User guide

Technical white paper | HP Enterprise Virtual Array Storage and VMware vSphere 4.x and 5.x configuration best practices

Using VMFS

VMFS is a high-performance cluster file system designed to eliminate single points of failure, while balancing storage

resources. This file system allows multiple vSphere 4.x hosts to concurrently access a single VMDK (Virtual Machine Disk

Format), as shown in Figure 19.

VMFS supports Fibre Channel SAN, iSCSI SAN, and NAS storage arrays.

Figure 19. VMFS datastore

Best practices for deploying VMFS

• To avoid spanning VMFS volumes, configure one VMFS volume per logical unit.

• In general limit VMs or VMDKs to 15-20 per volume.

• Either place I/O-intensive VMs on their own SAN volumes or use RDMs, which can minimize disk contention.

When placing I/O-intensive VMs on a datastore, start with no more than six to eight vSphere hosts per datastore. Monitor

performance on the vSphere hosts to ensure there is sufficient bandwidth to meet your application requirements and

that latency is acceptable.

Using RDM

As shown in Figure 20, RDM allows VMs to have direct access to Vdisks, providing support for applications such as MSCS

clustering or third-party storage management.

vSphere 4.x/5.x provides the following RDM modes, which support advanced VMware features like vMotion, High Availability

(HA), and Distributed Resource Scheduler (DRS):

• Virtual compatibility mode (vRDM):

– All I/O travels through the VMFS layer

– RDMs can be part of a VMware snapshot

• Physical compatibility mode (pRDM):

– All I/O passes directly through the underlying device

– pRDM requires the guest to use the virtual LSI Logic SAS controller

– pRDM is most commonly used when configuring MSCS clustering

There are some limitations when using RDM in conjunction with MSCS or VMware snapshots – for example, when

configuring an MSCS cluster between a physical Windows Server and a Windows virtual machine, you should utilize pRDM

because vRDM is not supported in that configuration. However, for a cluster between Windows Virtual Machine residing on

the same host (aka cluster in a box) you can use both vRDM and pRDM for the cluster configuration. Note that when

configuring Windows 2008 VMs in a cluster, you must use the LSISAS Virtual adapter for the shared pRDM.

For more information, refer to the VMware white paper, Setup for Failover Clustering and Microsoft Cluster Service.