The Disk Rate chart displays disk read and write rates for LUNs on a host, including average rates.

This chart is located in the Home view of the host Performance tab.

Table 1. Data Counters
Chart Label Description
Read Number of disk read commands completed on each disk on the host, per second. The aggregate number of all disk read commands is also displayed in the chart.

Read rate = blocksRead per second × blockSize

  • Counter: read
  • Stats Type: Rate
  • Unit: Kilobytes per second (KBps)
  • Rollup Type: Average
  • Collection Level: 3
Write Number of disk write commands completed on each disk on the host, per second. The aggregate number of all disk write commands is also displayed in the chart.

Write rate = blocksWritten per second × blockSize

  • Counter: write
  • Stats Type: Rate
  • Unit: Kilobytes per second (KBps)
  • Rollup Type: Average
  • Collection Level: 3

Chart Analysis

Use the disk charts to monitor average disk loads and to determine trends in disk usage. For example, you might notice a performance degradation with applications that frequently read from and write to the hard disk. If you see a spike in the number of disk read or write requests, check whether any such applications were running then.

The best ways to determine if your vSphere environment is experiencing disk problems is to monitor the disk latency data counters. You can 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 must be 0 -1 milliseconds. If the value is greater than 4 ms, the virtual machines on the host are trying to send more throughput to the storage system than the configuration supports. Check the CPU usage, and increase the queue depth.
  • 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 15 ms indicates probable 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.

If the disk latency values are high, or if you notice other problems with disk I/O performance, consider taking the following actions.

Table 2. Disk I/O Performance Enhancement Advice
# Resolution
1

Increase the virtual machine memory. It allows more operating system caching, which reduces I/O activity. Note: It might require you to increase the host memory. Increasing memory might reduce the need to store data because databases can utilize the 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.

2 Defragment the file systems on all guests.
3 Deactivate antivirus on-demand scans on the VMDK and VMEM files.
4 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. To increase throughput, consider array-side improvements.
5 Use Storage vMotion to migrate I/O-intensive virtual machines across multiple hosts.
6 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.
7 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 vSphere Storage.
8 For resource-intensive virtual machines, separate the virtual machine's physical disk drive from the drive with the system page file. It alleviates disk spindle contention during periods of high use.
9 On systems with sizable RAM, deactivate memory trimming by adding the line MemTrimRate=0 to the virtual machine's VMX file.
10 If the combined disk I/O is higher than a single HBA capacity, use multipathing or multiple links.
11 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.
12 Use the most current host hardware.