Re: How to create vhdx differencing disk using qemu-img

[John Snow]

On 3/17/21 10:37 PM, qi zhou wrote:
> When I create vhdx differencing disk using qemu-img, It says
>   qemu-img: xxx.vhd Backing file not supported for file format 'vhdx'
>
> The command I used is
> qemu-img create -f vhdx -b test.vhdx test-snapshot.vhdx
>
> Here is my questions
> 1. Is vhdx format [full] supported by qemu ?

We support raw and qcow2 fully, read-write.

Everything else is "read-only", though write support might work, we 
don't encourage its use in production environments.

> 2. If not, is there any easy way to implement differencing disk of vhdx in 
> qemu-img ?

the -b flag is generally for qcow2 files; I am not very familiar with 
VHDX but it appears as though we don't support it here.

I imagine it's 
https://www.altaro.com/hyper-v/hyper-v-differencing-disks-explained/ ?

I don't think we support those... ah, yeah, in block/vhdx.c:

typedef enum VHDXImageType {
    VHDX_TYPE_DYNAMIC = 0,
    VHDX_TYPE_FIXED,
    VHDX_TYPE_DIFFERENCING,   /* Currently unsupported */
} VHDXImageType;

> 3. Is there any other tools support vhdx on linux ?

Not that I'm aware of, but I can't say I've looked before.