High Availability and Fault Tolerance in VMware

Overview

High availability and fault tolerance are critical for ensuring business continuity and minimizing downtime in virtualized environments. VMware provides multiple technologies to protect against hardware failures and maintain service availability.

VMware High Availability (HA)

vSphere HA Architecture

vSphere HA automatically restarts virtual machines on alternative hosts when a server fails, providing cost-effective protection against unplanned outages.

Key Components:

• Master Agent: Coordinates HA activities on the cluster
• Slave Agents: Monitor and report on each host
• Admission Control: Ensures sufficient resources for failover
• Heartbeat Mechanism: Monitors cluster health

Enabling and Configuring vSphere HA

Cluster Configuration:

1. Create or Select Cluster

   • Navigate to Hosts and Clusters view
   • Right-click and select "New Cluster"

2. Enable vSphere HA

   • Go to cluster settings
   • Select "Turn ON vSphere HA"

3. Configure HA Settings

   • Host monitoring status
   • VM monitoring status
   • Admission control policies
   • Datastore heartbeat configuration

Admission Control Policies:

• Cluster resource percentage: Reserves resources for failover
• Number of host failures: Reserves resources based on host failures
• Specify failover host: Designates specific hosts for failover

HA Response Types

VM Failure Response:

• Restart VMs: Default response for most VM failures
• Power off: Immediate power-off for critical situations
• Leave Powered On: No action taken
• Disabled: HA disabled for specific VMs

Host Isolation Response:

• Power off: Power off VMs when host becomes isolated
• Shutdown and restart: Attempt graceful shutdown first
• Leave Powered On: Keep VMs running on isolated host

HA Best Practices

• Minimum 3 hosts: For proper master/slave election
• Resource reservation: Ensure sufficient resources for failover
• Network configuration: Separate management and VM networks
• Testing: Regular failover testing to verify functionality

VMware Fault Tolerance (FT)

FT Architecture

Fault Tolerance provides continuous availability for critical virtual machines by creating a secondary VM that runs in lockstep with the primary VM.

Key Features:

• Zero downtime: Continuous availability for critical applications
• Lockstep execution: Primary and secondary VMs stay synchronized
• Transparent failover: Automatic failover with no service interruption

FT Configuration Requirements

Hardware Requirements:

• Compatible processors: Same vendor and generation
• CPU features: VT-x/EPT or AMD-RVI/NPT enabled
• Shared storage: VMs must use shared storage
• Network configuration: Compatible network setup

Software Requirements:

• VMware vSphere Enterprise Plus: Required license level
• Compatible guest OS: Support for FT-enabled operating systems
• Single vCPU: Only single-vCPU VMs supported initially

Enabling FT

1. Verify Compatibility

   • Check host compatibility
   • Ensure shared storage
   • Verify VM configuration

2. Enable FT Logging

   • Configure dedicated network for FT logging
   • Ensure sufficient bandwidth

3. Turn On FT

   • Right-click VM and select "Fault Tolerance"
   • Select "Turn On Fault Tolerance"

FT Operational States

• Protected: Both primary and secondary VMs running
• Protecting: Secondary VM is being created
• Unprotected: FT is disabled or failed
• Incompatible: VM is not compatible with FT

Distributed Resource Scheduler (DRS)

DRS Architecture

DRS provides automated resource management by balancing workloads across hosts in a cluster.

Key Components:

• Service Engine: Makes resource allocation decisions
• Agent: Executes recommendations on hosts
• Load Balancer: Monitors and balances resource usage

DRS Configuration

Cluster Settings:

1. Enable DRS

   • Turn on DRS for the cluster
   • Select automation level

2. Automation Levels:

   • Manual: Only recommendations
   • Partially Automated: Recommendations for initial placement
   • Fully Automated: Automatic placement and load balancing

3. Migration Threshold

   • Level 1-5: Conservative to Aggressive
   • Determines frequency of migrations

DRS Rules and Affinity

VM-Host Affinity Rules:

• Must run on hosts in group: Mandatory placement
• Should run on hosts in group: Preferred placement
• Must not run on hosts in group: Mandatory exclusion
• Should not run on hosts in group: Preferred exclusion

VM-VM Affinity Rules:

• Keep virtual machines together: Anti-affinity rules
• Separate virtual machines: Affinity rules

DRS Resource Pools

• Hierarchical resource pools: Nested resource allocation
• Resource shares: Relative priority for resource allocation
• Resource reservations: Guaranteed minimum resources
• Resource limits: Maximum resource allocation

vSphere Replication

vSphere Replication Architecture

vSphere Replication provides efficient, application-aware replication of virtual machines to alternative sites.

Key Features:

• Application-consistent snapshots: Ensures data integrity
• Flexible scheduling: Multiple replication schedules
• Compression and throttling: Optimizes network usage
• Point-in-time recovery: Multiple recovery points

Configuring vSphere Replication

1. Enable Replication

   • Select VM for replication
   • Configure target site

2. Set Replication Schedule

   • Frequency of replication
   • Retention policy

3. Monitor Replication

   • Status of replication jobs
   • RPO compliance monitoring

Site Recovery Manager (SRM)

SRM Architecture

SRM automates disaster recovery processes and provides orchestration for failover and failback operations.

Components:

• SRM Server: Manages recovery plans
• Protection Groups: Collections of replicated VMs
• Recovery Plans: Orchestration of failover procedures

SRM Configuration Process

1. Install SRM

   • Deploy SRM appliances
   • Configure site pairing

2. Create Protection Groups

   • Define VMs to protect
   • Configure replication

3. Design Recovery Plans

   • Order of VM startup
   • Network mapping
   • Custom scripts

Storage vMotion and Live Migration

vSphere vMotion

vMotion enables live migration of running virtual machines between hosts without service interruption.

Requirements:

• Compatible CPUs: Similar processor families
• Shared storage: VM files accessible to both hosts
• Network connectivity: vMotion network configured
• Sufficient resources: Target host has adequate capacity

Storage vMotion

Storage vMotion allows moving virtual machine files between storage locations without downtime.

Use Cases:

• Storage maintenance: Moving VMs during storage upgrades
• Performance optimization: Moving to faster storage
• Space management: Freeing up space on full datastores

Monitoring and Alerting

HA Monitoring

Key Metrics:

• Host connectivity: Monitor host availability
• VM status: Track VM health
• Resource availability: Verify failover capacity
• Heartbeat status: Monitor cluster health

Alert Configuration

Critical Alerts:

• Host failure: Immediate notification
• HA agent failure: Monitor HA functionality
• Admission control violations: Resource constraint alerts
• VM restart failures: Failed VM recovery attempts

Troubleshooting HA and FT Issues

Common Problems

• HA not restarting VMs: Admission control or resource issues
• FT synchronization failures: Network or storage problems
• DRS not making recommendations: Constraint violations
• vMotion failures: Compatibility or network issues

Diagnostic Tools

• vSphere Client: Built-in health monitoring
• Log files: Detailed troubleshooting information
• esxtop: Real-time performance monitoring
• Network utilities: Connectivity verification

Resolution Steps

1. Identify root cause: Analyze symptoms and logs
2. Check configuration: Verify settings and requirements
3. Validate connectivity: Test network and storage paths
4. Apply fixes: Implement appropriate solutions
5. Test functionality: Verify repairs work

Best Practices for High Availability

Design Guidelines

• Redundant components: Multiple paths and devices
• Capacity planning: Adequate resources for failover
• Network segmentation: Separate critical network traffic
• Regular testing: Validate failover procedures

Performance Considerations

• Resource allocation: Balance protection with performance
• Network bandwidth: Ensure adequate capacity for FT and vMotion
• Storage performance: Account for replication overhead
• Monitoring overhead: Optimize monitoring impact

Security Considerations

• Access controls: Limit administrative access
• Network security: Secure management and replication networks
• Encryption: Protect replicated data
• Audit trails: Monitor HA and FT activities

Conclusion

High availability and fault tolerance features in VMware provide essential protection against hardware failures and service disruptions. Proper implementation of these technologies ensures business continuity and minimizes the impact of unplanned outages.

In the next article, we'll explore vCenter Server and centralized management, covering how to effectively manage VMware environments from a single console.