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

[John Snow <jsnow@redhat.com>]

On 02/13/2015 02:01 AM, Fam Zheng wrote:
> On Fri, 02/13 13:09, Wen Congyang wrote:
>> What is image fleecing?
>>
>
> It's the name of the feature which enables the built-in NBD server to exporting
> a thin point-in-time snapshot created via drive-backup sync=none.
>
> It's for host side data scanning tool to access a disk snapshot of running VM.
> The workflow in theory is:
>
> 1. guest uses "disk0" as its virtio-blk device.
>
> 2. in qmp, use blockdev-add (drive-backup) to add an empty "target0" qcow2
> image, that uses "disk0" as its backing file, and use nbd-server-add to export
> this empty image with NBD. This way, all reads coming from NBD client will
> produce data of "disk0".
>
> 3. in qmp, start blockdev-backup from "disk0" to "target0" with "sync=none".
> After this point, all guest data written to "disk0" will COW the original data
> to "target0", in other words, reading "target0" will effectively produce a
> point-in-time snapshot of the time when blockdev-backup started.
>
> 4. after step 3, the disk data seen by the NBD client is the stable snapshot.
> Because of the COW mechanism in blockdev-backup, "target0" is thin, and can be
> dropped once the inspection process is done.
>
>>   Are you implementing it now?
>
> I worked on it. Most parts of the series is merged, the remaining part is
> relatively small, namely to
>
> 1) enable adding "target0" in step 2 (currently in blockdev-add it's not
> possible to reference an existing drive as backing file);
>
> 2) enable "blockdev-backup" from "disk0" to "target0", which is obviously not
> possible because 1) is not done.
>
> I do have the patches at my tree, just that they need to be refreshed. :)
>
> https://github.com/famz/qemu/tree/image-fleecing
>
> Fam
>

I had intended to pick up these patches after I got incremental backup 
working, as Fam had started both of these projects and I inherited them 
-- though I hadn't begun work in earnest on refining and testing this 
particular feature yet.

--js