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spectVM
Enthusiast
Enthusiast

Question on physical RDM: SCSI vs Local ATA

Hi guys,

Using ESXi 5.1U1, I am able to attach disks located on an LSI2008 as physical RDM to my VMs.

However, the option is grayed out for disks located on the motherboard Intel ICH10 AHCI controller.

Why is that?

Thanks.

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a_p_
Leadership
Leadership

I assume the LSI controller presents the disks as "remote" disks rather than "local" disks. Anyway, afaik RDMs are only supported on shared storage.

André

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spectVM
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Enthusiast

Andre, thanks for the reply.

The question now is why does it matter to ESXi as far as physical RDM?

Clearly, the only difference is the disk controller. I can use vmkfstools to create the physical RDMs for the disks on the ICH10, but there is a mental comfort in being able to do it through the UI.

Is physical RDM safer if done through the LSI2008 than through the ICH10 controller?

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a_p_
Leadership
Leadership

Unfortunately I don't know the exact technical reasons for this. Maybe it's just restricted for support reasons!?

André

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spectVM
Enthusiast
Enthusiast

I have opted to doing physical RDM with the LSI2008 for now. It would be great though to get visibility into the technicals to understand the risks.

I have run physical RDM on Local SATA disks for years using the vmkfstools approach. However, I am a little leery this time around since I will be doing more low level access on the disks on this new setup.

It will be great if someone can shine in some technicals.

Thanks.

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spectVM
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Enthusiast

@vlho

Thanks for that link.

Local storage devices often do not support VPD page 0x83, and thus cannot be used for Raw Device Mappings (RDMs). The content of page 0x83 is used as a unique identifier for the device. For more information, see Creating Raw Device Mapping (RDM) is not supported for local storage (1017530).

So basically, VPD support is key. However, I am thinking this is purely to avoid the potential of mixing devices and sending a command to the wrong device.

Furthermore, I am guessing VMware is trying not to rely solely on disk serial numbers for identification. VPD combines various unique identifiers into one that is even more unique and fit for cluster usage.

If addressing is the only reason, then creating the RDM manually is just as safe as the RDM created through the vSphere client for as long as there is only one device per channel as it is in the case of SATA ports.

Opinions?

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