Re: qcow2: Zero-initialization of external data files

[Eric Blake <eblake@redhat.com>]

On 2/17/20 10:56 AM, Max Reitz wrote:
> Hi,
> 
> AFAIU, external data files with data_file_raw=on are supposed to return
> the same data as the qcow2 file when read.  But we still use the qcow2
> metadata structures (which are by default initialized to “everything
> unallocated”), even though we never ensure that the external data file
> is zero, too, so this can happen:
> 
> $ dd if=/dev/urandom of=foo.raw 64M
> [...]
> 
> $ sudo losetup -f --show foo.raw
> /dev/loop0
> 
> $ sudo ./qemu-img create -f qcow2 -o \
>      data_file=/dev/loop0,data_file_raw=on foo.qcow2 64M
> [...]
> 
> $ sudo ./qemu-io -c 'read -P 0 0 64M' foo.qcow2
> read 67108864/67108864 bytes at offset 0
> 64 MiB, 1 ops; 00.00 sec (25.036 GiB/sec and 400.5751 ops/sec)

This looks like a bug (and we should fix it for 5.0 if possible) - read 
of a data_file_raw=on should not treat unallocated clusters as reading 
0, but rather as reading whatever the raw data contains.

> 
> $ sudo ./qemu-io -c 'read -P 0 0 64M' -f raw foo.raw
> Pattern verification failed at offset 0, 67108864 bytes
> read 67108864/67108864 bytes at offset 0
> 64 MiB, 1 ops; 00.01 sec (5.547 GiB/sec and 88.7484 ops/sec)
> 
> I suppose this behavior is fine for blockdev-create because I guess it’s
> the user’s responsibility to ensure that the external data file is zero.
>   But maybe it isn’t, so that’s my first question: Is it really the
> user’s responsibility or should we always ensure it’s zero?

I'd argue that requiring the user to pre-zero the raw data file is 
undesirable; and that we should instead fix our code to not report the 
image as reading all zeroes when creating with data_file_raw=on.

> 
> My second question is: If we decide that this is fine for
> blockdev-create, should at least qcow2_co_create_opts() ensure the data
> file it just created is zero?

Having an option to make qemu force-zero the raw image during 
qcow2_co_create_opts seems reasonable, but for performance reasons, I 
don't think the flag should be on by default.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org