On Tue, Mar 10, 2020 at 07:19:25AM -0500, Eric Blake wrote:
> On 3/10/20 5:57 AM, Kashyap Chamarthy wrote:
(Slightly long e-mail, as it contains a bunch of tests and their
results; please bear with me.)
[...]
> > $> ~/build/qemu/qemu-img amend -o compat=v3 overlay1.qcow2
>
> This particular amend is not changing the backing file, and since I did not
> add warnings on the opening of an existing unsafe image, it is silent.
I see; okay, that's expected.
> Instead, you need to test a case where amend provokes a path that would
> change the backing file (perhaps as simple as '-o backing_file=./base.raw'),
> while omitting a format for the new backing file string.
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
> Look at patch 2/4 - that patch was written AFTER this patch in order to
> silence every warning that was introduced because of this patch, then
> rebased to occur first. My experience in writing 2/4 was that I indeed hit
> warnings through all four sub-commands.
Will look.
Thanks.
--
/kashyap
