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Re: [libvirt] [Qemu-devel] [PATCH v2] qemu-nbd: Deprecate qemu-nbd --partition



On Fri, Jan 25, 2019 at 05:48:37PM -0600, Eric Blake wrote:
> The existing qemu-nbd --partition code claims to handle logical
> partitions up to 8, since its introduction in 2008 (commit 7a5ca86).
> However, the implementation is bogus (actual MBR logical partitions
> form a sort of linked list, with one partition per extended table
> entry, rather than four logical partitions in a single extended
> table), making the code unlikely to work for anything beyond -P5 on
> actual guest images. What's more, the code does not support GPT
> partitions, which are becoming more popular, and maintaining device
> subsetting in both NBD and the raw device is unnecessary duplication
> of effort (even if it is not too difficult).
> 
> Note that obtaining the offsets of a partition (MBR or GPT) can be
> learned by using 'qemu-nbd -c /dev/nbd0 file.qcow2 && sfdisk --dump
> /dev/nbd0', but by the time you've done that, you might as well
> just mount /dev/nbd0p1 that the kernel creates for you instead of
> bothering with qemu exporting a subset.  Or, keeping to just
> user-space code, use nbdkit's partition filter, which has already
> known both GPT and primary MBR partitions for a while, and was
> just recently enhanced to support arbitrary logical MBR parititions.
> 
> Start the clock on the deprecation cycle, with examples of how
> to write device subsetting without using -P.
> 
> Signed-off-by: Eric Blake <eblake redhat com>
> 
> ---
> v2: actual nbdkit example [Rich], improved doc wording
> ---
>  qemu-deprecated.texi | 33 +++++++++++++++++++++++++++++++++
>  qemu-nbd.texi        |  6 ++++--
>  qemu-nbd.c           |  2 ++
>  3 files changed, 39 insertions(+), 2 deletions(-)

Reviewed-by: Stefano Garzarella <sgarzare redhat com>

Thanks,
Stefano


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