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

From: Kevin Wolf <kwolf@redhat.com>
To: Eric Blake <eblake@redhat.com>
Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH for-2.10 2/5] block: Allow reopen rw without BDRV_O_ALLOW_RDWR
Date: Fri, 4 Aug 2017 12:20:34 +0200	[thread overview]
Message-ID: <20170804102034.GB4108@localhost.localdomain> (raw)
In-Reply-To: <a7a5e25c-fef4-d508-4a9c-ff2d57c1b2ee@redhat.com>

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Am 04.08.2017 um 03:49 hat Eric Blake geschrieben:
> 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"}}

That's simply NBD not implementing .bdrv_reopen_*. In other words, not a
bug, but just a missing feature.

Kevin

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