On 1/25/19 5:48 PM, 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 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@xxxxxxxxxx> > > --- > v2: actual nbdkit example [Rich], improved doc wording Thanks; queued for my next NBD pull request. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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