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

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:

quoted

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