Re: [Qemu-devel] [PATCH for-2.10 2/5] block: Allow reopen rw without BDRV_O_ALLOW_RDWR

[Eric Blake <eblake@redhat.com>]

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On 08/04/2017 05:20 AM, Kevin Wolf wrote:
>>>
>>> Hmm, I wonder.  https://bugzilla.redhat.com/show_bug.cgi?id=1465320
>>> details a failure when starting qemu with a read-write NBD disk, then
>>> taking several snapshots (nbd <- snap1 <- snap2 <- snap3), then where
>>> intermediate commit (snap2 into nbd) works but live commit (snap3 into
>>> nbd) fails with a message that nbd does not support reopening.  I'm
>>> presuming that your series may help to address that; I'll give it a spin
>>> and see what happens.
>>
>> Nope, even with your patches, I'm still getting:
>>
>> {'execute':'block-commit','arguments':{'device':'drive-image1','top':'bar2'}}
>> {"return": {}}
>> {"timestamp": {"seconds": 1501811285, "microseconds": 439748}, "event":
>> "BLOCK_JOB_COMPLETED", "data": {"device": "drive-image1", "len":
>> 2097152, "offset": 2097152, "speed": 0, "type": "commit"}}
>>
>> {'execute':'block-commit','arguments':{'device':'drive-image1','top':'bar3'}}
>> {"error": {"class": "GenericError", "desc": "Block format 'nbd' used by
>> node '#block048' does not support reopening files"}}
> 
> That's simply NBD not implementing .bdrv_reopen_*. In other words, not a
> bug, but just a missing feature.

But why does intermediate commit work, while live commit fails?  Both
require that the image being committed into be read-write, and the NBD
image was opened read-write (even if it can be treated as read-only for
the duration of time that it is a backing image).  I understand missing
.bdrv_reopen making it impossible to change read-only to read-write, but
when missing it means you can never change read-write down to the
tighter read-only originally, then what is making live commit different
from intermediate in not being able to handle it?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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