BRRABill's Field Report With XenServer
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@olivier said:
XO will try to force migration, but from a recent to an older CPU, the result is half of the time a kernel panic (older to recent CPU is less problematic, you'll keep using existing instruction from the old CPU without exploding in flight, contrary to trying to use a recent instruction which doesn't exist on an older CPU)
So in this scenario, are you thinking it would be best to shut it down, and do an export/import?
I wish there was a way to copy from one XS to another. Instead of having to export it to my machine and then import it. As you have mentioned in other threads, it's a pain with larger VMs.
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Our "special" VM copy is doing this:
Check the "Copy" button in the VM view, select a destination SR, and you are done.
edit: original blog post: https://xen-orchestra.com/blog/vm-streaming-export-in-xenserver/
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@olivier said:
Our "special" VM copy is doing this:
Ooooh, that's what I am looking for!
Can you copy a live VM, or does it have to be shut down? (I ask because the option is there when it is running.) I tried that last night. It looked like it copied but threw up an error.
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If it's up, we are taking a snapshot AND copy it. So it doesn't matter.
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@olivier said:
If it's up, we are taking a snapshot AND copy it. So it doesn't matter.
Gotcha. So for stuff with a lot of transaction, not ideal.
But if I shut my VM down first, that would be perfect!
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It's always a trade off. At least, it will be a quiesce snapshot if your Windows VM support it.
But ideally, to avoid any risk, shutdown THEN copy is the safest solution.
Depends of risk level (and downtime!) you can accept (eg live migration is still possible, but you could possibly reboot at destination if CPU instructions are not correct)
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@olivier said:
Depends of risk level (and downtime!) you can accept (eg live migration is still possible, but you could possibly reboot at destination if CPU instructions are not correct)
I think I'll just do the shutdown route, but out of curiosity what kind of things would happen with mismatched CPU instructions? In the scope of a migration.
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Roughly, migration without storage is like this in XenServer:
- a snapshot is created on the origin host
- every new write is now streamed on both hosts (origin and destination)
- disks are copied
When this is done, it's a classical live migration:
- RAM is transfered
- VM is suspended a fraction of time on the origin host
- last RAM transactions are copie on the destination host
- VM is "resumed" (un-suspended) on destination
So your VM will continue its life without knowledge of the new hardware. Let's imagine you have a recent CPU on the origin host, with the "FOOBAR" instruction. Let's also imagine this "FOOBAR" instruction is not on the destination host CPU.
Your VM booted with this "FOOBAR" capable CPU, so for it, that's OK to call it. Imagine what happened when the call happen on the destination host (kernel is crash \o/).
More fun? Migrate a VM from two CPUs vendors (Intel/AMD), while running a Java program inside the VM. If you love fireworks, worth the shot.
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@olivier said:
More fun? Migrate a VM from two CPUs vendors (Intel/AMD), while running a Java program inside the VM. If you love fireworks, worth the shot.
Fireworks he says, more like fireworks going off on the freeway during rush hour
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@travisdh1 said:
Fireworks he says, more like fireworks going off on the freeway during rush hour
With a caravan of gas tankers all around your car.
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That's pretty much a good illustration.
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@olivier said:
That's pretty much a good illustration.
So in theory best practice would be having very similar hardware.
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Yes, totally. Sometimes it's not possible, so you got CPU masking as an alternative: http://support.citrix.com/article/CTX127059
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@BRRABill said:
@olivier said:
That's pretty much a good illustration.
So in theory best practice would be having very similar hardware.
Yes. That's actually the reason all hypervisors only do things like HA on similar hardware. They won't care if it's running on AMD or Intel CPU, but they wouldn't be able to migrate a VM between the two without at least a reboot.
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Is there a log in XO?
It flashes something up when it does something "like VM copy started", but I don't always see anything else.
Is there a place to checkto see the copy did indeed complete?
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You got tasks visible in the top menu (progress is broken due to a bug in XenServer).
Indeed there is a log in XO, but if you are using the sources, I can't tell where, depends on how you installed it.
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@olivier said:
You got tasks visible in the top menu (progress is broken due to a bug in XenServer).
Is there a way to know a task has completed successfully?
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The task list just near the main menu.
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@olivier said:
The task list just near the main menu.
When I mouse over that, it is greyed out and says "no running tasks".
