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John and all, about shared-volume implementation detail base on the
R/O volume, for now, it seems there are multiple choice for us:

a. Supporting R/O volume from Nova side. Ask Nova take care R/O volume
attaching but not Cinder, use hypervisors specific method to keep the
volume be attached in read only mode but actually the backend volume
(in cinder) is on R/W mode. It can be implemented as shared-volume
said "introduce a Read Only option that could be specified during
attach". So as we know, this is not a real R/O volume.

b. Supporting R/O volume from both Cinder and Nova sides. Nova part
just like I mentioned in above 'a' section. And in Cinder part, we can
give a native R/O volume support capability to Cinder, in this case,
Cinder can pass the read-write mode argument to backend
driver/storage, that is the volume can be attached under the "real"
read-only mode. We also have two choices here:
i. Allow client set this "read-only" mode option in volume creating
API calls, and cinder will not allow modify it after the volume
creating.
ii. Allow client mark a "read-only" flag to a volume on-demand,
(necessary checking is needed, such as an already attached volume will
not allow change its "read-write" mode), client can change volume from
R/O to R/W or reverse as they needed.

What's your thoughts?

Thanks,
Zhi Yan

On Tue, May 14, 2013 at 12:09 PM, John Griffith
<john.griffith at solidfire.com> wrote:
>
>
>
> On Mon, May 13, 2013 at 9:47 PM, lzy.dev at gmail.com <lzy.dev at gmail.com>
> wrote:
>>
>> Hi, Guys
>>
>> Form below link, it seems Xen can support R/O volume attaching also:
>> http://backdrift.org/xen-disk-hot-add-block-device-howto
>>
>> "xm block-attach <Domain> <BackDev> <FrontDev> <Mode> [BackDomain]"
>> the "mode" can be R/O and R/W (r and w).
>>
>> Any thoughts? if not I will update the etherpad to adding xen.
>>
>> Thanks,
>> Zhi Yan
>>
>> On Tue, May 14, 2013 at 2:26 AM, Martin, Kurt Frederick (ESSN Storage
>> MSDU) <kurt.f.martin at hp.com> wrote:
>> > Thanks Alessandro, I have also updated the etherpad
>> >
>> > (https://etherpad.openstack.org/summit-havana-cinder-multi-attach-and-ro-volumes)
>> > to include the latest findings regarding R/O volumes. It appears that a
>> > number of hypervisors do indeed allow for setting the volumes to read
>> > only.
>> >
>> > Regards,
>> >
>> > Kurt Martin
>> >
>> >
>> >
>> > From: Alessandro Pilotti [mailto:ap at pilotti.it]
>> > Sent: Monday, May 13, 2013 10:46 AM
>> > To: OpenStack Development Mailing List
>> > Subject: Re: [openstack-dev] [cinder] About Read-Only volume support
>> > Importance: High
>> >
>> >
>> >
>> > Hi guys,
>> >
>> >
>> >
>> > "Summit feedback: Not doing R/O volumes due to the limited hypervisor
>> > that can support setting the volume to R/O, currently only KVM has
>> > this capability".
>> >
>> >
>> >
>> > Hyper-V supports mounting R/O iSCSI volumes as well.
>> >
>> >
>> >
>> > Alessandro
>> >
>> >
>> >
>> >
>> >
>> > On May 13, 2013, at 13:22 , lzy.dev at gmail.com wrote:
>> >
>> >
>> >
>> > Hi All,
>> >
>> > In
>> >
>> > https://etherpad.openstack.org/summit-havana-cinder-multi-attach-and-ro-volumes,
>> > I saw a comment there:
>> > "Summit feedback: Not doing R/O volumes due to the limited hypervisor
>> > that can support setting the volume to R/O, currently only KVM has
>> > this capability".
>> >
>> > I agree there probably have some troubles cause R/O volumes support
>> > hard to implement.
>> > But maybe since I have not attended the summit, nova and cinder guys
>> > not notice there is a blueprint to plan to implement a cinder backend
>> > driver for glance
>> > (https://blueprints.launchpad.net/glance/+spec/glance-cinder-driver, I
>> > proposed), so I consider the R/O volumes support can be implemented
>> > gracefully.
>> > Under the case, the R/O volume stored in cinder will be created as an
>> > image, client can access it by glance via standard api, and nova can
>> > prepare the R/W image (base on R/O volume) for the instance normally.
>> >
>> > And more, I consider the R/O volume support and cinder driver for
>> > glance is valuable  because on nova side we can give some code changes
>> > to allow nova prepare instance disk via particular COW mechanism base
>> > on particular cinder backend store capability with more efficiency
>> > way, such as efficient snapshot.
>> >
>> > Thanks,
>> > Zhi Yan
>> >
>> > _______________________________________________
>> > OpenStack-dev mailing list
>> > OpenStack-dev at lists.openstack.org
>> > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>> >
>> >
>> >
>> >
>> > _______________________________________________
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>> > OpenStack-dev at lists.openstack.org
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>> >
>>
>> _______________________________________________
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>> OpenStack-dev at lists.openstack.org
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>
>
> Thanks Zhi Yan, I had some conversations with folks at the summit and the
> general concensus seemed to be that it was possible.  There's a BP for this
> that met a bit of objection:
> https://blueprints.launchpad.net/cinder/+spec/shared-volume
>
> perhaps we can work off of that and add some details to it.
>
> Thanks,
> John
>
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