Re: [Xen-users] Strange failures of Xen 4.3.1, PVHVM storage VM, iSCSI and Windows+GPLPV VM combination

[Adam Goryachev <mailinglists@xxxxxxxxxxxxxxxxxxxxxx>]

On 31/01/14 11:08, Kuba wrote:
> W dniu 2014-01-30 23:51, Adam Goryachev pisze:
> > On 31/01/14 00:50, Kuba wrote:
> > > Dear List,
> > >
> > > I am trying to set up a following configuration:
> > > 1. very simple Linux-based dom0 (Debian 7.3) with Xen 4.3.1 compiled
> > > from sources,
> > > 2. one storage VM (FreeBSD 10, HVM+PV) with SATA controller attached
> > > using VT-d, exporting block devices via iSCSI to other VMs and
> > > physical machines,
> > > 3. one Windows 7 SP1 64 VM (HVM+GPLPV) with GPU passthrough (Quadro
> > > 4000) installed on a block device exported from the storage VM (target
> > > on the storage VM, initiator on dom0).
> > >
> > > Everything works perfectly (including PCI & GPU passthrough) until I
> > > install GPLPV drivers on the Windows VM. After driver installation,
> > > Windows needs to reboot, boots fine, displays a message that PV SCSI
> > > drivers were installed and needs to reboot again, and then cannot
> > > boot. Sometimes it gets stuck at "booting from harddrive" in SeaBIOS,
> > > sometimes BSODs with "unmountable boot volume" message. All of the
> > > following I tried without GPU passthrough to narrow down the problem.
> > >
> > > The intriguing part is this:
> > >
> > > 1. If the storage VM's OS is Linux - it fails with the above symptoms.
> > > 2. If the block devices for the storage VM come directly from dom0
> > > (not via pci-passthrough) - it fails.
> > > 2. If the storage VM is an HVM without PV drivers (e.g. FreeBSD
> > > 9.2-GENERIC) - it all works.
> > > 3. If the storage VM's OS is Linux with kernel compiled without Xen
> > > guest support - it works, but is unstable (see below).
> > > 4. If the iSCSI target is on a different physical machine - it all 
> > > works.
> > > 5. If the iSCSI target is on dom0 itself - it works.
> > > 6. If I attach the AHCI controller to the Windows VM and install
> > > directly on the hard drive - it works.
> > > 7. If the block device for Windows VM is a disk, partition, file, LVM
> > > volume or even a ZoL's zvol (and it comes from a dom0 itself, without
> > > iSCSI)- it works.
> > >
> > > If I install Windows and the GPLPV drivers on a hard drive attached to
> > > dom0, Windows + GPLPV work perfectly. If I then give the same hard
> > > drive as a block device to the storage VM and re-export it through
> > > iSCSI, Windows usually boots fine, but works unstable. And by unstable
> > > I mean random read/write errors, sometimes programs won't start,
> > > ntdll.dll crashes, and after couple reboots Windows won't boot (just
> > > like mentioned above).
> > >
> > > The configurations I would like to achieve makes sense only with PV
> > > drivers on both storage and Windows VM. All of the "components" seem
> > > to work perfectly until all put together, so I am not really sure
> > > where the problem is.
> > >
> > > I would be very grateful for any suggestions or ideas that could
> > > possibly help to narrow down the problem. Maybe I am just doing
> > > something wrong (I hope so). Or maybe there is a bug that shows itself
> > > only in such a particular configuration (hope not)?
> > IMHO, it sounds like a resource issue... the domU providing the iSCSI,
> > plus the dom0 plus the domU (windows VM) are all asking for CPU, IRQ's,
> > etc, and someone isn't getting enough in time. It doesn't really help,
> > but we use a physical iSCSI server, the dom0 then connects to the iSCSI
> > and provides the disks to the VM's. Maybe look at assigning specific
> > exclusive CPU's to each dom0 and domU's, and see if you can still
> > reproduce the issue. Also, make absolutely sure that you don't have two
> > VM's accessing the same iSCSI.
> >
> > Regards,
> > Adam
> Dear Adam,
>
> Thank you for your reply. I will try assigning specific cores to the 
> VMs this weekend. I will also try to run all this on another machine 
> and try using something different then Windows for the second VM (why 
> didn't I think about it earlier?). When iSCSI target is on another 
> physical machine, everything works like a charm, but the whole point 
> is to make it all work on a single machine (everything faster than 1 
> Gbps is way beyond my reach, while iperf reports ~22 Gbps between dom0 
> and storage VM...)
> Once again thank you for your suggestions, I will report back with 
> more results. In the meantime please clarify one thing for me - is 
> there something inherently wrong with what I'm trying to do?
I don't see anything "inherently" wrong, but I would question why you 
would want to do this? Why not let dom0 use the scsi controller, and 
export the disks as physical devices into the various VM's? You are 
adding a whole bunch of "ethernet" overhead to both domU's plus the 
dom0. Considering the storage is physically local.
The problem you have is the VM's will not be portable to another 
machine, because they are both tied to the physical pci devices, and the 
block devices are not available on any other physical machine anyway.
A 1Gbps ethernet provides 100MB/s (actually closer to 130MB/s), so 
simply bonding 2 x 1Gbps ethernet can usually provide more disk 
bandwidth than the disks can provide.
In my setup, the iSCSI server uses 8 x 1Gbps ethernet bonded, and the 
xen machines use 2 x 1Gbps bonded for iSCSI, plus 1 x 1Gbps which is 
bridged to the domU's and for dom0 "management". You can get a couple of 
cheap-ish 1Gbps network cards easily enough, and your disk subsystem 
probably won't provide more than 200MB/s anyway (we can get a max of 
2.5GB/s read from the disk subsystem, but the limited bandwidth for each 
dom0 helps to stop any one domU from stealing all the disk IO). In 
practice, I can run 4 x dom0 and obtain over 180MB/s on each of them in 
parallel.
Regards,
Adam

--
Adam Goryachev Website Managers www.websitemanagers.com.au

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