Virtual Storage Devices Test

Virtual hard disk (VHD) is a disk image file format for storing the complete contents of a hard drive. It  replicates an existing hard drive and includes all data and structural elements.

On the Hyper-V management operating system, virtual hard disks can have a capacity of up to 2040 gigabytes and can be of any of the following types:

• Fixed: A fixed virtual hard disk is a disk that occupies physical disk space on the management operating system equal to the maximum size of the disk, regardless of whether a virtual machine requires the disk space. A fixed virtual hard disk takes longer to create than other types of disks because the allocated size of the .vhd file is determined when it is created. This type of virtual hard disk provides improved performance compared to other types because fixed virtual hard disks are stored in a contiguous block on the management operating system.

• Dynamically expanding: A dynamically expanding virtual hard disk is a disk in which the size of the .vhd file grows as data is written to the disk. This type provides the most efficient use of disk space. You will need to monitor the available disk space to avoid running out of disk space on the management operating system.

• Differencing: A differencing virtual hard disk stores the differences from the virtual hard disk on the management operating system. This allows you to isolate changes to a virtual machine and keep a virtual hard disk in an unchanged state. The differencing disk on the management operating system can be shared with virtual machines and, as a best practice, must remain read-only. If it is not read-only, the virtual machine‘s virtual hard disk will be invalidated.

The performance of a virtual hard disk is judged by the speed with which it processes I/O requests. Slowdowns in I/O processing can cause read-write requests to the virtual hard disks to queue up, thereby increasing the I/O load on the virtual hard disks and degrading the overall performance of the VMs using those virtual hard disks. Moreover, since virtual hard disk files (.vhd files) are typically stored in the physical disks of the Hyper-V host, excessive I/O activity on the virtual hard disk will also impact the performance of the corresponding physical disks.

Using the Virtual Storage Devices test, administrators can periodically monitor the I/O activity on each virtual hard disk assigned to every VM on Hyper-V. This way, probable delays in the processing of I/O requests can be proactively detected, and the physical disks and VMs that may be impacted by these latencies can be isolated.

Target of the test : A Hyper-V / Hyper-V VDI server

Agent executing the test : An internal agent

Output of the test : One set of results for each virtual disk hosted by a physical disk and assigned to a VM

Configurable parameters for the test

Parameter	Description
Test period	This indicates how often should the test be executed.
Host	Specify the HOST for which this test is to be configured.

Measurements reported by the test:

Measurement	Description	Measurement Unit	Interpretation
Read throughput	Indicates the rate at which data is read from this virtual hard disk that is stored by this physical disk and used by this VM.	Bytes/Sec	While an abnormal increase in the value of these measures could indicate a high level of read-write activity on a virtual hard disk, a consistent decrease in the value of these measures could be indicative of a processing bottleneck probably caused by slowdowns in a virtual hard disk. When a physical disk experiences slowdowns, you can compare the value of these measures across all virtual hard disks that are located on that physical disk, so that you can instantly identify which virtual hard disk is contributing to the processing delay.
Write throughput	Indicates the rate at which data is written to this virtual hard disk that is stored by this physical disk and used by this VM.	Bytes/Sec
Total throughput	Indicates the rate at which data is read from/written to this virtual hard disk that is stored by this physical disk and used by this VM.	KBps
Read IOPS	Indicates the rate at which data reads are performed on this virtual hard disk that is stored by this physical disk and used by this VM.	Reads/sec
Write IOPS	Indicates the rate at which data writes are performed on this virtual hard disk that is stored by this physical disk and used by this VM.	Writes/sec
Total IOPS	Indicates the total number of data reads and data writes performed per second on this virtual hard disk that is stored by this physical disk and used by this VM.	Operations/sec
Errors count	Indicates the number of error events triggered on this virtual hard disk that is stored by this physical disk and used by this VM.	Number
Flush count	Represents the total number of flush operations that have occurred on this virtual device.	Number
Read count	Indicates the number of read operations performed on this virtual hard disk that is stored by this physical disk and used by this VM.	Number
Write count	Indicates the number of write operations performed on this virtual hard disk that is stored by this physical disk and used by this VM.	Number