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

[Fam Zheng <famz@redhat.com>]

On Wed, 02/25 16:36, Wen Congyang wrote:
> On 02/25/2015 10:46 AM, Fam Zheng wrote:
> > On Tue, 02/24 15:50, 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.
> >>
> >> I don't understand this. In which function, the hidden target is created automatically?
> >>
> > 
> > It's to be determined. This part is only in my mind :)
> 
> Does hidden target is only used for COLO?
> 

I'm not sure I get your question.

In this case yes, this is a dedicate target that's only written to by COLO's
secondary VM.

In other general cases, this infrastructure could also be used for backup or
image fleecing.

Fam

> 
> > 
> > Fam
> > 
> >>
> >>>
> >>> 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 >>>>
> >>>
> >>> 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
> >>> .
> >>>
> >>
> > .
> > 
> 
>