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

[Eric Blake]

On 08/03/2017 10:21 AM, Eric Blake wrote:
> On 08/03/2017 10:02 AM, Kevin Wolf wrote:
>> BDRV_O_ALLOW_RDWR is a flag that tells whether qemu can internally
>> reopen a node read-write temporarily because the user requested
>> read-write for the top-level image, but qemu decided that read-only is
>> enough for this node (a backing file).
>>
>> bdrv_reopen() is different, it is also used for cases where the user
>> changed their mind and wants to update the options. There is no reason
>> to forbid making a node read-write in that case.
> 
> 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"}}

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

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