From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [RFC-v2 0/4] tcm_vhost+cmwq fabric driver code for-3.6
Date: Wed, 18 Jul 2012 19:00:15 +0300
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To: Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>, Anthony Liguori <aliguori@us.ibm.com>, linux-scsi <linux-scsi@vger.kernel.org>, kvm-devel <kvm@vger.kernel.org>, lf-virt <virtualization@lists.linux-foundation.org>, Anthony Liguori <aliguori@linux.vnet.ibm.com>, target-devel <target-devel@vger.kernel.org>, Paolo Bonzini <pbonzini@redhat.com>, Zhi Yong Wu <wuzhy@cn.ibm.com>, Christoph Hellwig <hch@lst.de>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
List-Id: virtualization@lists.linuxfoundation.org

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