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Re: [libvirt] ESX and inaccessible storage pools



On Mon, Apr 19, 2010 at 11:09:17AM +0100, Daniel P. Berrange wrote:
> On Sun, Apr 18, 2010 at 10:55:04PM +0200, Matthias Bolte wrote:
> > I'm currently implementing the storage driver for ESX and have a
> > mapping problem.
> > 
> > libvirt storage pools can be either running or inactive just like the
> > domain. ESX is a bit different here. A datastore is always active. It
> > can be inaccessible, e.g. if an NFS is mounted as datastore but the
> > NFS server went unreachable. I discussed this with Daniel on IRC and
> > he summarized it like this:
> > 
> >   <DV> they are defined, the system knows them, but they are not
> > available for use
> > 
> > The question is, what should the ESX driver do with inaccessible datastores?
> > 
> > Option 1: They could just be represented as inactive storage pools,
> > but libvirt's semantic for inactive storage pools includes that the
> > user can start an inactive storage pool. That's not possible for an
> > inaccessible datastore, so the driver would report an error if a user
> > tries to start an inaccessible storage pool.
> 
> I don't think this is a good match, because 'inactive' implies that
> the system hasn't even connected to the NFS server.
> 
> > Option 2: Just ignore inaccessible datastores and make the ESX storage
> > driver to pretend that they don't exist. But this would hide
> > information from the user.
> 
> Agreed, hiding info is not good.
> 
> > Option 3: Extend libvirt's API for inaccessible storage pools.
> 
> This is the only one left !
> 
> IMHO, the 'inaccessible' data store is active. A little noticed fact
> is that the storage pool state is actually more than just a boolean.
> At least in the public API we define
> 
>   typedef enum {
>     VIR_STORAGE_POOL_INACTIVE = 0, /* Not running */
>     VIR_STORAGE_POOL_BUILDING = 1, /* Initializing pool, not available */
>     VIR_STORAGE_POOL_RUNNING = 2,  /* Running normally */
>     VIR_STORAGE_POOL_DEGRADED = 3, /* Running degraded */
>   } virStoragePoolState;
> 
> 
> THe 'BUILDING' state is to indicate cases where the storage is being
> constructed in some way.
> 
> The 'RUNNING' state is the normal runtime state.
> 
> The 'DEGRADED' state indicates that it is fully operational, but in a 
> degraded state. eg, one of the members of a RAID array failed, or one
> of the SCSI paths failed, etc.
> 
> The DEGRADED state doesn't exactly map to your NFS example though because
> in case of a non-responsive NFS server, you can't use the pool.
> 
> Thus I'd suggest just adding a 5th option
> 
>   VIR_STORAGE_POOL_INACCESSIBLE = 4 /* Running, but not accessible */

Agreed; I didn't read your response before I posted mine.  This sounds
like the right way to to it.

Dave


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