On Wed, 02/25 17:10, 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 >>>>
>
> Why nbd target has backing image ever?
It's not strictly necessary, depending on your VM disk configuration. (for
example at the time of vm booting, your image already points to a backing file,
etc.
Fam
