Re: [PATCH v3 1/4] block: Add trivial backing_fmt support to qcow, sheepdog, vmdk

On 3/9/20 10:21 AM, Kevin Wolf wrote: Do we know what formats qcow, sheepdog, and vmdk expect to probe? I'm wondering if we can compromise by checking that the requested backing image has the specified format, and error if it is not, rather than completely ignoring it - but at the same time, the image formats have no where to record a backing format. I'm guessing that qcow works with either raw or qcow as backing format (and anything else is odd - a qcow2 backing to a qcow is unusual, and would be better to reject). I'm not sure if sheepdog can be backed by anything but another sheepdog, similarly, I'm not sure if a vmdk can be backed by anything but another vmdk. If so, it should be simple enough to do a v4 of this patch which requires -F to be a known-acceptable probe type for these images. Still, the point of this patch is that I want to add -F into all the iotests, and without something along the lines of this patch, all of those iotests are broken for these image formats. Patch 2 is a lot harder to write if we have to make our use of -F conditional on the image format in question. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org