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Re: [libvirt] [PATCH] qemu-nbd: Deprecate qemu-nbd --partition

On 1/23/19 3:55 PM, Richard W.M. Jones wrote:
> On Wed, Jan 23, 2019 at 03:19:53PM -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 maintenance
>> burden.  And nbdkit has just added code to properly handle an
>> arbitrary number of MBR partitions, along with its existing code
>> for handling GPT partitions.
>> Note that obtaining the offsets of a partition 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.
>> Start the clock on the deprecation cycle, with an example of how
>> to write device subsetting without using -P.

>> +For example, if partition 1 is 100MiB starting at 1MiB, the old command
>> +
>> + example{qemu-nbd -P 1 -f qcow2 file.qcow2}
>> +
>> +can be rewritten as:
>> +
>> + example{qemu-nbd --image-opts driver=raw,offset=1M,size=100M,file.driver=qcow2,file.backing.driver=file,file.backing.filename=file.qcow2}
>> +
>> +Alternatively, the @code{nbdkit} project provides a more powerful
>> +partition filter on top of its nbd plugin, which can be used to select
>> +an arbitrary MBR or GPT partition on top of any other full-image NBD
>> +export.
> You might want to add the actual command here.

Good idea - as long as we are deprecating something, telling the user
how to get the same functionality (in this case, user-space partition
detection, without involving /dev/nbd) is worth the extra effort.

>  Unfortunately nbdkit
> cannot read qcow2 files meaning (as you note already) that you have to
> forward the connection through the nbdkit-nbd-plugin to qemu-nbd.
> This worked for me:
>   qemu-nbd -t -k /tmp/sock -f qcow2 file.qcow2 &
>   nbdkit -f --filter=partition nbd socket=/tmp/sock partition=1 &

Is the -f necessary? Otherwise, yes, this looks reasonable.  I'll add it
for v2.

> If you drop the requirement to demonstrate this with qcow2 then the
> command would be just this:
>   nbdkit --filter=partition file disk.raw partition=1

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

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