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
> >>> .
> >>>
> >>
> > .
> >
>
>
