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



On Wed, Jan 23, 2019 at 04:18:16PM -0600, Eric Blake wrote:
> 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.

It's not necessary, but it makes nbdkit behave the same way with
respect to remaining in the foreground as qemu-nbd.

Rich.

> > 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
> 




-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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