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

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.

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 &

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