Re: [PATCH v2 2/2] iotests/244: Test preallocation for data-file-raw

[Eric Blake <eblake@redhat.com>]

On 3/26/21 9:55 AM, Max Reitz wrote:
> Three test cases:
> (1) Adding a qcow2 (metadata) file to an existing data file, see whether
>     we can read the existing data through the qcow2 image.
> (2) Append data to the data file, grow the qcow2 image accordingly, see
>     whether we can read the new data through the qcow2 image.
> (3) At runtime, add a backing image to a freshly created qcow2 image
>     with an external data file (with data-file-raw).  Reading data from
>     the qcow2 image must return the same result as reading data from the
>     data file, so everything in the backing image must be ignored.
>     (This did not use to be the case, because without the L2 tables
>     preallocated, all clusters would appear as unallocated, and so the
>     qcow2 driver would fall through to the backing file.)
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  tests/qemu-iotests/244     | 104 +++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/244.out |  59 +++++++++++++++++++++
>  2 files changed, 163 insertions(+)
> 

> +
> +# We cannot use qemu-img to create the qcow2 image, because it would
> +# clear the data file.  Use the blockdev-create job instead, which will
> +# only format the qcow2 image file.

Well, perhaps we could use qemu-img to create a qcow2 pointing to a
temporary file, then rewrite it to point to the real data file, but that
feels hackish, and your approach worked.  And besides, while we have
qemu-img rebase -u to rewrite the backing file, I don't know if we have
a similar qemu-img command for rewriting the data file.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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