Re: [Qemu-devel] [PATCH COLO v3 01/14] docs: block replication's description

[Dr. David Alan Gilbert]

* Wen Congyang (address@hidden) wrote:
> Signed-off-by: Wen Congyang <address@hidden>
> Signed-off-by: Paolo Bonzini <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 | 153 
> +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 153 insertions(+)
>  create mode 100644 docs/block-replication.txt
> 
> diff --git a/docs/block-replication.txt b/docs/block-replication.txt
> new file mode 100644
> index 0000000..4426ffc
> --- /dev/null
> +++ b/docs/block-replication.txt
> @@ -0,0 +1,153 @@
> +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 COLO block replication from many basic
> +blocks that are already in QEMU.
> +
> +         virtio-blk       ||
> +             ^            ||                            .----------
> +             |            ||                            | Secondary
> +        1 Quorum          ||                            '----------
> +         /      \         ||
> +        /        \        ||
> +   Primary      2 NBD  ------->  2 NBD
> +     disk       client    ||     server                                      
>    virtio-blk
> +                          ||        ^                                        
>         ^
> +--------.                 ||        |                                        
>         |
> +Primary |                 ||  Secondary disk <--------- hidden-disk 4 
> <--------- active-disk 3
> +--------'                 ||        |          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 secondary disk receives writes from the primary VM through QEMU's
> +embedded NBD server (speculative write-through).
> +
> +3) 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
> +be qcow2.

If active_disk is empty, how do you get an initial copy of the primary's
disk contents over to the secondary?

It would be interesting to consider how this would change to support
recovery back to the point where we have a pair of machines after the 
primary failed (continuous ft);  somehow the drive on the secondary would
have to flip over to being a quorum set and look like the current primary.

Dave

> +
> +4) 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().
> +
> +== 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.
> +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,\
> +         children.0.file.filename=1.raw,\
> +         children.0.driver=raw,\
> +         children.1.file.driver=nbd+colo,\
> +         children.1.file.host=xxx,\
> +         children.1.file.port=xxx,\
> +         children.1.file.export=xxx,\
> +         children.1.driver=raw,\
> +         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=qcow2+colo,file=active_disk.qcow2,export=xxx,\
> +         backing_reference.drive_id=nbd_target1,\
> +         backing_reference.hidden-disk.file.filename=hidden_disk.qcow2,\
> +         backing_reference.hidden-disk.driver=qcow2,\
> +         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.
> -- 
> 2.1.0
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK