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