HP Integrity Virtual Machines 4.2: Installation, Configuration, and Administration

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Figure 7-1 Integrity VM Storage I/O Stack

HP-UX

Interface Driver

HP-UX

Interface Driver

HP-UX

Interface Driver

Physical Adapter Physical Adapter Physical Adapter

Physical Media Physical Media Physical Media

VM Host Driver Services

File Systems Layer

Virtual File Disk

Logical Volume Managers Layer

Virtual LvDisk

Disk Drivers Layer

Virtual Disk

Attached Devices

Integrity Vm Passthrough

Drivers Layer

For a virtual I/O operation to be completed, it has to travel round trip between the virtual storage

adapter and the VM Host physical storage device. The longer the path is, the longer it takes for

virtual I/O to be completed. As shown in Figure 7-1, a virtual I/O operation must traverse each

software layer in order, from where it originates to the physical media. For example, a virtual

I/O operation for a Virtual FileDisk must traverse any logical volume managers the file system

is on and the disk drivers that control the whole disk. Therefore, in general, the higher the virtual

media is in the VM Host I/O stack, the slower it operates.

The simplified I/O stack in Figure 7-1 does not completely illustrate all the choices that can affect

the performance:

• Performance of different software layers differs.

• The interfaces to each software layer are different, allowing Integrity VM different ways to

send I/O through the layers. For example, whole disks can achieve higher throughput rates

than logical volumes and file systems.

• The I/O layer might have features to help performance increase beyond a lower layer. For

example, a file system's buffer cache may help a Virtual FileDisk perform better on some

I/O workloads than the other virtual device types, which have no such caching.

For further information on tuning performance at each software layer on the VM Host, see the

Integrity VM white papers on the HP Documentation website at http://docs.hp.com.

When you configure virtual devices, consider how the virtual media maps to the physical storage.

All virtual media connects to a piece of physical media somewhere in the data center. You can

help ensure the best performance by understanding the impact of the physical storage and the

way I/O accesses it.

104 Creating Virtual Storage Devices