Re: [RFC-v2 0/4] tcm_vhost+cmwq fabric driver code for-3.6

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 18, 2012 at 11:53:38AM -0400, Christoph Hellwig wrote:
> On Wed, Jul 18, 2012 at 08:42:21AM -0500, Anthony Liguori wrote:
> > 
> > If you add support for a new command, you need to provide userspace
> > a way to disable this command.  If you change what gets reported for
> > VPD, you need to provide userspace a way to make VPD look like what
> > it did in a previous version.
> > 
> > Basically, you need to be able to make a TCM device behave 100% the
> > same as it did in an older version of the kernel.
> > 
> > This is unique to virtualization due to live migration.  If you
> > migrate from a 3.6 kernel to a 3.8 kernel, you need to make sure
> > that the 3.8 kernel's TCM device behaves exactly like the 3.6 kernel
> > because the guest that is interacting with it does not realize that
> > live migration happened.
> 
> I don't think these strict live migration rules apply to SCSI targets.
> 
> Real life storage systems get new features and different behaviour with
> firmware upgrades all the time, and SCSI initiators deal with that just
> fine.
>  I don't see any reason to be more picky just because we're
> virtualized.


Presumably initiators are shut down for target firmware upgrades?
With virtualization your host can change without guest shutdown.
You can also *lose* commands when migrating to an older host.

-- 
MST