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Re: [Xen-users] Status of PVSCSI

  • To: xen-users@xxxxxxxxxxxxx
  • From: "Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>
  • Date: Thu, 11 Aug 2016 15:11:00 -0400
  • Delivery-date: Thu, 11 Aug 2016 19:12:30 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

On 2016-08-11 13:15, Jon Schewe wrote:
On 8/11/16 12:07 PM, Austin S. Hemmelgarn wrote:
On 2016-08-11 11:47, Jon Schewe wrote:
On 8/5/16 10:38 AM, Jon Schewe wrote:
On 8/5/16 10:27 AM, Simon Hobson wrote:
Jon Schewe <jschewe@xxxxxxx> wrote:

I'm looking to move my backups into a VM and PVSCSI seems like the
right way to get the tape library and drive visible to the VM.
So what front end can I use pvscsi with?
Can you use PCI passthrough of the SCSI controller ?

I have considered that, however I have a couple of issues with it:
1) When I tested it, after shutting down the domU the dom0 couldn't
attach to the SCSI controller again until I rebooted. Perhaps that was
because I shutdown the domU before calling pci-detach.

2) I see that I need to detach the pci devices before save and restore.
That will make things hard when rebooting the dom0 for updates. I
usually just run "service xendomains stop" and then run the updates and
reboot. This suspends the domUs and everything is fine on startup. If
this process will automatically handle the detach and attach calls,
I may be able to use it.

Does anyone know if pci devices need to be explicitly detached before
shutdown or if xen will do the right thing on shutdown of the VM?
In my experience, it depends on the device.  Some devices work fine
(mostly really simple dumb devices, such as some really cheap GbE
NIC's), but many need hardware re-initialization before they will work
(and some like certain GPU's actually need a full power cycle before
they work reliably).  For many of those that need hardware
re-initialization,, you can unload and reload the driver in Domain-0
to reinitialize the hardware, but only after you've detached the
device from the VM and removed the reservations.

Also if I call "/etc/init.d/xendomains stop" and the VMs are saved, will
that do the right thing or will I need to manually detach pci devices
that are passed through?
You would have to detach the devices if you want to use them elsewhere
before restoring the VM.  Think along the same lines as accessing the
VM's disk image while it's stopped, the OS won't be expecting the
change, and it may lead to data loss or other odd behavior.  Saving a
VM is essentially the same in this case as putting a laptop to sleep,
so you really should avoid doing anything with any hardware for the VM
(real or emulated) when it's saved and not running, and more
importantly, don't power cycle any of the hardware attached to it
until it's been shut down properly.

In my use case I'm thinking that I would dedicate the PCI device (SCSI
controller here) to one VM and not  move it to another VM or domain-0.
Given that assumption, it should all work pretty seamlessly, correct?

It generally should, but like I said, you might have issues if you leave the device attached while the machine is saved and then reboot the host (or the host crashes, or otherwise has to restart). I have no personal experience passing through SCSI controllers here though, just NIC's (which usually have no issues), so I there may be involved factors that I don't know about.

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