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

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(a)redhat.com&gt; --- 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(-)