Is it possible to restrict to map rbd image on different client hosts same time?

Is it possible to restrict to map rbd image on different client hosts same time?

[Aleksei Gutikov]

Hello

I've just created a bug about list of watchers of rbd image and list of 
clients actually using this rbd image: http://tracker.ceph.com/issues/20943

During reproducing of this bug several questions raised:

1) Does watcher in rbd status output should be treated as 100% reliable 
mark that disk was mapped?

2) And in other way: Does watcher absence should be treated as 100% 
reliable mark that disk was not mapped?

3) And finally: Is it possible to restrict to map disk on several 
clients in the same time (exclusive mapping)?



Thanks!

-- 
Aleksei

Re: Is it possible to restrict to map rbd image on different client hosts same time?

[Ilya Dryomov]

On Tue, Aug 8, 2017 at 11:42 AM, Aleksei Gutikov
<aleksey.gutikov@synesis.ru> wrote:
> Hello
>
> I've just created a bug about list of watchers of rbd image and list of
> clients actually using this rbd image: http://tracker.ceph.com/issues/20943
>
> During reproducing of this bug several questions raised:
>
> 1) Does watcher in rbd status output should be treated as 100% reliable mark
> that disk was mapped?

No.  A watch generally means that the image is "open" in rw mode, but
not necessarily by the kernel client.  rbd CLI tool opens images in
rw mode when performing maintenance operations, you can do it yourself
through the API, there are various external daemons, etc.

>
> 2) And in other way: Does watcher absence should be treated as 100% reliable
> mark that disk was not mapped?

No.  A watch can be temporarily lost and you can catch the client in
the process of reestablishing the watch.

>
> 3) And finally: Is it possible to restrict to map disk on several clients in
> the same time (exclusive mapping)?

Yes.  Use "rbd map -o exclusive" (supported since kernel 4.12).

Thanks,

                Ilya

Re: Is it possible to restrict to map rbd image on different client hosts same time?

[Aleksei Gutikov]

Ilya, thank you for your suggestion, it works for me.

But during testing I realized that what actually I need is not just 
exclusive lock, but some kind of idempotent exclusive lock.
I mean the last who exclusively mapped image should kick out others.
Is it possible now?

Thanks
-- 
Aleksei

Re: Is it possible to restrict to map rbd image on different client hosts same time?

[Ilya Dryomov]

On Tue, Aug 8, 2017 at 6:35 PM, Aleksei Gutikov
<aleksey.gutikov@synesis.ru> wrote:
>
> Ilya, thank you for your suggestion, it works for me.
>
> But during testing I realized that what actually I need is not just
> exclusive lock, but some kind of idempotent exclusive lock.
> I mean the last who exclusively mapped image should kick out others.
> Is it possible now?

No, I don't think there is anything like that out of the box.

What are you trying to accomplish?

Thanks,

                Ilya