colo1',
        'child': 'children.1'
    }
  }
  { 'execute': 'human-monitor-command',
    'arguments': {
        'command-line': 'drive_del xxxx'
    }
  }
  Note: there is no qmp command to remove the blockdev now

Secondary:
  The primary host is down, so we should do the following thing:
  { 'execute': 'nbd-server-stop' }

TODO:
1. Continuous block replication
2. Shared disk
Commit	Line	Data
68365a38 WC	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	/ \ \|\|
69	/ \ \|\|
70	Primary 2 filter
71	disk ^ virtio-blk
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	\|\| drive-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 drive-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	== Failure Handling ==
110	There are 7 internal errors when block replication is running:
111	1. I/O error on primary disk
112	2. Forwarding primary write requests failed
113	3. Backup failed
114	4. I/O error on secondary disk
115	5. I/O error on active disk
116	6. Making active disk or hidden disk empty failed
117	7. Doing failover failed
118	In case 1 and 5, we just report the error to the disk layer. In case 2, 3,
119	4 and 6, we just report block replication's error to FT/HA manager (which
120	decides when to do a new checkpoint, when to do failover).
121	In case 7, if active commit failed, we use replication failover failed state
122	in Secondary's write operation (what decides which target to write).
123
124	== New block driver interface ==
125	We add four block driver interfaces to control block replication:
126	a. replication_start_all()
127	Start block replication, called in migration/checkpoint thread.
128	We must call block_replication_start_all() in secondary QEMU before
129	calling block_replication_start_all() in primary QEMU. The caller
130	must hold the I/O mutex lock if it is in migration/checkpoint
131	thread.
132	b. replication_do_checkpoint_all()
133	This interface is called after all VM state is transferred to
134	Secondary QEMU. The Disk buffer will be dropped in this interface.
135	The caller must hold the I/O mutex lock if it is in migration/checkpoint
136	thread.
137	c. replication_get_error_all()
138	This interface is called to check if error happened in replication.
139	The caller must hold the I/O mutex lock if it is in migration/checkpoint
140	thread.
141	d. replication_stop_all()
142	It is called on failover. We will flush the Disk buffer into
143	Secondary Disk and stop block replication. The vm should be stopped
144	before calling it if you use this API to shutdown the guest, or other
145	things except failover. The caller must hold the I/O mutex lock if it is
146	in migration/checkpoint thread.
147
148	== Usage ==
149	Primary:
150	-drive if=xxx,driver=quorum,read-pattern=fifo,id=colo1,vote-threshold=1,\
151	children.0.file.filename=1.raw,\
152	children.0.driver=raw
153
154	Run qmp command in primary qemu:
155	{ 'execute': 'human-monitor-command',
156	'arguments': {
157	'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'
158	}
159	}
160	{ 'execute': 'x-blockdev-change',
161	'arguments': {
162	'parent': 'colo1',
163	'node': 'nbd_client1'
164	}
165	}
166	Note:
167	1. There should be only one NBD Client for each primary disk.
168	2. host is the secondary physical machine's hostname or IP
169	3. Each disk must have its own export name.
170	4. It is all a single argument to -drive and you should ignore the
171	leading whitespace.
172	5. The qmp command line must be run after running qmp command line in
173	secondary qemu.
174	6. After failover we need remove children.1 (replication driver).
175
176	Secondary:
177	-drive if=none,driver=raw,file.filename=1.raw,id=colo1 \
178	-drive if=xxx,id=topxxx,driver=replication,mode=secondary,top-id=topxxx\
179	file.file.filename=active_disk.qcow2,\
180	file.driver=qcow2,\
181	file.backing.file.filename=hidden_disk.qcow2,\
182	file.backing.driver=qcow2,\
183	file.backing.backing=colo1
184
185	Then run qmp command in secondary qemu:
186	{ 'execute': 'nbd-server-start',
187	'arguments': {
188	'addr': {
189	'type': 'inet',
190	'data': {
191	'host': 'xxx',
192	'port': 'xxx'
193	}
194	}
195	}
196	}
197	{ 'execute': 'nbd-server-add',
198	'arguments': {
199	'device': 'colo1',
200	'writable': true
201	}
202	}
203
204	Note:
205	1. The export name in secondary QEMU command line is the secondary
206	disk's id.
207	2. The export name for the same disk must be the same
208	3. The qmp command nbd-server-start and nbd-server-add must be run
209	before running the qmp command migrate on primary QEMU
210	4. Active disk, hidden disk and nbd target's length should be the
211	same.
212	5. It is better to put active disk and hidden disk in ramdisk.
213	6. It is all a single argument to -drive, and you should ignore
214	the leading whitespace.
215
216	After Failover:
217	Primary:
218	The secondary host is down, so we should run the following qmp command
219	to remove the nbd child from the quorum:
220	{ 'execute': 'x-blockdev-change',
221	'arguments': {
222	'parent': 'colo1',
223	'child': 'children.1'
224	}
225	}
226	{ 'execute': 'human-monitor-command',
227	'arguments': {
228	'command-line': 'drive_del xxxx'
229	}
230	}
231	Note: there is no qmp command to remove the blockdev now
232
233	Secondary:
234	The primary host is down, so we should do the following thing:
235	{ 'execute': 'nbd-server-stop' }
236
237	TODO:
238	1. Continuous block replication
239	2. Shared disk