ide vs scsi vm disks

[JamesConaway]

I have read many places that it is best practice to use scsi disk vs IDE for virtual machines. What I have not been able to find is exactly the reason why?

Performance is always given for the reason, but no details on why scsi performs better than IDE. I would like to get the elevator pitch answer to this question if possible to answer customer questions.

Can someone explain?

[FredPeterson · Accepted solution]

It helps to understand what the SCSI protocol does vs the PATA protocol.

Basically SCSI allows parallel execution of commands, PATA is serial.

[davidmarkley]

Counter-intuitive if you ask me.

This might help: http://communities.vmware.com/docs/DOC-10971.pdf

[JamesConaway]

The performance overhead of SCSI over IDE comes from structure of the bus, not the drive. The nature of the SCSI bus allows it much better performance when doing data hungry tasks such as multi-tasking. The SCSI bus controller is capable of controlling the drives without any work by the processor. Also, all drives on a SCSI chain are capable of operating at the same time. With IDE, one is limited to two drives in a chain, and these drives cannot work at the same time. In essence, they must “take turns”.

Better performance due to the design of the technology

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