Disk Interface; I/O Paths - Hitachi Adaptable Modular Storage 2000 Best Practices Manual

Because dynamic VHDs have more overhead, best practice is to use fixed VHDs in most circumstance. For
heavy application workloads such as Exchange or SQL, create multiple fixed VHDs and isolate applications
files such as database and logs on their own VHDs.
Pass-through Disks
A Hyper-V pass-through disk is a physical disk or LU that is mapped or presented directly to the guest OS.
Hyper-V pass-through disks normally provide better performance than VHD disks.
After the pass-through disk is visible to and offline within the parent partition, it can be made available to the
child partition using the Hyper-V Manager. Pass-through disks have the following characteristics:
• Must be in the offline state from the Hyper-V parent perspective, except in the case of clustered or highly
available virtual machines
• Presented as raw disk to the parent partition
• Cannot be dynamically expanded
• Do not allow the capability to take snapshots or utilize differencing disks

Disk Interface

Hyper-V supports both IDE and SCSI controllers for both VHD and pass-through disks. The type of controller
you select is the disk interface that the guest operating system sees. The disk interface is completely
independent of the physical storage system.
Table 1 summarizes disk interface considerations and restrictions.
Table 1. Disk Interface Considerations
Disk Considerations
Interface
IDE All child partitions must boot from an IDE
device.
A maximum of four IDE devices are available
for each child partition.
Virtual DVD drives can only be created as an
IDE device.
SCSI Best choice for all volumes based on I/O
performance.
Requires that Integration Services be
installed on the child partition.
Can define a maximum of four SCSI
controllers per child partition.

I/O Paths

The storage I/O path is the path that a disk I/O request generated by an application within a child partition must
take to a disk on the storage system. Two storage configurations are available, based on the type of disk
selected for deployment.
VHD Disk Storage Path
With VHD disks, all I/O goes through two complete storage stacks, once in the child partition and once in the
parent partition. The guest application disk I/O request goes through the storage stack within the guest OS and
onto the parent partition file system.
Restrictions
None.
A maximum of one device per IDE controller for a
maximum of four devices per child partition.
None.
None.
Guest OS specific.
A maximum of 64 devices per SCSI controller for a
maximum of 256 devices per child partition.
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