[Qemu-devel] [PATCH COLO-BLOCK v7 13/17] docs: block replication's description

[Wen Congyang]

Signed-off-by: Wen Congyang <address@hidden>
Signed-off-by: Yang Hongyang <address@hidden>
Signed-off-by: zhanghailiang <address@hidden>
Signed-off-by: Gonglei <address@hidden>
---
 docs/block-replication.txt | 179 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 179 insertions(+)
 create mode 100644 docs/block-replication.txt

diff --git a/docs/block-replication.txt b/docs/block-replication.txt
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+Block replication
+----------------------------------------
+Copyright Fujitsu, Corp. 2015
+Copyright (c) 2015 Intel Corporation
+Copyright (c) 2015 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 (COurse-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 Primary VM 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 at the time of checkpoint, the disk modification operations of
+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
+
+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 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 be 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 be an empty disk, and
+the driver supports bdrv_make_empty() and backing file.
+
+== Failure Handling ==
+There are 6 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
+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).
+There is one internal error when doing failover:
+1. Commiting the data in active disk/hidden disk to secondary disk failed
+We just to report this error to FT/HA manager.
+
+== New block driver interface ==
+We add three block driver interfaces to control block replication:
+a. bdrv_start_replication()
+   Start block replication, called in migration/checkpoint thread.
+   We must call bdrv_start_replication() in secondary QEMU before
+   calling bdrv_start_replication() in primary QEMU. The caller
+   must hold the I/O mutex lock if it is in migration/checkpoint
+   thread.
+b. bdrv_do_checkpoint()
+   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. bdrv_stop_replication()
+   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. 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,no-connect=on,\
+         children.0.file.filename=1.raw,\
+         children.0.driver=raw,\
+         children.1.file.driver=nbd,\
+         children.1.file.host=xxx,\
+         children.1.file.port=xxx,\
+         children.1.file.export=xxx,\
+         children.1.driver=replication,\
+         children.1.mode=primary,\
+         children.1.ignore-errors=on
+  Note:
+  1. NBD Client should not be the first child of quorum.
+  2. There should be only one NBD Client.
+  3. host is the secondary physical machine's hostname or IP
+  4. Each disk must have its own export name.
+  5. It is all a single argument to -drive, and you should ignore
+     the leading whitespace.
+
+Secondary:
+  -drive if=none,driver=raw,file=1.raw,id=nbd_target1 \
+  -drive if=xxx,driver=replication,mode=secondary,export=xxx,\
+         file.file.filename=active_disk.qcow2,\
+         file.driver=qcow2,\
+         file.backing_reference.drive_id=nbd_target1,\
+         file.backing_reference.hidden-disk.file.filename=hidden_disk.qcow2,\
+         file.backing_reference.hidden-disk.driver=qcow2,\
+         file.backing_reference.hidden-disk.allow-write-backing-file=on
+  Then run qmp command:
+    nbd-server-start host:port
+  Note:
+  1. The export name for the same disk must be the same in primary
+     and secondary QEMU command line
+  2. The qmp command nbd-server-start must be run before running the
+     qmp command migrate on primary QEMU
+  3. Don't use nbd-server-start's other options
+  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.
-- 
2.4.3