Installation guide

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.
Table 22-7. Disk I/O Performance Enhancement Advice
# Resolution
1 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.
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. Consider array-side improvements
to increase throughput.
5 Use Storage VMotion to migrate I/O-intensive virtual machines across multiple ESX/ESXi 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 the Fibre Channel SAN
Configuration Guide.
8 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.
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 ESX/ESXi host hardware.
Memory Performance
Use the vSphere Client memory performance charts to monitor memory usage of clusters, hosts, virtual
machines, and vApps. Use the guidelines below to identify and correct problems with memory performance.
To ensure best performance, the host memory must be large enough to accommodate the active memory of
the virtual machines. Note that the active memory can be smaller than the virtual machine memory size. This
allows you to over-provision memory, but still ensures that the virtual machine active memory is smaller than
the host memory.
A virtual machine's memory size must be slightly larger than the average guest memory usage. This enables
the host to accommodate workload spikes without swapping memory among guests. Increasing the virtual
machine memory size results in more overhead memory usage.
If a virtual machine has high ballooning or swapping, check the amount of free physical memory on the host.
A free memory value of 6% or less indicates that the host cannot meet the memory requirements. This leads
to memory reclamation which may degrade performance. If the active memory size is the same as the granted
memory size, demand for memory is greater than the memory resources available. If the active memory is
consistently low, the memory size might be too large.
Chapter 22 Working with Performance Statistics
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