Re: [Qemu-devel] [Qemu-block] [RFC PATCH COLO v2 01/13] docs: block replication's description

[Eric Blake <eblake@redhat.com>]

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On 03/25/2015 03:36 AM, Wen Congyang wrote:
> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
> ---
>  docs/block-replication.txt | 147 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 147 insertions(+)
>  create mode 100644 docs/block-replication.txt
> 

Grammar review only (I'll leave the technical review to others)

> diff --git a/docs/block-replication.txt b/docs/block-replication.txt
> new file mode 100644
> index 0000000..874ed8e
> --- /dev/null
> +++ b/docs/block-replication.txt
> @@ -0,0 +1,147 @@
> +Block replication
> +----------------------------------------
> +Copyright Fujitsu, Corp. 2015
> +Copyright (c) 2015 Intel Corporation
> +Copyright (c) 2015 HUAWEI TECHNOLOGIES CO.,LTD.

Space after comma in English writing.

> +
> +This work is licensed under the terms of the GNU GPL, version 2 or later.
> +See the COPYING file in the top-level directory.
> +
> +The block replication is used for continuous checkpoints. It is designed

Sounds better as either of:
The block replication feature is...
Block replication is...

> +for COLO that Secondary VM is running. It can also be applied for FT/HA

Please define COLO and FT/HA on first use (okay to abbreviate elsewhere
in the document, but the first use should not assume the acronym is
well-known)

s/for COLO that/for COLO (COurse-grain LOck-stepping), where the/

> +scene that Secondary VM is not running.

s/for FT/HA scene that/for the FT/HA (Fault-tolerance/High Assurance)
scenario, where the/

> +
> +This document gives an overview of block replication's design.
> +
> +== Background ==
> +High availability solutions such as micro checkpoint and COLO will do
> +consecutive checkpoint. The VM state of Primary VM and Secondary VM is

s/checkpoint/checkpoints/

> +identical right after a VM checkpoint, but becomes different as the VM
...
> +
> +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 dirver supports bdrv_make_empty().

s/dirver/driver/

> +
> +== 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 transfered to

s/transfered/transferred/

> +   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 when failover. We will flush the Disk buffer into

s/when/on/

> +   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

This command line looks like multiple arguments because of the leading
whitespace on succeeding lines.  I don't know if there is any better way
to format it, though, to make it obvious that it is all a single
argument to -drive.

> +  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.

Maybe a note 5 to call out the formatting aspect of the command line?

> +
> +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.
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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