vSphere Fault Tolerance Part I : Introduction

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Table of Contents

introduction

In our conclusion of High Availability article , we have mentioned one disadvantage of HA : which downtime

We said that downtime normally 5 minutes and could be  extended to 10 minutes  , which could be acceptable for some VMs

But unfortunately 

In some high critical application VM , it’s NOT acceptable

>> so we have to find  a solution that could decrease downtime = 0 [Yes ZERO]

VMware provide feature FT [ Fault Tolerance ] that could make downtime = ZERO, and   data loss = ZERO

FT  is usually used for

  • business-critical applications that must be available all the time.
  • It is also sometimes used for applications that have no native capability for clustering.

How FT work 

The concept of Fault tolerance is  to make 2 redundant copy of  VM [primary and secondary]  in two ESXI servers

and   any change to primary VM will be immediate synchronized  to secondary VM through dedicated [port group] network connection ,so both will identical 

Both VM will use same IP address

  so  if primary VM down accidentally >> ESXI immediately will change connection to secondary replica VM

FT requirement

(FT) has the requirements  at three levels :

  • cluster requirements  
  • ESXI host requirements 
  • and virtual machine requirements 

let us to details each one of them : 

  cluster requirements

  • the ESXi hosts in the cluster must have access to the same datastores and networks. >> Done 
  • a minimum of two FT-certified ESXi hosts with the same FT version or host build number must be used.>> Done
  • the ESXi hosts must have FT logging and vMotion networking configured.>> Done : new port group called FT for FT sunch
  • vSphere HA must be enabled on the cluster.>> Done already from prev article 
  • host certificate checking must be enabled in the vCenter Server settings.  >> Done

 ESXI  host requirements

  • the hosts must have processors from an FT-compatible processor group.>> Done all esxi server in pioneers.lab are HP Gen10 
  • the hosts must be licensed for Fault Tolerance.>> Done currently we use trial license which open all features 
  • the configuration for each host must have Hardware Virtualization (HV) enabled in the BIOS.>> Done
  • the hosts must be certified for FT in the VMware HCL.>> Done HP servers 

  virtual machine requirements

  • only VMs with a single vCPU are supported with vSphere FT.>> Done
  • the VMs must be running a supported guest OS.>> Done >>we will make for running windows10 VM 
  • the VM files must be stored on shared storage that is accessible to all applicable ESXi hosts. Fibre Channel, FCoE, iSCSI, and NFS for shared storage are supported by FT.>> Done windows10 VM in ISCSI storage as we configured previously 
  • the VM’s virtual disks must be in thick provisioned format or a Virtual mode RDM. Physical mode RDMs are not supported.>> Done NO RDM currently used 
  • the VM must not have any snapshots.>> Done NO snapshot created for VM 
  • the VM must not be a linked clone.>> Done no Link cloe exist 
  • the VM cannot have any USB devices, sound devices, serial ports, or parallel ports in its configuration.>> Done NO peripheral used 
  • vSphere Fault Tolerance is not supported with a 2TB+ VMDK.>> Done VM is NO more than 100GB

FT disadvantages

at first glance : you will say FT cost double RAM and double CPU >> this is NOT the only disadvantage of FT 

vSphere FT has some disadvantages. including but NOT limited to 

  • increased resource usage. An FT-protected VM will use twice as much resources. For example, if the primary VM uses 2GB of RAM, the secondary VM will also use 2GB of RAM.
  • only virtual machines with a single vCPU are compatible with Fault Tolerance.
  •  vSphere FT require License NOT like HA .
  • the VM must not have any snapshots.

FT limitations

beside of some disadvantages above >> FT has limitation 

FT does NOT support some Features 

  • Storage vMotion
  • Linked clones
  • Virtual Volume DataStore.[vVOL]
  • Storage-based policy management. Storage policies are supported for vSAN storage.
  • I/O filters.
  • Disk encryption.
  • VBS enabled VMs.

FT maximum hardware supported :

  • Virtual Disk = 16
  • Disk size =2TB
  • Virtual CPU per VM =4
  • RAM per FT VM = 64GB
  • VM per host = 4
  • Virtual CPU per Host = 8

FT license

  • FT is supported in ALL vSphere License [Standard, vSphere Enterprise and vSphere Enterprise Plus Editions.]
  • VSphere Standard and Enterprise. Allows up to 2 vCPUs
  • VSphere Enterprise Plus. Allows up to 8 vCPUs

some FT considerations

  • You can use vSphere Fault Tolerance with vSphere Distributed Resource Scheduler (DRS) in vSphere 6.7. >>  BUT NOT required
  • FT is Based on VM  >> while HA is based on ESXI host
  • Downtime [RTO] = 0 second
  • FT could be enabled either VM is powered-on or powered-off
  • If you are using NFS to access shared storage, use dedicated NAS hardware with at least a 1Gbit NIC to obtain the network performance required for Fault Tolerance to work properly.
  • The memory reservation of a FT VM is set to the VM’s memory size when Fault Tolerance is turned on. >> so please  Ensure that a resource pool containing fault tolerant VMs has memory resources above the memory size of the virtual machines.
  • To ensure redundancy and maximum Fault Tolerance protection, >> you should have a minimum of three hosts in the cluster. In a failover situation, this provides a host that can accommodate the new Secondary VM that is created.

Conclusion

in this articles we taken an overview of Fault tolerance FT , with advantages and disadvantages , 

also we got some limitation of FT 

and finally some considerations that could help you when practice FT 

 

next article  FT Part II : we will see how to practice FT and test it 

 

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