HP Integrity Virtual Machines Installation, Configuration, and Administration Version A.03.50

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 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.
It is important to know exactly where the virtual media is located on physical storage devices.
With Integrity VM, a single physical disk might be sliced into logical volumes or files. Slicing
up physical disks increases utilization, but it can affect the performance of the physical device.
The guest OS treats the virtual disk as a whole disk, not as a part of a physical one. Over-slicing
physical storage can overload a physical device's ability to handle virtual I/O that is meant for
whole disks. Figure 7-2 shows a common mistake of overdriving physical storage with multiple
guest OS boot disks, which are often I/O intensive.
Figure 7-2 Overdriving Physical Storage Hurts Performance
Guest
Boot Disk
Guest
Boot Disk
Overdriven
Physical Storage
Guest
Boot Disk
Provide workloads that the physical devices can handle for all the virtual devices layered on top
of them. Use performance tools on the VM Host, like sar(1M), to see how the physical storage is
keeping up with the virtual device demands.
The way the virtual media I/O gets to the physical storage backing it is also an important
consideration. As shown in Figure 7-1, all virtual I/O goes through a general VM Host I/O services
layer that routes the virtual I/O to the correct VM Host interface driver. The interface driver then
controls the physical I/O adapter to issue virtual I/O to the physical storage device. By load
balancing across these physical adapters, virtual I/O bottlenecks can be eliminated at the physical
hardware layers, thereby increasing performance. Load balancing can be done by installing a
multipath solution on the VM Host. See Section 7.2.1.3: “VM Storage Multipath Solutions”
(page 88) for help with selecting a multipath solution for a virtual media type.
The performance of attached devices is largely determined by the type of physical device attached
to the virtual machine. Tapes, media changers, and CD/DVD burners are inherently slow devices,
not significantly impacted by the software overhead of Integrity VM.
7.2 Configuring Integrity VM Storage 87