Failover Cluster

You are currently browsing the archive for the Failover Cluster category.

This post is part of the Failover Cluster Checklist series.

Validation process

Beware: Running cluster validation may cause failovers or offline cluster resources. 

Running cluster validation in production is not recommended, especially if you are troubleshooting an unstable cluster. I once took down a six-node Hyper-V cluster by running validation. It broke the storage connection to all nodes, and all the VMs crashed as a result. I recommend scheduling an outage, or at least informing the powers that be before you press the validate button.

  • Start the “Validate Configuration” wizard


  • Add servers. If you run this wizard on an existing cluster, the existing cluster nodes may be pre-added. Make changes as necessary.
  • Select “Run All tests”
  • Wait for the test to complete (5-60 minutes depending on configuration)

If you are troubleshooting a specific validation error or warning, re-running a full validation test may be time-consuming. Try re-running just the failing test until you get it working again, but remember to run a full validation afterwards to make sure you haven’t “fixed” something else.

Common errors in the report

Software updates missing, Validate software update levels

You can usually ignore this on a new cluster, as you should run windows update after installing the cluster anyway. On an existing cluster, fix as soon as possible. Sometimes, this rule will generate false positives due to the cluster nodes not being patched at the same time. This may lead to different KB numbers as one update may supersede another. You may have to remove patches from some nodes to correct this.


More than one VLAN on the same MAC address due to teaming.

Also known as a Converged networking setup. Not recommended. Make sure you have at least 2 completely independent networks.

Node reachable by only one interface

Make sure the network is highly available (NIC TEAM).


No disks


This is normal if you have a cluster configuration without shared storage. Otherwise, this warning points to mis-configured storage.

MPIO related errors or warnings

The SAN connection is mis-configured or faulty, or you need a DSM update. Check with your SAN admin.

SCSI-3 persistent reservations


Your SAN does not support clustered storage pools with the current firmware. If you do not plan to use storage pools in your cluster, this warning can safely be ignored.

Print This Post Print This Post


This post is part of the Failover Cluster Checklist series.

 What to do

For Windows 2008 automount should be disabled, for 2012 it should be enabled by default. If you have drive letter assignment problems, try disabling automount.

You can check and change this setting from an elevated command prompt:

  • Diskpart
  • Automount
  • Automount disable/enable

Sample from Windows 2012:



Automount can cause several issues in failover clusters, mostly related to drive letter and mount point mappings. The issues are more prevalent when using iSCSI storage due to the way iSCSI volumes are mounted. If you have FC storage where the LUN IDs are different on the cluster nodes, similar problems may occur. FC LUN IDs are configured in the SAN interface, not on the server.

  • I have experienced issues on Win 2008 and 2008R2 where the drive letter mappings of cluster disks change on failover if automount is enabled. I have seen volumes that are configured with a mount point suddenly being assigned a driveletter instead on failover, thus resulting in a failed resource. Cluster services may fail or behave erratically, as there are settings and configuration items who are bound to drive letters.
  • When you add storage to a cluster, automount could assign a driveletter to the new volume that is already in use on another node. This may lie dormant for quite some time, until you try to fail over the two drives to the same node. Failover cluster validation will spot this error.

I have yet to experience any automount related issues on Windows 2012, thus the reccomendation to keep automount enabled. The default setting is enabled, both on Win2008 and Win2012.

Print This Post Print This Post

This post is part of the Failover Cluster Checklist series.

For servers with a large amount of RAM, the page file may get very large if you do not change the default settings. Such a large page file is rarely required. To get a more sensible starting point, calculate a page file size using the following formula, and set a fixed page file size.

8GiB + 1GiB for each 8 GiB above 8GiB.

64GiB RAM 8 + (64-8)/8 = 15 15*1024=15360
128GiB RAM 8 + (128-8)/8 = 23 23*1024=23552
256iGB RAM 8 + (256-8)/8 = 39 39 * 1024 = 39936
384GiB RAM 8+(384-8)/8 = 55 55 * 1024 = 56320

I cannot remember where I first saw this formula, but I have seen it used in several posts and books. The actual value is not really important anymore, the most important point is to limit the size of the page file to keep it from filling up your local drives. This has become even more important with the use of local SSD drives, as they tend to be rather small to keep costs down. I do however not recommend setting a value lower than 8GiB, even if you have two terabytes of RAM running downhill at warp speed with turbo engaged. Windows 2012 seems to have more reasonable automation algorithms than those found in Windows 2008, but I see no reason to trust the automation,  as they are probably controlled by the mood of a nearby squirrel. Furthermore, if you are ever in a situation where you need more than the prescribed default page file size on a physical server with more than 64GiB of RAM, the page file is not going to save you. It will only slow down the inevitable decent into the abyss of dreadful performance from which a hard reboot may be the only way out.

Changing the page file size on Windows 2008-2012





Print This Post Print This Post

This post is part of the Failover Cluster Checklist series.

If you have to use a proxy server to reach the internet for patches etc from your datacenter, you should define a fixed proxy setting on your cluster nodes. If you do not, some installers will fail or complain about not being able to download the latest updates.

In internet explorer for the user you use during installation


From cmd or powershell for all users and programs without explicit proxy settings

Only complete this step if you get errors after setting proxy settings for the user. MSSQL cluster installations require this setting in some environments, and it is recommended for cluster aware updating.

netsh winhttp set proxy [proxy server name:port] bypass-list=”*.[AD domain fqdn];local”

Sample: netsh winhttp set proxy bypass-list=”*;local”


Print This Post Print This Post


In this guide, a fabric is a separate network infrastructure, be it SAN, WAN or LAN. A network may or may not be connected to a dedicated fabric. Some fabrics have more than one network.

The cluster nodes should be connected to each other over at least two independent networks/fabrics. The more independent the better. Ideally, the networks should share no components at all, but as a minimum they should be connected to separate NICs in the server. Ergo, if you want to use NIC teaming you should have at least 4 physical network ports on at least two separate NICs. The more the merrier, but be aware that as with all other forms of redundancy, higher redundancy equals higher complexity.

If you do not have more than one network port or only one network team, do not add an additional virtual network adapter/vlan for “heartbeat purposes”. The most prevalent network faults today are caused by someone unplugging the wrong cable, deactivating the wrong switch port or other user errors. Having separate vlans over the same physical infrastructure rarely offers any protection from this. You are better off just using the one adapter/team.

Previously, each Windows cluster needed a separate heartbeat network used to detect node failures. From Windows 2008 and newer (and maybe also on 2003) the “heartbeat” traffic is sent over all available networks between the cluster nodes unless we manually block it on specific cluster networks. Thus, we no longer need a separate dedicated heartbeat network, but adding a second network ensures that the cluster will survive failures on the primary network. Some cluster roles such as Hyper-V require multiple networks, so check what the requirements are for your specific implementation.

Quick takeaway

If you are designing a cluster and need a quick no-nonsense guideline regarding networks, here it comes:

  • If you use shared storage, you need at least 3 separate fabrics
  • If you use local storage, you need at least 2 separate fabrics

All but a few clusters I have been troubleshooting have had serious shortcomings and design failures in the networking department. The top problems:

  • Way to few fabrics.
  • Mixing storage and network traffic on the same fabric
  • Mixing internal and external traffic on the same fabric
  • Outdated faulty NIC firmware and drivers
  • Bad, poorly designed NICs from Qlogic and Emulex
  • Converged networking

Do not set yourself up for failure.


If you haven’t implemented IPv6 yet in your datacenter, you should disable IPv6 on all cluster nodes. If you don’t, you run a high risk of unnecessary failovers due to IPv6 to IPv4 conversion mishaps on the failover cluster virtual adapter. As long as IPv6 is active on the server, the failover cluster virtual adapter will use IPv6, even if none of the cluster networks have a valid IPv6 address. This causes all heartbeat traffic to be converted to/from IPv4 on the fly, which sometimes will fail. If you want to use IPv6, make sure all cluster nodes and domain controllers have a valid IPv6 address that is not link local (fe80:), and make sure you have routers, switches and firewalls that support IPv6 and are configured properly. You will also need IPv6 dns in the active directory domain.

Disabling IPv6

Do NOT disable IPv6 on the network adapters. The protocol binding for IPv6 should be enabled:


Instead, use the DisabledComponents registry setting. See Disable IPv6 for details.


Storage networks

If you use IP-based storage like ISCSI, SMB or FCOE, make sure you do not mix it with other traffic. Dedicated physical adapters should always be used for storage traffic. Moreover, if you are one of the unlucky few using FCOE you should seriously consider converting to FC or SMB3.

Hyper-V networks

In a perfect world, you should have six or more separate networks/fabrics for Hyper-v clusters. Sadly though, the world is seldom perfect. The absolute minimum for production clusters is two networks. Using only one network in production will cause nothing but trouble, so please do not try. Determining whether or not to use teaming also complicates matters further. As a general guide, I would strongly recommend that you always have a dedicated storage fabric with HA, that is teaming or MPIO, unless you use local storage on the cluster nodes. The storage connection is the most important one in any form of cluster. If the storage connection fails, everything else falls apart in seconds. For the other networks, throughput is more important than high availability. If you have to make a choice between HA and separate fabrics, chose separate fabrics for all other networks than the storage network.

7 Physical networks/fabrics

· Internal/Cluster/CSV (if local)/Heartbeat

· Public network for VMs

· VM Host management

· Live Migration

· 2*Storage (ISCSI, FC, SMB3)

· Backup

5 Physical networks/fabrics

· Internal/Cluster/CSV (if local)/Heartbeat/Live Migration

· Public network for vm, VM guest management

· VM Host management

· 2*Storage (ISCSI, FC, SMB3)

4 Physical networks/fabrics

· Internal/Live Migration

· Public & Management

· 2*Storage



Most blade server chasses today have a total of six fabric backplanes, grouped in three groups where each group connects to a separate adapter in the blade. Thus, each network adapter or FC HBA is connected to two separate fabrics/backplanes. The groups could be named A,B and C, with the fabrics named A1, A2, B1 and so on. Each group should have identical backplanes, that is the backplane in A1 should be the same model as the backplane in A2.

If we have Fibrechannel (FC) backplanes in group A, and 10G Ethernet backplanes in group B & C, we have several possible implementations. Group A will always be storage in this example, as FC is a dedicated storage network.


Here, we have teaming implemented on both B and C. Thus, we use the 4 networks configuration from above, splitting our traffic in Internal and Public/Management. This implementation may generate some conflicts during Live Migrations, but in return we get High Availability for all groups.


By splitting group B and C in two single ports, we get 5 fabrics and a more granulated separation of traffic at the cost of High Availability.

Hyper-V trunk adapters/teams on 2012

If you are using Hyper-V virtual switches bound to a physical port or team on you Hyper-V hosts, Hyper-V Extensible Virtual Switch should be the only bound protocol. Note: Do not change these settings manually, Hyper-V manager will change the settings automatically when you configure the virtual switch. If you bind the Hyper-V Extensible virtual switch protocol manually, creation of the virtual switch may fail.


Teaming in Windows 2012

In Windows 2012 we finally got native support for nic teaming. You access the nic teaming dialog from Server Manager. You can find a short description of the features here:, and a more detailed one here: Windows Server 2012 NIC Teaming (LBFO) Deployment and Management.

Native teaming support rids us of some of the problems related to unstable vendor teaming drivers, and makes setup of nic teaming a unified experience no matter what nics you are using. Note: never use nic teaming on ISCSI networks. Use MPIO instead.

A note on Active/Active teaming

It is possible to use active active teaming, thus aggregating the bandwidth of two or more adapters to support higher throughput. This is a fantastic technology, especially on 1G ethernet adapters where bandwidth congestion can become a problem. There is, however a snag; a lot of professional datacenters have a complete ban on active/active teaming due to years of teaming problems. I have my self been victim of unstable active/active teams, so I know this to be a real issue. I do think this is less of a problem in Windows 2012 than it was on previous versions, but there may still be configurations that just does not work. The more complex your network infrastructure is, the less likely active/active teaming is to work. Connecting all members in the team to the same switch increases the chance of success. This also makes the team dependent on a single switch of course, but if the alternative is bandwidth congestion or no teaming at all, it does not really matter.

I recommend talking to your local network specialist about teaming before creating a design dependent on active/active teaming.

Using multiple vlans per adapter or team

It has become common practice to use more than one vlan per team, or even more than one vlan per adapter. I do not recommend this for clusters, with the exception of adapters/teams connected to a Hyper-V switch. An especially stupid thing to do is mixing ISCSI traffic with other traffic on the same physical adapter. I have dealt with the aftermath of such a setup, and it does not look pretty unless data corruption is your kind of fun. And if you create a second vlan just to get an internal network for cluster heartbeat traffic on the same physical adapters you are using for client connections, you are not really achieving anything other than making your cluster more complex. The cluster validation report will even warn you about this, as it will detect more than one interface with the same MAC address.

Print This Post Print This Post

Tags: , ,

Binding for IPv6 should be enabled on all physical nics, uinless they are part of a team, in which case it should already be disabled. Virtual team nics should have IPv6 enabled if IPv4 is enabled. disabled on team virtual nics. The important point is to avoid a situation where the IPv4 binding is enabled, but not IPv6.

Physical nics should look like this:


Or like this:


But never like this:


IPv6 should be disabled through the creation of the DisabledComponents registry key.


You can use the following powershell command to read the registry key:

(get-ItemProperty -Path HKLM:\SYSTEM\CurrentControlSet\services\TCPIP6\Parameters -Name DisabledComponents).DisabledComponents


And you can use this command to create or update it if it doesn’t exist or has the wrong value. A reboot is required to apply the changes.

New-ItemProperty -Path HKLM:\SYSTEM\CurrentControlSet\services\TCPIP6\Parameters -Name DisabledComponents -PropertyType DWord -Value 0xffffffff


Print This Post Print This Post

Newer entries »

%d bloggers like this: