Disk Alignment - OS Test

In a SAN environment, the smallest hardware unit used by a SAN storage array to build a LUN out of multiple physical disks is called a chunk or a stripe. To optimize I/O, chunks are usually much larger than sectors. Thus, a SCSI I/O request that intends to read a sector in reality reads one chunk.

On top of this, in a Windows environment, NTFS is formatted in blocks ranging from 1MB to 8MB. The file system used by the guest operating system optimizes I/O by grouping sectors into so called clusters (allocation units).

Figure 5 shows these three layers at issue. There are the SAN blocks at the bottom, then the VMFS blocks in the middle, and then the NTFS blocks used by the Windows VM. 

Figure 5 : The SAN, VMFS, and NTFS blocks

If these three layers are not aligned, your SAN may be working harder than it needs to. For example, a call to read a single NTFS block may require the SAN to read three blocks as shown below:

Figure 6 : Unaligned partitions

An unaligned partition therefore, results in a track crossing and an additional I/O, incurring a penalty on latency and throughput. The additional I/O (especially if small) can impact system resources significantly on some host types.

What would hence be ideal is for the three layers in Figure 6 above to be aligned so that a single NTFS block requires only one SAN block to be read as illustrated below:

Figure 7 : Aligned partitions

An aligned partition ensures that the single I/O is serviced by a single device, eliminating the additional I/O and resulting in overall performance improvement.

Therefore, whenever users to Windows virtual desktops complaint that the virtual desktop is running slower than usual, you may want to check the disk alignment to determine whether the slowdown can be attributed to one/more unaligned disk partitions. This test enables you to perform such a check.

This test is disabled by default. To enable the test, go to the ENABLE/DISABLE TESTS page using the menu sequence : Agents -> Tests -> Enable/Disable, pick Cloud Desktops as the desired Component type, set Performance as the Test type, choose the test from the DISABLED TESTS list, and click on the < button to move the test to the ENABLED TESTS list. Finally, click the Update button.

Target of the test : A Windows virtual desktop on the cloud

Agent deploying the test : A remote agent

Outputs of the test : One set of results for each disk partition on every Windows virtual desktop on the cloud

Configurable parameters for the test
Parameter Description

Test Period

How often should the test be executed.


The nick name of the Cloud Desktops component for which this test is to be configured.


Refers to the port at which the specified host listens to. By default, this is NULL.

Inside View Using

To obtain the 'inside view' of performance of the cloud-hosted Windows desktops - i.e., to measure the internal performance of the Windows virtual desktops - this test uses a light-weight eG VM Agent software deployed on each of the desktops. Accordingly, this parameter is by default set to eG VM Agent (Windows).

Report Powered OS

This flag is relevant only for those tests that are mapped to the Inside View of Desktops layer. If this flag is set to Yes (which is the default setting), then the 'inside view' tests will report measures for even those Windows virtual desktops that do not have any users logged in currently. Such desktops will be identified by their name and not by the username_on_virtualdesktopname. On the other hand, if this flag is set to No, then this test will not report measures for those Windows virtual desktops to which no users are logged in currently.  

Is Cloud VMs

Since this test runs for a 'Cloud Desktops' component, this flag is set to Yes by default.

DD Frequency

Refers to the frequency with which detailed diagnosis measures are to be generated for this test. For instance, if you set to 1:1, it means that detailed measures will be generated every time this test runs, and also every time the test detects a problem.

Detailed Diagnosis

To make diagnosis more efficient and accurate, the eG suite embeds an optional detailed diagnostic capability. With this capability, the eG agents can be configured to run detailed, more elaborate tests as and when specific problems are detected. To enable the detailed diagnosis capability of this test for a particular server, choose the On option. To disable the capability, click on the Off option.

The option to selectively enable/disable the detailed diagnosis capability will be available only if the following conditions are fulfilled:

  • The eG manager license should allow the detailed diagnosis capability

  • Both the normal and abnormal frequencies configured for the detailed diagnosis measures should not be 0.

Measurements made by the test
Measurement Description Measurement Unit Interpretation

Disk partition alignment status

Indicates whether this disk partition is aligned or not.


If the partition is unaligned, this test reports the value Partition is not aligned. For an aligned partition, this test reports the value Partition is aligned.

The numeric values that correspond to the above-mentioned measure values are described in the table below:

Measure Value Numeric Value
Partition is aligned 100
Partition is not aligned 0


By default, this measure reports one of the Measure Values listed in the table above. The graph of this measure however will represent the disk alignment status using the numeric equivalents - 100 or 0.

If a partition is found to be misaligned, you can use the detailed diagnosis of this test to figure out the caption, device ID, logical partition name, and block size of the faulty partition.

If a partition is found to be misaligned, you can use the detailed diagnosis of this test to figure out the caption, device ID, logical partition name, and block size of the faulty partition.

Figure 8 : The detailed diagnosis of the Disk partition alignment status measure