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 DirectPath Questions 
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Joined: Fri Jan 21, 2011 10:07 pm
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Post DirectPath Questions
I remember reading one post here where is was mentioned that DirectPath accessed several resources as a block on a particular motherboard.

Is this a common problem with DirectPath? Obviously I would want to select a board where this is not an issue ...

My next question regards how DirectPath works - lets say that I am passing a video card through to a VM. If the VM is powered off will the card also be off or will it still be powered on but just idle until the VM is turned on again?

How much control does vSphere have over DirectPath devices versus the VM that they are passed through to?

Lastly is there a sense of what kind of hardware work well with DirectPath versus that which will not? I am talking about hardware not on the VMware HCL here. I see that there is at least one video card that works so on that basis can reasonable estimates be made as to what other cards might work?

Obviously my understanding of how DirectPath works is extremely minimal - I am just trying to understand that if the technology is designed to pass hardware through to a VM what is it about particular hardware that present vSphere with issues?


Thu Jan 27, 2011 8:43 pm
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Post Re: DirectPath Questions
> Is this a common problem with DirectPath? Obviously I would want to select a board where this is not an issue ...

When the VM is powered an the device is reset. With PCI anything else on the bus is also reset hence with PCI devices anything on the same bus is treated as a single unit. If you see the attached image that is the case with Intel NIC and graphics adapter. PCIe devices don't share a bus thus they all show up as individual items.

> My next question regards how DirectPath works - lets say that I am passing a video card through to a VM. If the VM is powered off will the card also be off or will it still be powered on but just idle until the VM is turned on again?

It just sits idle. You could even use it in a second VM as long as both VMs don't try to access it at the same time.

> How much control does vSphere have over DirectPath devices versus the VM that they are passed through to?

The VM has full control over it.

> Lastly is there a sense of what kind of hardware work well with DirectPath versus that which will not? I am talking about hardware not on the VMware HCL here. I see that there is at least one video card that works so on that basis can reasonable estimates be made as to what other cards might work?

It varies from device to device. One thread on VMware's forums details an issue with Windows 7 VMs, graphics cards and over 2 GB of memory. With just 2 GB of memory the VMs run OK, but over 2 GB and the VM doesn't start.


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Dave Mishchenko
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Thu Jan 27, 2011 11:03 pm
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Joined: Fri Jan 21, 2011 10:07 pm
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Post Re: DirectPath Questions
Thanks for the reply Dave.

Good to know that PCI-x is not a shared bus in DirectPath - I was mulling over the pros and cons of running media center and win 7 in VMs each of them with access to their own video cards ... the only con being the constant power draw from the idle video cards when they are not in use (or if the VM is powered down).

Thanks for highlighting the issue with Win 7 / graphics cards and memory > 2Gb - very handy to know about that limitation!

Thanks again!


Fri Jan 28, 2011 12:21 am
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