Re: SMP/DRBVD issues ...

[Avi Kivity <avi@redhat.com>]

On 12/06/2009 04:43 AM, Gareth Bult wrote:
> Hi,
>
> I'm new to the list so apologies if this is known / fixed, but I've not been able to find satisfactory answers in the archives.
>
> I'm running a number of boxes with KVM on the stock Ubuntu 9.10 kernel.
> Generally it works very well and I have live migration working on DRBD volumes - very impressed - good job!
>
> However I have a number of issues that I note others have also reported, for which I've not seen fixes;
>
> a. SMP, it appears on Ubuntu 9.10 at the very least - does not work. Whereas setting -smp 2 does actually
>     start two kvm threads, the overall performance of the VM is slower than if you use -smp 1, AND the combined
>     kvm threads use way more CPU on the host than they should.
>
>     [note; this is using virt-manager to setup and maintain, CPU's are AMD Phenom II X4 @ 3.2G ]
>
>     I think that some of the performance hit comes down to processes not being tagged to specific CPU's -
>     I've noticed on Zen that if you run a 4 thread guess on a 4 core CPU with nothing else running, so it doesn't
>     need to move threads between cores, you get quite a large performance boost.
>     But, this doesn't really cover the huge impact on the host. The guest can be showing 15% CPU util when
>     configured with 4 cores, while the host is showing 280%.
>
>     I can supply more information if needed, but the problem seems to blatant I'm hoping people already know
>     about it and that can someone can supply some details re; a way forward.
>
>    

We haven't observed this; what kind of guest is it?

> b. DRBD and migration, in order to make this work both hosts it appears must be configured for with the
>      parameter 'allow-two-primaries'.  This makes me a little nervous, but it does seem to work. There is
>      however one massive flaw, KVM does not seem to be DRBD aware and with two volumes on two machines,
>      it's possible to start two instances of a given virtual machine. i.e. neither instance successfully locks the
>      volume to prevent another instance also starting on it. As you will guess, this has a detrimental effect on
>      the underlying volume.
>
>      Is there some way to make KVM apply a lock to a DRBD device such that another instance of the VM cannot
>      be started on another host?  (incidentally, XEN does this 'out of the box' for drbd volumes, so I'm guessing
>      it is possible somehow...? )
>
>      Note; it's nice to configure VM's to auto start on a given machine so in the event of a power failure the VM will
>      boot with the host. However, if this machine fails and you need to start the VM on an alternative machine,
>      when the original machine recovers / reboots, it will attempt (and succeed) in auto booting the same VM
>      leaving you with two copies of the same VM and a screwed guest image.
>
>    

This needs support at the management layer.  qemu has no way of knowing 
whether you want to share the disk between two guests or not.

-- 
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