Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] commit: Add top-node/base-node options

[Peter Krempa]

On Wed, Sep 05, 2018 at 08:48:15 -0500, Eric Blake wrote:
> On 09/05/2018 07:38 AM, Peter Krempa wrote:
> 
> > block-commit is able to reopen the format layers and works as expected.
> > 
> > Unfortunately though the 'read-only' option is actually useful as the
> > curl-driver does not work without it:
> > 
> > -blockdev 
> > {"driver":"http","url":"http://ftp.sjtu.edu.cn:80/ubuntu-cd/12.04/ubuntu-12.04.5-alternate-amd64.iso","node-name":"libvirt-2-storage","discard":"unmap"}:
> >  curl block device does not support writes
> > 
> > We obviously can encode that knowledge into libvirt but it will be hard
> > to undo if qemu eventually supports writes in the curl driver.
> > 
> > Which other protocol drivers don't support writes? in case we have to go
> > this way.
> 
> When an NBD server exported an image as read-only, the NBD block client
> cannot request write permissions.  But that's a runtime discovery process,
> not a limitation of the block driver itself.

Hmmm, that's unfortunate. Because in some cases we don't know this fact
upfront in libvirt and we also don't know whether an user might attempt
to block-commit at some time.

We probably do need a way to specify that we want
'read-write-if-possible' behaviour.

Peter

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