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

[Wen Congyang]

On 03/25/2015 11:38 PM, Eric Blake wrote:
> On 03/25/2015 03:36 AM, Wen Congyang 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 | 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.

Yes, but I am not sure I can change it. HUAWEI always use this way.
You can find it in bootdevice.c

Thanks
Wen Congyang

> 
>> +
>> +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.
>>
>