I wanted to discuss about one of the most talked about new features in vSphere. The new fault tolerance feature allows you to have a primary Virtual machine on one ESX host and a mirror of that Virtual Machine on another in lock step with the primary. If the primary virtual machine goes down for any reason, the mirror Virtual machine with takeover in 2 milliseconds. Since the memory and processor are in lockstep with each other, you do not lose any transactions. This sounds great but there are some restrictions that you have to take into account. Fault Tolerance (FT) provides continuous availability for applications by creating a live shadow instance of a virtual machine that is in virtual lockstep with the primary instance. By allowing instantaneous failover between the two instances in the event of hardware failure, VMware Fault Tolerance eliminates even the smallest of data loss or disruption.

The guest operating system support for VMware FT is different for vSphere in general. For specific guest operating system version information, see the Guest Operating System Installation Guide.

VMware collaborated with AMD and Intel in providing an efficient VMware FT capability on modern x86 processors. The collaboration required changes in both the performance counter architecture and virtualization hardware assists of both Intel and AMD. These changes could only be included in recent processors from both vendors: 3rd-Generation AMD Opteron based on the AMD Barcelona, Budapest and Shanghai processor families; and Intel Xeon processors based on the Penryn and Nehalem microarchitectures and their successors.

Some system vendors are certifying that their systems work with FT. You can find details on the at FT-certified systems.

You can also check processor, operating system, and virtual machine configuration compliance with VMware FT by downloading and running the VMware SiteSurvey utility from Champion Solutions Group Tools Section. It highlights compliance issues and describes how to correct them.

There are a number of requirements which must be met before FT can be setup.

• CPUs: Limited processors must be the same family (no mix/match)
• Requires Intel 31xx, 33xx, 52xx, 54xx, 55xx, 74xx or AMD 13xx,23xx, 83xx series of processors
• SMP virtual machines are not supported.
• Hardware Virtualization must be enabled in the BIOS.
• Hosts must be in a VMware High Availability enabled cluster.
• Storage: shared storage (FC, iSCSI, or NAS).
• Network: minimum of 3 NICs for various types of traffic (ESX Management/VMotion, virtual machine traffic, FT logging).
• GigE required for VMotion and FT logging.
• Minimized single points of failures in the environment. For example, NIC teaming, multiple network switches, storage multipathing.
• Primary and secondary hosts must be running the same build of ESX.

For VMware FT to perform as expected, it must run in an environment that meets specific pre-requisites.

• The primary and secondary fault tolerant virtual machines must be in a VMware HA cluster.
• Primary and secondary virtual machines must not run on the same host. FT automatically places the secondary virtual machine on a different host.
• Virtual machine files must be stored on shared storage.
• Shared storage solutions include NFS, FC, and iSCSI.
• For virtual disks on VMFS-3, the virtual disks must be thick, meaning they cannot be thin or sparsely allocated.
• Turning on VMware FT automatically converts the virtual machine to thick-eager zeroed disks.
• Virtual Raw Disk Mapping (RDM) is supported. Physical RDM is not supported.
• Multiple gigabit Network Interface Cards (NICs) are required.
• A minimum of two VMKernel Gigabit NICs dedicated to VMware FT Logging and VMotion.
• The FT Logging interface is used for logging events from the primary virtual machine to the secondary FT virtual machines.
• For best performance, use 10Gbit NIC rather than 1Gbit NIC, and enable the use of jumbo frames.
• VMware FT requires that Hardware Virtualization (HV) be turned on in the BIOS. The process for enabling HV varies among BIOSs. Contact your vendor for specifics.

VMware FT provides more continuity than VMware HA since FT does not require a virtual machine restart, and the secondary virtual machine immediately comes online with all, or almost all state information preserved.

Virtual machines protected by FT are not handled by VMware HA for restart priority. It is considered disabled in the restart priority.

I hope this helps when you are considering Fault Tolerance for your vSphere environment.

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Kevin Vogl July 20 2009 02:00:00 PM

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