From: Max Reitz <mreitz@redhat.com>
To: John Snow <jsnow@redhat.com>, qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH 7/7] iotests: Disable 126 for some vmdk subformats
Date: Wed, 14 Aug 2019 16:01:13 +0200 [thread overview]
Message-ID: <8cdb2e48-5c9b-289a-0a2c-49cd0e6b623d@redhat.com> (raw)
In-Reply-To: <5496dd9a-1a78-4a5b-b84b-fb870ad409e5@redhat.com>
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On 14.08.19 00:26, John Snow wrote:
>
>
> On 8/13/19 10:00 AM, Max Reitz wrote:
>> On 12.08.19 23:33, John Snow wrote:
>>>
>>>
>>> On 7/25/19 11:57 AM, Max Reitz wrote:
>>>> Several vmdk subformats do not work with iotest 126, so disable them.
>>>>
>>>> (twoGbMaxExtentSparse actually should work, but fixing that is a bit
>>>> difficult. The problem is that the vmdk descriptor file will contain a
>>>> referenc to "image:base.vmdk", which the block layer cannot open because
>>>
>>> reference
>>>
>>>> it does not know the protocol "image". This is not trivial to solve,
>>>> because I suppose real protocols like "http://" should be supported.
>>>> Making vmdk treat all paths with a potential protocol prefix that the
>>>> block layer does not recognize as plain files seems a bit weird,
>>>> though. Ignoring this problem does not seem too bad.)
>>>>
>>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>>> ---
>>>> tests/qemu-iotests/126 | 6 ++++++
>>>> 1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/tests/qemu-iotests/126 b/tests/qemu-iotests/126
>>>> index 9b0dcf9255..8e55d7c843 100755
>>>> --- a/tests/qemu-iotests/126
>>>> +++ b/tests/qemu-iotests/126
>>>> @@ -33,6 +33,12 @@ status=1 # failure is the default!
>>>>
>>>> # Needs backing file support
>>>> _supported_fmt qcow qcow2 qed vmdk
>>>> +# (1) Flat vmdk images do not support backing files
>>>> +# (2) Split vmdk images simply fail this test right now. Fixing that
>>>> +# is left for another day.
>>>
>>> Which one? :)
>>
>> Hmmmm? Fixing refers to #2. #1 is not a bug or missing feature, it’s
>> just how it is. (This test needs backing files, so...)
>>
>> If you mean “which are which“, then the ones with *Flat are flat images
>> (:-)), and the ones with twoGbMaxExtent* are split.
>>
>
> "Which day" ;)
>
>>>> +_unsupported_imgopts "subformat=monolithicFlat" \
>>>> + "subformat=twoGbMaxExtentFlat" \
>>>> + "subformat=twoGbMaxExtentSparse"
>>>> # This is the default protocol (and we want to test the difference between
>>>> # colons which separate a protocol prefix from the rest and colons which are
>>>> # just part of the filename, so we cannot test protocols which require a prefix)
>>>>
>>>
>>> What exactly fails?
>>
>> Interestingly I only now noticed that the test passes with “vmdk: Use
>> bdrv_dirname() for relative extent paths” (patch 2) reverted...
>>
>>> Does the VMDK driver see `image:` and think it's a
>>> special filename it needs to handle and fails to do so?
>> No. Whenever the block layer sees a parsee filename[1] with a colon
>> before a slash, it thinks everything before the colon is a protocol
>> prefix. For example:
>>
>
> Actually, I think we're on the same page here. I maybe meant to type
> "block layer" instead of "VMDK driver", but it does look like it does
> special processing on this sort of filename that breaks in this case.
>
>> $ qemu-img info foo:bar
>> qemu-img: Could not open 'foo:bar': Unknown protocol 'foo'
>>
>> This test is precisely for this. How can you specify an image filename
>> that has a colon in it (without using -blockdev)? One way is to prepend
>> it with “./”, the other is “file:”.
>>
>> Now with split VMDKs, we must write something in the header file to
>> reference the extents. What vmdk does for an image like
>> “image:foo.vmdk” is it writes “image:foo-s001.vmdk” there.
>>
>> When it tries to open that extent, what happens depends on whether
>> “vmdk: Use bdrv_dirname() for relative extent paths” (patch 2) is applied:
>>
>> --- Before that patch ---
>>
>> vmdk takes the descriptor filename, which, thanks to some magic in the
>> block layer, is always “./image:foo.vmdk”, even when you gave it as
>> “file:image:foo.vmdk” (the “file:” is stripped because it does nothing,
>> generally, and the “./” is then prepended because of the false protocol
>> prefix “image:”).
>>
>> It then invokes path_combine() with that path and the path given in the
>> descriptor file (“image:foo-s001.vmdk”). This yields
>> “./image:foo-s001.vmdk”, which actually works.
>>
>> --- After that patch ---
>>
>> OK, what I messed up is that I just took the extent path to be an
>> absolute path if it has a protocol prefix. (Because that’s how we
>> usually do it.) Turns out that vmdk never did that, and path_combine()
>> actually completely ignores protocol prefixes in the relative filename.
>>
>> I suppose I could do the same and just drop the path_has_protocol() from
>> patch 2. But that’d be a bit broken, as I wrote in the commit
>> message... If the descriptor file refers to an extent on
>> “http://example.com/extent.vmdk”, I suppose that should not be
>> interpreted as a relative path, but actually work...
>>
>> But anyway, I guess if it’s a bit broken already, I might just keep it
>> that way.
>>
>>
>> tl;dr: Turns out patch 2 broke this test, because it (accidentally)
>> tried to fix something that I consider broken. If I just keep it broken
>> (I didn’t know it was), this test will continue to work and probably
>> nobody will care because, well, it already is broken and nobody cares.
>>
>
> So which kinda-broken thing are you making the case for? Are you
> re-spinning in light of your discovery or... are we fine with the state
> of things as they land here?
>
> (Sorry, it wasn't clear to me which way you were leaning.)
I’m going to respin and drop the “path_has_protocol(fname)” condition
from patch 2.
Max
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prev parent reply other threads:[~2019-08-14 14:02 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-25 15:57 [Qemu-devel] [PATCH 0/7] vmdk: Misc fixes Max Reitz
2019-07-25 15:57 ` [Qemu-devel] [PATCH 1/7] iotests: Fix _filter_img_create() Max Reitz
2019-08-12 19:32 ` [Qemu-devel] [Qemu-block] " John Snow
2019-07-25 15:57 ` [Qemu-devel] [PATCH 2/7] vmdk: Use bdrv_dirname() for relative extent paths Max Reitz
2019-08-12 20:17 ` [Qemu-devel] [Qemu-block] " John Snow
2019-07-25 15:57 ` [Qemu-devel] [PATCH 3/7] iotests: Keep testing broken " Max Reitz
2019-08-12 20:21 ` [Qemu-devel] [Qemu-block] " John Snow
2019-07-25 15:57 ` [Qemu-devel] [PATCH 4/7] vmdk: Reject invalid compressed writes Max Reitz
2019-08-12 20:26 ` [Qemu-devel] [Qemu-block] " John Snow
2019-08-12 21:03 ` Max Reitz
2019-08-12 21:16 ` John Snow
2019-08-13 12:58 ` Max Reitz
2019-07-25 15:57 ` [Qemu-devel] [PATCH 5/7] iotests: Disable broken streamOptimized tests Max Reitz
2019-07-25 15:57 ` [Qemu-devel] [PATCH 6/7] iotests: Disable 110 for vmdk.twoGbMaxExtentSparse Max Reitz
2019-08-12 21:26 ` [Qemu-devel] [Qemu-block] " John Snow
2019-07-25 15:57 ` [Qemu-devel] [PATCH 7/7] iotests: Disable 126 for some vmdk subformats Max Reitz
2019-07-25 17:00 ` Eric Blake
2019-07-26 7:52 ` Max Reitz
2019-08-12 21:33 ` [Qemu-devel] [Qemu-block] " John Snow
2019-08-13 14:00 ` Max Reitz
2019-08-13 22:26 ` John Snow
2019-08-14 14:01 ` Max Reitz [this message]
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