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Re: [Qemu-devel] qcow2 resize with snapshots

From: zhangzhiming
Subject: Re: [Qemu-devel] qcow2 resize with snapshots
Date: Sat, 7 May 2016 10:47:13 +0800

thank you for your reply, and i am glad to join to the development of qemu.
i will try my best to finish this new function.

have a good day!


> On May 3, 2016, at 4:44 PM, Kevin Wolf <address@hidden> wrote:
> [ Cc: qemu-block ]
> Am 29.04.2016 um 10:59 hat zhangzm geschrieben:
>> hi, i want to implement the function of qcow2 resize which has
>> snapshots.
>> each snapshot of qcow2 will have a separate total size, and when apply
>> a snapshot, the image can be shrunk, and the total size of image will
>> change after apply to a snapshot with different size.
>> now, there is a disk_size value in struct QcowSnapshot, i only need to
>> change the size of current active image layer when apply a snapshot
>> with different size, and the io request will be limit in the range of
>> active layer.
> Yes, I think today the qcow2 format provides everything that is needed
> to implement this. You need to make sure that we have a v3 image so that
> the virtual disk size is actually stored in the snapshot (this field did
> not exist in v2 images yet).
> What you need to consider is that loading a snapshot becomes similar to
> resizing an image then and you need to do the same things for it. For
> example, we need to figure out what to do with associated dirty bitmaps
> (adapt them to the new size like in bdrv_truncate()?), call resize
> callbacks so that the guest devices actually see the changes size and
> possibly also consider the BLOCK_OP_TYPE_RESIZE blockers to prevent a
> size change while the image is in use.
>> and i want my code merged into the master of qemu.
> The wiki has a few tips on how to submit patches for qemu:
> http://wiki.qemu.org/Contribute/SubmitAPatch
> For a patch (or patch series) like this, you will want to CC at least
> qemu-block and qemu-devel, plus possibly a few individual people.
> Kevin

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