6.7

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.
n
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.
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 1‑57. 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.
vSphere Monitoring and Performance
VMware, Inc. 49