VMware: SATA Controller Support in ESX 3.5 (1008673)

ESX 3.5 supports multiple SATA controllers. Some of these controllers are supported only in the native Advanced Host Controller Interface (AHCI)/SATA mode with SATA hard-drive, and some are only supported in IDE/ATA mode with SATA Optical Drive. For other controllers, VMware provides support in AHCI/SATA or IDE/ATA mode on both types of drives, but you need to be aware of certain feature limitations and trade-offs associated with each mode.

ESX 3.5 Update 3 introduced support for two new SATA Controllers:

• Broadcom HT1000
• Intel ICH-7

ESX 3.5 Update 4 added support for the following SATA controllers:

• Intel ICH-9
• Intel ICH-10

For controllers that support both AHCI/SATA and IDE/ATA you need to be aware of the following trade offs:

• If you’re using AHCI/SATA mode on SATA optical drive, the guest operating system does not have access to the optical drive. Only VMkernel and the service console have access to optical drive.
• If you are using IDE/ATA mode, VMFS is not supported.
  ESX 3.5 only exports IDE CD-ROM devices to the guest operating system. It does not export SCSI CD-ROM to the guest operating system. In other words, the guest operating system can attach CD-ROM devices via IDE controller only. You cannot attach CD-ROM devices via SCSI controller because ESX does not support connecting SCSI CD-ROMs or ISO images. This is true even if underlying ESX storage driver exposes SCSI interface to VMkernel.
  While running in AHCI/SATA mode, ESX loads its AHCI drivers which enable I/O access to a CD-ROM or hard-drive connected through a I/O Controller Hub (ICH). AHCI is a SCSI-based driver that exports SCSI interface to VMkernel. However, the guest operating system does not have direct CD-ROM access because this is a SCSI-based CD-ROM.
  While running in IDE/ATA mode, ESX loads its IDE-based drivers which enable I/O access to a CD-ROM or hard-drive connected through a I/O Controller Hub (ICH). Guest operating systems have CD-ROM access because this is an IDE-based CD-ROM. However, I/O speed to the hard drive is slower than in AHCI/SATA mode, which is why most high-end servers do not use IDE/ATA mode for hard drive use.
  For hard drive usage, note that, by design, VMFS datastores cannot be created on an IDE-based interface. On the other hand, VMFS can be created on a SCSI-based drive.

The following table and diagram summarizes the behavior. The text highlighted in bold are known limitations.

* This is a known limitation.
 

ESX 3.5 automatically determines which mode to run (either IDE/ATA or AHCI/SATA) based on the BIOS setting in the machine.

Frequently Asked Questions

1. Does the guest operating system/CD-ROM access limitation only apply to ESX 3i (embedded)? What about classic ESX 3.5?
   This limitation applies to both ESX 3i (embedded) and ESX 3.5.
2. In past ESX 3.5 releases, the guest operating system was always able to have direct access to the CD-ROM. Why are you saying that the guest operating system may not have CD-ROM access anymore in ESX 3.5 Update 4? This sounds like a regression. What has changed?
   In the past, the guest operating system had CD-ROM access because ESX 3.5 was using IDE/ATA interface and not AHCI/SATA interface. In ESX 3.5 Update 4, if you continue to use IDE/ATA interface, the guest operating system will continue to have CD-ROM access. This is not a regression, as no functionality has changed. What has changed recently is the introduction of AHCI/SATA support in U3, which now gives the server vendor the choice to configure their I/O controller to either in IDE/ATA or AHCI/SATA. If a server is configured with the ICH10 controller to use AHCI/SATA mode, for example, then the guest operating system will no longer have access to CD-ROM.
3. Are there any plans for VMware to address the guest operating system CD-ROM limitation?
   Yes, VMware is working on a changed design. However, enabling the guest operating system to recognize SCSI-based CD-ROM is a major engineering effort. Code changes are required in ESX and Virtual Infrastructure Management (VIM), and will be completed after the ESX 3.5 Update 4 release.
4. Earlier, it was mentioned that we can create VMFS if we use AHCI/SATA mode. If so, why did VMware not claim VMFS support when using SATA controller running in AHCI/SATA mode?
   VMware might decide to add support in the near future. There is no strong need to have VMFS support on a SATA drive, because native SATA protocol does not support reserve/release. Reserve/release is needed if VMFS is used as clustered file system in a shared disk environment.

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Daniel Buonocore

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