On Thu, 2014-07-24 at 11:06 +0100, Daniel P. Berrange wrote:
> On Wed, Jul 23, 2014 at 10:32:44PM -0700, Nicholas A. Bellinger wrote:
> > *) vhost-scsi doesn't support migration
> > 
> > Since it's initial merge in QEMU v1.5, vhost-scsi has a migration blocker
> > set.  This is primarily due to requiring some external orchestration in
> > order to setup the necessary vhost-scsi endpoints on the migration
> > destination to match what's running on the migration source.
> > 
> > Here are a couple of points that Stefan detailed some time ago about what's
> > involved for properly supporting live migration with vhost-scsi:
> > 
> > (1) vhost-scsi needs to tell QEMU when it dirties memory pages, either by
> > DMAing to guest memory buffers or by modifying the virtio vring (which also
> > lives in guest memory).  This should be straightforward since the
> > infrastructure is already present in vhost (it's called the "log") and used
> > by drivers/vhost/net.c.
> > 
> > (2) The harder part is seamless target handover to the destination host.
> > vhost-scsi needs to serialize any SCSI target state from the source machine
> > and load it on the destination machine.  We could be in the middle of
> > emulating a SCSI command.
> > 
> > An obvious solution is to only support active-passive or active-active HA
> > setups where tcm already knows how to fail over.  This typically requires
> > shared storage and maybe some communication for the clustering mechanism.
> > There are more sophisticated approaches, so this straightforward one is just
> > an example.
> > 
> > That said, we do intended to support live migration for vhost-scsi using
> > iSCSI/iSER/FC shared storage.
> > 
> > *) vhost-scsi doesn't support qcow2
> > 
> > Given all other cinder drivers do not use QEMU qcow2 to access storage
> > blocks, with the exception of the Netapp and Gluster driver, this argument
> > is not particularly relevant here.
> > 
> > However, this doesn't mean that vhost-scsi (and target-core itself) cannot
> > support qcow2 images.  There is currently an effort to add a userspace
> > backend driver for the upstream target (tcm_core_user [3]), that will allow
> > for supporting various disk formats in userspace.
> > 
> > The important part for vhost-scsi is that regardless of what type of target
> > backend driver is put behind the fabric LUNs (raw block devices using
> > IBLOCK, qcow2 images using target_core_user, etc) the changes required in
> > Nova and libvirt to support vhost-scsi remain the same.  They do not change
> > based on the backend driver.
> > 
> > *) vhost-scsi is not intended for production
> > 
> > vhost-scsi has been included the upstream kernel since the v3.6 release, and
> > included in QEMU since v1.5.  vhost-scsi runs unmodified out of the box on a
> > number of popular distributions including Fedora, Ubuntu, and OpenSuse.  It
> > also works as a QEMU boot device with Seabios, and even with the Windows
> > virtio-scsi mini-port driver.
> > 
> > There is at least one vendor who has already posted libvirt patches to
> > support vhost-scsi, so vhost-scsi is already being pushed beyond a debugging
> > and development tool.
> > 
> > For instance, here are a few specific use cases where vhost-scsi is
> > currently the only option for virtio-scsi guests:
> > 
> >   - Low (sub 100 usec) latencies for AIO reads/writes with small iodepth
> >     workloads
> >   - 1M+ small block IOPs workloads at low CPU utilization with large
> >     iopdeth workloads.
> >   - End-to-end data integrity using T10 protection information (DIF)
> 
> IIUC, there is also missing support for block jobs like drive-mirror
> which is needed by Nova.
> 

This limitation can be considered an acceptable trade-off in initial
support by some users, given the already considerable performance +
efficiency gains that vhost logic provides to KVM/virtio guests.

Others would like to utilize virtio-scsi to access features like SPC-3
Persistent Reservations, ALUA Explicit/Implicit Multipath, and
EXTENDED_COPY offload provided by the LIO target subsystem.

Note these three SCSI features are enabled (by default) on all LIO
target fabric LUNs, starting with v3.12 kernel code.

> From a functionality POV migration & drive-mirror support are the two
> core roadblocks to including vhost-scsi in Nova (as well as libvirt
> support for it of course). Realistically it doesn't sound like these
> are likely to be solved soon enough to give us confidence in taking
> this for the Juno release cycle.
> 

The spec is for initial support of vhost-scsi controller endpoints
during Juno, and as mentioned earlier by Vish, should be considered a
experimental feature given caveats you've highlighted above.

We also understand that code will ultimately have to pass Nova + libvirt
upstream review in order to be merged, and that the approval of any
vhost-scsi spec now is not a guarantee the feature will actually make it
into any official Juno release.

Thanks,

--nab


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