[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description
From: |
Fam Zheng |
Subject: |
Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description |
Date: |
Thu, 26 Feb 2015 16:44:47 +0800 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Thu, 02/26 14:38, 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 :)
>
> What about this:
> -drive file=nbd-target,if=none,id=nbd-target0 \
> -drive
> file=active-disk,if=virtio,driver=qcow2,backing.file.filename=hidden-disk,backing.driver=qcow2,backing.backing=nbd-target0
>
It's close. I suppose backing.backing is referencing another drive as its
backing_hd, then you cannot have the other backing.file.* option - they
conflict. It would be something along:
-drive file=nbd-target,if=none,id=nbd-target0 \
-drive file=hidden-disk,if=none,id=hidden0,backing.backing=nbd-target0 \
-drive file=active-disk,if=virtio,driver=qcow2,backing.backing=hidden0
Or for simplicity, s/backing.backing=/backing=/g
Yes, adding these "backing=$drive_id" option is also exactly what we expect
in order to support image-fleecing, but we haven't figured how to allow that
without breaking other qmp operations like block jobs, etc.
Fam
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, (continued)
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Hongyang Yang, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/24
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/24
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/25
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/25
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/25
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/26
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description,
Fam Zheng <=
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/26
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/26
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/26
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/26
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/25
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/25
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/25
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/25
[Qemu-devel] [RFC PATCH 04/14] Add new block driver interfaces to control disk replication, Wen Congyang, 2015/02/11