Archive for February 2013

Analyze HA cluster capacity to determine optimum cluster size

February 4, 2013 Objective 4 Business Continuity No comments

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Cluster Capacity Recommendations

  • How many hosts do you have
  • How many VMs do you have
  • Do you have reservations on your VMs
  • Remember the limit of 32 hosts per cluster
  • Do you have Oracle clustering considerations (CPU Licensing)?
  • Do you have to have separate clusters because of large VMs?
  • By pooling together hosts into larger clusters, DRS is far more efficient at VM placement and providing resource management. It also allows for more efficient HA policy management since the absorption of spare capacity needed for infrequent host failures is now spread out over a larger set of hosts
  • Check all hosts can see shared storage
  • Check your networking. Inconsistency in the network configuration may result in virtual machines losing network connectivity after redistribution, in the event they are moved to a physical host lacking the required VLAN.
  • It’s critical to ensure there is complete network redundancy in all paths between hosts in the cluster. The greatest risk with VMware HA implementations is a false isolation event where an ESX host is incorrectly identified as being offline, triggering an isolation response.
  • DRS load balancing is not instantaneous, so balancing virtual machines with rapidly oscillating load with more consistent VMware HA and DRS Capacity Planning workload characteristics will likely require a larger cluster to ensure a more rapid redistribution response.
  • There are also performance considerations as to the number of hosts that should concurrently access a single LUN, as well as storage IO considerations. Too many hosts with concurrent access to a single LUN can inflict a performance penalty due to LUN-level SCSI reservations associated with virtual machine file operations and LUN metadata updates.
  • Although an HA cluster can contain up to sixteen hosts, recommended cluster sizes generally fall somewhere between eight and twelve, based on the individual organization’s needs.
  • VMware ESX host hardware in the cluster should be as homogeneous as possible, specifically in terms of memory capacity, CPU clock speed, and core count. Because DRS relies on VMotion to migrate running workloads, all hosts must be VMotion compatible
  • It is best to have hosts of the same spec so you don’t create a skewed HA Failover situation.

Analyse performance metrics to calculate host failure requirements

February 4, 2013 Objective 4 Business Continuity No comments

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Performance Metrics to use

  • Use the inbuilt vCenter Performance charts

The vCenter Performance charts have 3 advanced settings which allow you to see Effective CPU Resources, Effective Memory Resources and Current Failover level

ClusterPerf

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  • Use ESXTOP/RESXTOP

ESXTOP and RESXTOP can be used in batch mode to monitor hosts over a day maximum. You can monitor all statistics or select the counters you want to monitor. The resulting csv file can then be imported into EXCEL, ESXPLOT or Perfmon on a Windows server to analyse the results. From these, you can probably see if there are any resources that may restrict the choice of cluster setting you apply. If you are low on resources then selecting a Specify Failover Host setting is probably not the best idea as wasting a host as a hot standby when you are already contrained for resources will take even more resource away

Analyze vSphere environment to determine appropriate HA admission control policy

February 4, 2013 Objective 4 Business Continuity No comments

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Choosing an Admission Control Policy

You should choose a vSphere HA admission control policy based on your availability needs and the characteristics of your cluster. When choosing an admission control policy, you should consider a number of factors.

  • Avoiding Resource Fragmentation

Resource fragmentation occurs when there are enough resources in aggregate for a virtual machine to be failed over. However, those resources are located on multiple hosts and are unusable because a virtual machine can run on one ESXi host at a time. The Host Failures Cluster Tolerates policy avoids resource fragmentation by defining a slot as the maximum virtual machine reservation. The Percentage of Cluster Resources policy does not address the problem of resource fragmentation. With the Specify Failover Hosts policy, resources are not fragmented because hosts are reserved for failover.

  • Flexibility of Failover Resource Reservation

Admission control policies differ in the granularity of control they give you when reserving cluster resources for failover protection. The Host Failures Cluster Tolerates policy allows you to set the failover level as a number of hosts. The Percentage of Cluster Resources policy allows you to designate up to 100% of cluster CPU or memory resources for failover. The Specify Failover Hosts policy allows you to specify a set of failover hosts.

  • Heterogeneity of Cluster

Clusters can be heterogeneous in terms of virtual machine resource reservations and host total resource capacities. In a heterogeneous cluster, the Host Failures Cluster Tolerates policy can be too conservative because it only considers the largest virtual machine reservations when defining slot size and assumes the largest hosts fail when computing the Current Failover Capacity. The other two admission control policies are not affected by cluster heterogeneity.

Understand interactions between DRS and HA

February 4, 2013 Objective 4 Business Continuity No comments

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Using vSphere HA and DRS Together

Using vSphere HA with Distributed Resource Scheduler (DRS) combines automatic failover with load balancing. This combination can result in a more balanced cluster after vSphere HA has moved virtual machines to different hosts.
When vSphere HA performs failover and restarts virtual machines on different hosts, its first priority is the immediate availability of all virtual machines. After the virtual machines have been restarted, those hosts on which they were powered on might be heavily loaded, while other hosts are comparatively lightly loaded.
vSphere HA uses the virtual machine’s CPU and memory reservation to determine if a host has enough spare capacity to accommodate the virtual machine.

In a cluster using DRS and vSphere HA with admission control turned on, virtual machines might not be evacuated from hosts entering maintenance mode. This behavior occurs because of the resources reserved for restarting virtual machines in the event of a failure. You must manually migrate the virtual machines off of the hosts using vMotion.
In some scenarios, vSphere HA might not be able to fail over virtual machines because of resource constraints. This can occur for several reasons.

  • HA admission control is disabled and Distributed Power Management (DPM) is enabled. This can result in DPM consolidating virtual machines onto fewer hosts and placing the empty hosts in standby mode leaving insufficient powered-on capacity to perform a failover.
  • VM-Host affinity (required) rules might limit the hosts on which certain virtual machines can be placed.
  • There might be sufficient aggregate resources but these can be fragmented across multiple hosts so that they can not be used by virtual machines for failover.

In such cases, vSphere HA can use DRS to try to adjust the cluster (for example, by bringing hosts out of standby mode or migrating virtual machines to defragment the cluster resources) so that HA can perform the failovers.
If DPM is in manual mode, you might need to confirm host power-on recommendations. Similarly, if DRS is in manual mode, you might need to confirm migration recommendations. If you are using VM-Host affinity rules that are required, be aware that these rules cannot be violated. vSphere HA does not perform a failover if doing so would violate such a rule.

Configure HA Related alarms and monitor a HA Cluster

February 4, 2013 Objective 4 Business Continuity No comments

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Monitoring a cluster

  • Highlight the cluster
  • Select Summary
  • You will see the following page

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  • Click Advanced Runtime. This tells you the current state of your cluster including the slot information and host information

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  • Click on Cluster Operational Status to see informational and warning messages associated with your cluster

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  • Click Cluster Status and tab through the below options

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Configuring Alarms

You may setup custom alerts of your own to monitor the health of your HA cluster. However, VMware provides a number of default alerts for you described below. These alarms may be updated to take custom actions. To access these settings, connect to the vCenter server using the vSphere client:

  1. Enter the Host and Clusters view (Ctrl + Shift + H)
  2. Highlight the vCenter Server
  3. Click on the Alarms tab
  4. Select the Definitions view
  5. Select one of the below described HA default alarms
  6. Right click the alarm and select Edit Settings
  7. Click the Actions tab

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Default System Actions

  1. Send a Notification email
  2. Send a Notification trap
  3. Run a command
  4. Power on VM
  5. Power off VM
  6. Suspend VM
  7. Reset VM
  8. Migrate VM
  9. Reboot Guest VM
  10. Shutdown Guest VM

To Configure a Sender Email Account for the vCenter Server

  1. Enter the Server Settings dialog (Ctrl + Shift + I)
  2. Click on Mail
  3. Enter an SMTP Server
  4. Enter a Sender Account
  5. Note. Additional settings will be required to allow vCenter to use a pre-defined account to send emails. Check with your third party email server documentation when setting up this account.

To Configure SNMP

  1. Enter the Server Settings dialog (Ctrl + Shift + I)
  2. Click on SNMP
  3. Enter up to four different Receiver URLs and Community Strings for your SNMP environment

Configure customised isolation response settings

February 4, 2013 HA, Objective 4 Business Continuity No comments

HA Isolation Responses

As seen in the below diagram, when HA detects a failure on one of the hosts, a response is triggered to deal with the Virtual Machines on that host

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Host Isolation Responses

First of all we need to look at the Host Monitoring which is a selectable box within the HA Settings

Host Monitoring

The restarting by VMware HA of virtual machines on other hosts in the cluster in the event of a host isolation or host failure is dependent on the “host monitoring” setting. If host monitoring is disabled, the restart of virtual machines on other hosts following a host failure or isolation is also disabled. Disabling host monitoring also impacts VMware Fault Tolerance because it controls whether HA will restart a Fault Tolerance (FT) secondary virtual machine after an event. Essentially a host will always perform the programmed host isolation response when it determines it is isolated. The host monitoring setting determines if virtual machines will be restarted elsewhere following this event.

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Isolation  Responses

When an isolation response is triggered, the isolated host must determine whether it must take any action based upon the configuration settings for the isolation response for each virtual machine that is powered on. The isolation response setting provides a means to dictate the action desired for the powered-on virtual machines maintained by a host when that host is declared isolated. There are three possible isolation response values that can be configured and applied to a cluster or individually to a specific virtual machine.These are

  • Leave Powered On
  • Power Off
  • Shut Down

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Leave Powered On

With this option, virtual machines hosted on an isolated host are left powered on. In situations where a host loses all management network access, it might still have the ability to access the storage subsystem and the virtual machine network. Selecting this option enables the virtual machine to continue to function if this were to occur. This is now the default isolation response setting in vSphere High Availability 5.0.

Power Off
When this isolation response option is used, the virtual machines on the isolated host are immediately stopped. This is similar to removing the power from a physical host. This can induce inconsistency with the file system of the OS used in the virtual machine. The advantage of this action is that VMware HA will attempt to restart the virtual machine more quickly than when using the third option.

Shut Down

Through the use of the VM Tools package installed within the guest operating system of a virtual machine, this option attempts to gracefully shut down the operating system with the virtual machine before powering off the virtual machine. This is more desirable than using the Power Off option because it provides the OS with time to commit any outstanding I/O activity to disk. HA will wait for a default of 300 seconds (5 minutes) for this graceful shutdown to occur. If the OS is not gracefully shut down by this time, it will initiate a power-off of the virtual machine. Changing the das.isolationshutdowntimeout attribute will modify this timeout if it is determined that more time is required to gracefully shut down an OS. The Shut Down option requires that the VM Tools package be installed in the guest OS. Otherwise, it is equivalent to the Power Off setting.

Best Practices

From a best practices perspective, Leave Powered On is the recommended isolation response setting for the majority of environments. Isolated hosts are a rare event in a properly architected environment, given the redundancy built in. In environments that use network-based storage protocols, such as iSCSI and NFS, the recommended isolation response is Power Off. With these environments, it is highly likely that a network outage that causes a host to become isolated will also affect the host’s ability to communicate to the datastores.
An isolated host will initiate the configured isolation response for a running virtual machine if either of the following is true

  • The host lost access to the datastore containing the configuration (.vmx) file for the virtual machine
  • The host still has access to the datastore and it determined that a master is responsible for the virtual machine.

To determine this, the isolated host checks for the accessibility of the “home datastore” for each virtual machine and whether the virtual machines on that datastore are “owned” by a master, which is indicated by a master’s having exclusively locked a key file that HA maintains on the datastore. After declaring itself as being isolated, the isolated host releases any locks it might have held on any datastores. It then checks periodically to see whether a master has obtained a lock on the datastore. After a lock is observed on the datastore by the isolated host, the HA agent on the isolated host applies the configured isolation response. Ensuring that a virtual machine is under continuous protection by a master provides an additional layer of protection. Because only one master can lock a datastore at a given time, this significantly reduces chances of “split-brain” scenarios. This also protects against situations where a complete loss of the management networks without a complete loss of access to storage would make all the hosts in a cluster determine they were isolated.
In certain environments, it is possible for a loss of the management network to also affect access to the heartbeat datastores. This is the case when the heartbeat datastores are hosted via NFS that is tied to the management network in some manner. In the event of a complete loss of connectivity to the management network and the heartbeat datastores, the isolation response activity resembles that observed in vSphere 4.x.
In this configuration, the isolation response should be set to Power Off so another virtual machine with access to the network can attempt to power on the virtual machine.
There is a situation where the isolation response will likely take an extended period of time to transpire. This occurs when all paths to storage are disconnected, referred to as an all-paths-down (APD) state, and the APD condition does not impact all of the datastores mounted on the host. This is due to the fact that there might be outstanding write requests to the storage subsystem that must time out. Establishing redundant paths to the storage subsystem will help prevent an APD situation and this issue.

Configure HA Redundancy

February 4, 2013 Objective 4 Business Continuity No comments

HA in vSphere 5

The way HA works in vSphere 5 is quite different to the way it worked in vSphere 4. vSphere HA now uses a new tool called FDM (Fault Domain Manager) which has been developed to replace AAM (Automated Availability Manager) AAM had limitations in reliance on name resolution and scalability limits. Improvements such as

  • FDM uses a Master/Slave architecture which does not rely on Primary/Secondary host designations
  • FDM uses both the management network and storage devices for communications
  • FDM introduces support for IPv6
  • FDM addresses the issues of network partition and network isolation

How it works

  • When vSphere HA is enabled, the vSphere HA agents enter an election to pick a vSphere HA Master.
  • The vSphere HA master monitors slave hosts and will restart VMs in the event of a failover
  • The vSphere HA master monitors the power state of all protected machines and if the VM fails, it will be restarted
  • The vSphere HA master manages the list of hosts that are members of the cluster and manages the adding/removing of hosts into a cluster
  • The vSphere HA master manages the list of protected VMs
  • The vSphere HA master caches the cluster configuration. The master notifies and informs slave hosts of changes in the cluster
  • The vSphere HA master sends heartbeat messages to the slave hosts so the slaves know the master is still alive
  • The vSphere HA master reports state information to vCenter (Only the master does this)

HA  Process

  • The hosts within an HA cluster constantly heartbeat with the host designated as the master over the management network. The first step in determining whether a host is isolated is detecting a lack of these heartbeats.
  • After a host stops communicating with the master, the master attempts to determine the cause of the issue.
  • Using heartbeat datastores, the master can distinguish whether the host is still alive by determining if the affected host is maintaining heartbeats to the heartbeat datastores. This enables the master to differentiate between a management network failure, a dead host and a partitioned/isolated situation.
  • The time elapsed before the host declares itself isolated varies depending on the role of the host (master or slave) at the time of the loss of heartbeats.
  • If the host was a master, it will declare itself isolated within 5 seconds.
  • If the host was a slave, it will declare itself isolated in 30 seconds.
  • The difference in time is due to the fact that if the host was a slave, it then must go through an election process to identify whether any other hosts exist or if the master host simply died. This election process starts for the slave at 10 seconds after the loss of heartbeats is detected.
  • If the host sees no response from another host during the election for 15 seconds, the HA agent on a host then elects itself as a master, checks whether it is isolated and, if so, drops into a startup state.
  • In short, a host will begin to check to see whether it is isolated whenever it is a master in a cluster with more than one other host and has no slaves. It will continue to do so until it becomes a master with a slave or connects to a master as a slave.
  • At this point, the host will attempt to ping its configured isolation addresses to determine the viability of the network. The default isolation address is the gateway specified for the management network.
  • Advanced settings can be used to modify the isolation addresses used for your particular environment. The option das. isolationaddress[X] (where X is 1–10) is used to configure multiple isolation addresses.
  • Additionally das. usedefaultisolationaddress is used to indicate whether the default isolation address (the default gateway) should be used to determine if the host is network isolated. If the default gateway is not able to receive ICMP ping packets, you must set this option to “false.”
  • It is recommended to set one isolation address for each management network used, keeping in mind that the management network links should be redundant, as previously mentioned.
  • The isolation address used should always be reachable by the host under normal situations, because after 5 seconds have elapsed with no response from the isolation addresses, the host then declares itself isolated.
  • After this occurs, it will attempt to inform the master of its isolated state by use of the heartbeat datastores.

What does HA use?

  • Management Network
  • Datastore Heartbeats

Management Network

Ideally the Management network should be set up as a fully redundant network team at the adapter level or at the Management network level. It can either be setup as Etherchannel with Route based on IP Hash or in an Active/Standby configuration allowing for failover should one network card fail

In the event that the vSphere HA Master cannot communicate with a slave through the management network isolation address, it can then check its Heartbeat Datastores to see if the host is still up and running. This helps vSphere HA deal with Network Partitioning and Network Isolation

Switch Setup

Requirements:

  • Two physical network adaptors
  • VLAN trunking
  • Two physical switches

The vSwitch should be configured as follows:

  • Load balancing = route based on the originating virtual port ID (default)
  • Failback = no
  • vSwitch0: Two physical network adaptors (for example: vmnic0 and vmnic2)
  • Two port groups (for example, vMotion and management)

In this example, the management network runs on vSwitch0 as active on vmnic0 and as standby on vmnic2. The vMotion network runs on vSwitch0 as active on vmnic2 and as standby on vmnic0
Each port group has a VLAN ID assigned and runs dedicated on its own physical network adaptor. Only in the case of a failure is it switched over to the standby network adaptor. Failback is set to “no” because in the case of physical switch failure and restart, ESXi might falsely recognize that the switch is back online when its ports first come online. In reality, the switch might not be forwarding on any packets until it is fully online. However, when failback is set to “no” and an issue arises, both your management network and vMotion network will be running on the same network adaptor and will continue running until you manually intervene.

Network Partitioning

This is what happens when one or more of the slaves cannot communicate with the vSphere HA Master even though they still have network connectivity. Checking the Datastore heartbeats is then used to see whether the slave hosts are alive

When a management network failure occurs for a vSphere HA cluster, a subset of the cluster’s hosts might be unable to communicate over the management network with the other hosts. Multiple partitions can occur in a cluster.
A partitioned cluster leads to degraded virtual machine protection and cluster management functionality. Correct the partitioned cluster as soon as possible.

Virtual machine protection. vCenter Server allows a virtual machine to be powered on, but it is protected only if it is running in the same partition as the master host that is responsible for it. The master host must be communicating with vCenter Server. A master host is responsible for a virtual machine if it has exclusively locked a system-defined file on the datastore that contains the virtual machine’s configuration file.

Cluster management. vCenter Server can communicate with only some of the hosts in the cluster, and it can connect to only one master host. As a result, changes in configuration that affect vSphere HA might not take effect until after the partition is resolved. This failure could result in one of the partitions operating under the old configuration.

Advanced Cluster Settings which can be used

  • das.isolationaddressx Used to configure multiple isolation addresses.
  • das.usedefaultisolationaddress Set to true/false and used in the cse where a default gateway is not pingable, in which case this set to false in conjunction with configuring another address for das.isolationaddress
  • das.failuredetectiontime. Increase to 30 seconds (30000) to decrease the likelyhood of a false positive

NOTE: If you change the value of any of the following advanced attributes, you must disable and then re-enable vSphere HA before your changes take effect.

Network Isolation

This is where one or more slave hosts have lost all management network connectivity. Isolated hosts can’t communicate with the master or other slaves. The slave host uses the heartbeat datastore to notify the master that it is isolated via special binary file called host-X-poweron file.

Datastore Heartbeating

By default, vCenter will automatically select two datastores to use for storage heartbeats. An algorithm designed to maximize availability and redundancy of the storage heartbeats selects these datastores. This algorithm attempts to select datastores that are connected to the highest number of hosts. It also attempts to select datastores that are hosted on different storage arrays/NFS servers. A preference is given to VMware vSphere VMFS–formatted datastores, although NFS-hosted datastores can also be used.

  • Highlight your cluster
  • Click Edit Settings
  • Select Datastore Heartbeating

Datastore Heartbeat

  • Select only from my preferred datastores restricts HA to using only those selected from the list
  • Select any of the cluster datastores disables the selection of datastores from the list. Any cluster datastore can be used by HA for heartbeating
  • Select any of the cluster datastores taking into account my preferences is a mix of the previous 2 options. The Admin selects the preferred datastores that HA should use. vSphere selects which ones to use from these. If any become unavailable, HA will choose another from the list.
  • The vSphere HA Cluster Status box will show you which datastores are being used

Datastore Heartbeat2

VMware Availability Guide

http://pubs.vmware.com

vSphere High Availability Deployment Best Practices

http://www.vmware.com/files/pdf/techpaper/vmw-vsphere-high-availability.pdf