Re: [PATCH v3 4/4] qemu-img: Deprecate use of -b without -F

On Tue, Mar 10, 2020 at 07:19:25AM -0500, Eric Blake wrote: (Slightly long e-mail, as it contains a bunch of tests and their results; please bear with me.) [...] I see; okay, that's expected. I couldn't work out the black magic to change the backing file via 'qemu-img amend'. It is surely not this: $> ~/build/qemu/qemu-img amend -o 'backing_file=./bar.qcow2' -o another_base.qcow2 qemu-img: Expecting one image file name Let's try something else: give a *non-existent* "bar.qcow2" to '-o': $> ~/build/qemu/qemu-img amend -o 'backing_file=./bar.qcow2' another_base.qcow2 qemu-img: Could not open 'another_base.qcow2': Could not open backing file: Failed to get shared "write" lock Is another process using the image [./another_base.qcow2]? That's strange; there is no live QEMU process on this host (let alone one that is using another_base.qcow2): $> pgrep qemu-system-x86 $> echo $? 1 Probably it is just complaning about the non-existent bar.qcow2 file? Then I'd expect it to say as much. On IRC you pointed out iotest 082 to look for help. There I don't see a way to change the backing file. But only a combination of 'amend' + 'rebase': run_qemu_img amend -f $IMGFMT \ -o backing_fmt=$IMGFMT,backing_file="$TEST_IMG",,\? "$TEST_IMG" run_qemu_img rebase -u -b "" -f $IMGFMT "$TEST_IMG" (I know you can use 'rebase' alone to change the backing file format.) Note to self: we really need to document 'amend' much better, in which scenarios it is useful, and contrast it with 'rebase'. - - - Meanwhile, I've done a bunch of tests with 'amend'. Here are the results. Scenario: base.raw <-- overlay1.qcow2 ------------------------------------- Without "-f raw", the warning is provoked when trying to amend the backing file (let's ignore for a moment that you can't seem to amend a raw file): $> ~/build/qemu/qemu-img amend -o compat=v3 base.raw WARNING: Image format was not specified for 'base.raw' and probing guessed raw. Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted. Specify the 'raw' format explicitly to remove the restrictions. qemu-img: Format driver 'raw' does not support option amendment With "-f raw", the warning is not triggerred (correctly so?): $> ~/build/qemu/qemu-img amend -o compat=v3 -f raw base.raw qemu-img: Format driver 'raw' does not support option amendment And these two tests (one with relative path; the other with absolute path) don't trigger the warning either (on IRC you said the following is _supposed_ to trigger a warning): $> ~/build/qemu/qemu-img amend \ -o 'backing_file=base.raw' -f qcow2 overlay1.qcow2 $> ~/build/qemu/qemu-img amend \ -o 'backing_file=./base.raw' -f qcow2 overlay1.qcow2 'qemu-img info' of the above disk image chain: $> ~/build/qemu/qemu-img info --backing-chain overlay1.qcow2 image: overlay1.qcow2 file format: qcow2 virtual size: 4 GiB (4294967296 bytes) disk size: 196 KiB cluster_size: 65536 backing file: ./base.raw backing file format: raw Format specific information: compat: 1.1 lazy refcounts: false refcount bits: 16 corrupt: false image: ./base.raw file format: raw virtual size: 4 GiB (4294967296 bytes) disk size: 778 MiB Scenario: another_base.qcow2 <-- overlay1_of_ab.qcow2 ----------------------------------------------------- With and w/o specifying the aAmend the backing file (none of these provoke the warning -- expected?): $> ~/build/qemu/qemu-img amend another_base.qcow2 $> ~/build/qemu/qemu-img amend -f qcow2 another_base.qcow2 Tests to amend the overlay file (none of these provoke the warning -- expected, per your previous reply): $> ~/build/qemu/qemu-img amend overlay1_of_ab.qcow2 $> ~/build/qemu/qemu-img amend -f qcow2 overlay1_of_ab.qcow2 'qemu-img info' of the above disk image chain: $> ~/build/qemu/qemu-img info --backing-chain overlay1_of_ab.qcow2 image: overlay1_of_ab.qcow2 file format: qcow2 virtual size: 4 GiB (4294967296 bytes) disk size: 196 KiB cluster_size: 65536 backing file: ./another_base.qcow2 Format specific information: compat: 1.1 lazy refcounts: false refcount bits: 16 corrupt: false image: ./another_base.qcow2 file format: qcow2 virtual size: 4 GiB (4294967296 bytes) disk size: 293 MiB cluster_size: 65536 Format specific information: compat: 1.1 lazy refcounts: false refcount bits: 16 corrupt: false Strange command-line -------------------- Does this make sense? What is this even trying to do ... $> ~/build/qemu/qemu-img amend \ -o 'backing_file=./another_base.qcow2' another_base.qcow2 $> echo $? 0 Will look. Thanks. -- /kashyap