6.7

n
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.
n
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 1‑51. 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 Disable 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, disable 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.
Disk Rate (KBps)
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.
vSphere Monitoring and Performance
VMware, Inc. 44