On 02/12/2015 06:26 PM, f...@redhat.com wrote:
> On Thu, 02/12 18:11, Wen Congyang wrote:
>> On 02/12/2015 05:44 PM, Fam Zheng wrote:
>>> On Thu, 02/12 17:33, Wen Congyang wrote:
>>>> On 02/12/2015 04:44 PM, Fam Zheng wrote:
>>>>> On Thu, 02/12 15:40, Wen Congyang wrote:
>>>>>> On 02/12/2015 03:21 PM, Fam Zheng wrote:
>>>>>>> Hi Congyang,
>>>>>>>
>>>>>>> On Thu, 02/12 11:07, Wen Congyang wrote:
>>>>>>>> +== 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 in the Disk buffer.
>>>>>>>
>>>>>>> I'm a little confused by the tenses ("will be" versus "are") and terms.
>>>>>>> I am
>>>>>>> reading them as "s/will be/are/g"
>>>>>>>
>>>>>>> Why do you need this buffer?
>>>>>>
>>>>>> We only sync the disk till next checkpoint. Before next checkpoint,
>>>>>> secondary
>>>>>> vm write to the buffer.
>>>>>>
>>>>>>>
>>>>>>> If both primary and secondary write to the same sector, what is saved
>>>>>>> in the
>>>>>>> buffer?
>>>>>>
>>>>>> The primary content will be written to the secondary disk, and the
>>>>>> secondary content
>>>>>> is saved in the buffer.
>>>>>
>>>>> I wonder if alternatively this is possible with an imaginary "writable
>>>>> backing
>>>>> image" feature, as described below.
>>>>>
>>>>> When we have a normal backing chain,
>>>>>
>>>>> {virtio-blk dev 'foo'}
>>>>> |
>>>>> |
>>>>> |
>>>>> [base] <- [mid] <- (foo)
>>>>>
>>>>> Where [base] and [mid] are read only, (foo) is writable. When we add an
>>>>> overlay
>>>>> to an existing image on top,
>>>>>
>>>>> {virtio-blk dev 'foo'} {virtio-blk dev 'bar'}
>>>>> | |
>>>>> | |
>>>>> | |
>>>>> [base] <- [mid] <- (foo) <---------------------- (bar)
>>>>>
>>>>> It's important to make sure that writes to 'foo' doesn't break data for
>>>>> 'bar'.
>>>>> We can utilize an automatic hidden drive-backup target:
>>>>>
>>>>> {virtio-blk dev 'foo'}
>>>>> {virtio-blk dev 'bar'}
>>>>> |
>>>>> |
>>>>> |
>>>>> |
>>>>> v
>>>>> v
>>>>>
>>>>> [base] <- [mid] <- (foo) <----------------- (hidden target)
>>>>> <--------------- (bar)
>>>>>
>>>>> v ^
>>>>> v ^
>>>>> v ^
>>>>> v ^
>>>>> >>>> drive-backup sync=none >>>>
>>>>>
>>>>> So when guest writes to 'foo', the old data is moved to (hidden target),
>>>>> which
>>>>> remains unchanged from (bar)'s PoV.
>>>>>
>>>>> The drive in the middle is called hidden because QEMU creates it
>>>>> automatically,
>>>>> the naming is arbitrary.
>>>>>
>>>>> It is interesting because it is a more generalized case of image fleecing,
>>>>> where the (hidden target) is exposed via NBD server for data scanning
>>>>> (read
>>>>> only) purpose.
>>>>>
>>>>> More interestingly, with above facility, it is also possible to create a
>>>>> guest
>>>>> visible live snapshot (disk 'bar') of an existing device (disk 'foo') very
>>>>> cheaply. Or call it shadow copy if you will.
>>>>>
>>>>> Back to the COLO case, the configuration will be very similar:
>>>>>
>>>>>
>>>>> {primary wr}
>>>>> {secondary vm}
>>>>> |
>>>>> |
>>>>> |
>>>>> |
>>>>> |
>>>>> |
>>>>> v
>>>>> v
>>>>>
>>>>> [what] <- [ever] <- (nbd target) <------------ (hidden buf disk)
>>>>> <------------- (active disk)
>>>>>
>>>>> v ^
>>>>> v ^
>>>>> v ^
>>>>> v ^
>>>>> >>>> drive-backup sync=none >>>>
>>>>
>>>> What is active disk? There are two disk images?
>>>
>>> It starts as an empty image with (hidden buf disk) as backing file, which in
>>> turn has (nbd target) as backing file.
>>
>> It's too complicated..., and I don't understand it.
>> 1. What is active disk? Use raw or a new block driver?
>
> It is an empty qcow2 image with the same lenght as your Secondary Disk.
I test qcow2_make_empty()'s performance. The result shows that it may
take about 100ms(normal sata disk). It is not acceptable for COLO. So
I think disk buff is necessary(just use it to replace qcow2).
Thanks
Wen Congyang
>
>> 2. Hidden buf disk use new block driver?
>
> It is an empty qcow2 image with the same lenght as your Secondary Disk, too.
>
>> 3. nbd target is hidden buf disk's backing image? If it is opened read-only,
>> we will
>> export a nbd with read-only BlockDriverState, but nbd server needs to
>> write it.
>
> NBD target is your Secondary Disk. It is opened read-write.
>
> The patches to enable opening it as read-write, and starting drive-backup
> between it and hidden buf disk, are all work in progress (the core concept) of
> image fleecing.
>
> Fam
>
>>>>>
>>>>> The workflow analogue is:
>>>>>
>>>>>>>> + 1) Primary write requests will be copied and forwarded to
>>>>>>>> Secondary
>>>>>>>> + QEMU.
>>>>>
>>>>> Primary write requests are forwarded to secondary QEMU as well.
>>>>>
>>>>>>>> + 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 in the Disk buffer.
>>>>>
>>>>> Before Primary write requests are written to (nbd target), aka the
>>>>> Secondary
>>>>> disk, the orignal sector content is read from it and copied to (hidden buf
>>>>> disk) by drive-backup. It obviously will not overwrite the data in (active
>>>>> disk).
>>>>>
>>>>>>>> + 3) Primary write requests will be written to Secondary disk.
>>>>>
>>>>> Primary write requests are written to (nbd target).
>>>>>
>>>>>>>> + 4) Secondary write requests will be buffered in the Disk buffer
>>>>>>>> and it
>>>>>>>> + will overwrite the existing sector content in the buffer.
>>>>>
>>>>> Secondary write request will be written in (active disk) as usual.
>>>>>
>>>>> Finally, when checkpoint arrives, if you want to sync with primary, just
>>>>> drop
>>>>> data in (hidden buf disk) and (active disk); when failover happends, if
>>>>> you
>>>>> want to promote secondary vm, you can commit (active disk) to (nbd
>>>>> target), and
>>>>> drop data in (hidden buf disk).
>>>>>
>>>>> Fam
>>>>> .
>>>>>
>>>>
>>>>
>>> .
>>>
>>
> .
>
