Use the vSphere Client disk performance charts to monitor disk I/O usage for clusters, hosts, and virtual machines. Use the guidelines below to identify and correct problems with disk I/O performance.

The virtual machine disk usage (%) and I/O data counters provide information about average disk usage on a virtual machine. Use these counters to monitor trends in disk usage.

The best ways to determine if your vSphere environment is experiencing disk problems is to monitor the disk latency data counters. You use the Advanced performance charts to view these statistics.

The kernelLatency data counter measures the average amount of time, in milliseconds, that the VMkernel spends processing each SCSI command. For best performance, the value should be 0-1 milliseconds. If the value is greater than 4ms, the virtual machines on the ESX/ESXi host are trying to send more throughput to the storage system than the configuration supports. Check the CPU usage, and increase the queue depth or storage.

The deviceLatency data counter measures the average amount of time, in milliseconds, to complete a SCSI command from the physical device. Depending on your hardware, a number greater than 15ms indicates there are probably problems with the storage array. Move the active VMDK to a volume with more spindles or add disks to the LUN.

The queueLatency data counter measures the average amount of time taken per SCSI command in the VMkernel queue. This value must always be zero. If not, the workload is too high and the array cannot process the data fast enough.

Disk I/O Performance Enhancement Advice




Increase the virtual machine memory. This should allow for more operating system caching, which can reduce I/O activity. Note that this may require you to also increase the host memory. Increasing memory might reduce the need to store data because databases can utilize system memory to cache data and avoid disk access.

To verify that virtual machines have adequate memory, check swap statistics in the guest operating system. Increase the guest memory, but not to an extent that leads to excessive host memory swapping. Install VMware Tools so that memory ballooning can occur.


Defragment the file systems on all guests.


Disable antivirus on-demand scans on the VMDK and VMEM files.


Use the vendor's array tools to determine the array performance statistics. When too many servers simultaneously access common elements on an array, the disks might have trouble keeping up. Consider array-side improvements to increase throughput.


Use Storage VMotion to migrate I/O-intensive virtual machines across multiple ESX/ESXi hosts.


Balance the disk load across all physical resources available. Spread heavily used storage across LUNs that are accessed by different adapters. Use separate queues for each adapter to improve disk efficiency.


Configure the HBAs and RAID controllers for optimal use. Verify that the queue depths and cache settings on the RAID controllers are adequate. If not, increase the number of outstanding disk requests for the virtual machine by adjusting the Disk.SchedNumReqOutstanding parameter. For more information, see the Fibre Channel SAN Configuration Guide.


For resource-intensive virtual machines, separate the virtual machine's physical disk drive from the drive with the system page file. This alleviates disk spindle contention during periods of high use.


On systems with sizable RAM, disable memory trimming by adding the line MemTrimRate=0 to the virtual machine's .VMX file.


If the combined disk I/O is higher than a single HBA capacity, use multipathing or multiple links.


For ESXi hosts, create virtual disks as preallocated. When you create a virtual disk for a guest operating system, select Allocate all disk space now. The performance degradation associated with reassigning additional disk space does not occur, and the disk is less likely to become fragmented.


Use the most current ESX/ESXi host hardware.