[ovirt-devel] vdsm disabling logical volumes

[Nir Soffer]

----- Original Message -----
> From: "Jiri Moskovcak" <jmoskovc at redhat.com>
> To: "Nir Soffer" <nsoffer at redhat.com>
> Cc: devel at ovirt.org
> Sent: Sunday, May 4, 2014 9:23:49 PM
> Subject: Re: [ovirt-devel] vdsm disabling logical volumes
> 
> On 05/04/2014 07:57 PM, Nir Soffer wrote:
> > ----- Original Message -----
> >> From: "Jiri Moskovcak" <jmoskovc at redhat.com>
> >> To: devel at ovirt.org
> >> Sent: Sunday, May 4, 2014 8:08:33 PM
> >> Subject: [ovirt-devel] vdsm disabling logical volumes
> >>
> >> Greetings vdsm developers!
> >>
> >> While working on adding ISCSI support to the hosted engine tools, I ran
> >> into a problem with vdms. It seems that when stopped vdsm deactivates
> >> ALL logical volumes in it's volume group and when it starts it
> >> reactivates only specific logical volumes. This is a problem for hosted
> >> engine tools as they create logical volumes in the same volume group and
> >> when vdsm deactivates the LVs the hosted engine tools don't have a way
> >> to reactivate it, because the services drop the root permissions and are
> >> running as vdsm and apparently only root can activate LVs.
> >
> > Can you describe what volumes are you creating, and why?
> 
> We create hosted-engine.lockspace (for sanlock) and
> hosted-engine.metadata (keeps data about hosted engine hosts)

Do you create these lvs in every vdsm vg? Is this part of the domain structure
used by hosted engine, or it has nothing to do with the storage domain?

> 
> >
> >> So far the
> >> only suitable solution seems to be to change vdsm to only
> >> deactivate/activate it's own LVs.
> >
> > This sounds reasonable. You can add a list of hosted engine lv names
> > and skip these volumes when deactivating vdsm volumes.
> 
> - this sounds a bit suboptimal, vdsm already has list of it's LVs, so it
> can just disable only LVs known to it, otherwise we would have to change
> the list everytime we add some LV to the group

vdsm has a list of special lvs, that needs special treatment. Otherwise, it
consider any other lv as owned by vdsm, and will deactivate them when they are
not used.

I agree that this will create a dependency, but this can also be solved. 
For example, vdsm can load the list from a file installed by hosted engine,
like the typical conf.d directories.

> 
> >
> > Another solution is to tag hosted engine lvs, and have vdsm ignore
> > lvs that contains this tag.
> 
> - this sounds good, because if we teach vdsm to ignore LVs with some tag
> we can add new LVs in future without changing vdsm. This however applies
> also to the solution where vdsm only disables it's own LVs, 

vdsm own lvs are *all* lvs in vdsm vgs. We can implement something like this
using some historic tags we keep (e.g. RHAT_*), but I'd rather add new tag with 
clear semantic than use some random historic value we have.

> so it
> depends on vdsm devels which solution they find better. I think the
> solution without tags is better, because is simpler and others (like
> hosted-engine) can just createlv and don't bother with tags..

I think that a generic tag like OVIRT_IGNORE is an easy solution for everyone.

Federico, what do you think?

Nir