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Re: [Qemu-devel] qcow2 resize with snapshots

From: Kevin Wolf
Subject: Re: [Qemu-devel] qcow2 resize with snapshots
Date: Tue, 3 May 2016 10:44:32 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

[ 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:

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.


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