Hi there! Today I`ll continue talking about disaster recovery functions, provided by VmWare, and I`d like to cover question of requirements and limitations of Fault Tolerance technology. As always, I`ll be terse and clear. Requirements for servers:
CPU:
- FT is available with following editions: Advanced, Enterprise, Enterprise Plus.
- The same CPU model, family, and stepping must be used on both active and passive servers.
- Supported CPUs are: AMD Barcelona+, Intel Penryn+. Also you can use VmWare Site Survey to identify if your host is FT capable. List of CPUs: 3300 Series, 5200 Series (DP), 5400 Series, 7400 Series; Intel Xeon based on Core i7 Microarchitecture Category: 3500 Series, 5500 Series; AMD 3rd Generation Opteron Category: 1300 and 1400 Series, 2300 and 2400 Series (DP), 8300 and 8400 Series (MP). List of supported OSes is available here. Need to note that all hosts have to use the same set of enabled/disabled functions. All hosts must use hardware virtualization.
- Hosts that are running active and passive VM must be running approximately on one CPU frequency, otherwise passive VM will be often restarted. The reason of possible difference in CPU frequency – power saving technologies.
Storage:
- VMs protected with FT must be on shared storage (NAS or SAN).
- This version of vSphere Fault Tolerance can`t be used with DRS
- Extensive load on VMFS volume can lead to failover, therefore FT VMs must be located on storage, which is not abused with often vMotion operations, VMs reloading and turning on/off.
- For now VMware Consolidated Backup (VCB) can not be used with Fault Tolerance (check this theme).
Memory:
- FT disables using of NPT/EPT (Nested Page Tables/Extended Page Tables) on the ESX host, therefore be sure, that you do not need it.
- Resource pool, containing FT VMs must have excess memory above the memory size of the virtual machines, because FT uses all memory reservation. If there is not enough, VMs will be not available to use memory overhead.
- When FT is turned on vCenter Server sets memory reservation to the memory size of the virtual machine. Mind that while FT is turned on it`s not possible to change memory reservation, size, limit, or shares.
Network:
- FT requires a dedicated 1 Gb network between hosts, while 10Gb network is required if FT is used for many VMs. It is better to use 9k JumboFrames for better perfomance. Each host must have a VMotion and a Fault Tolerance Logging NIC, which must be on different subnets.
- Ports 8100 and 8200 (TCP – out, UDP – in/out) must be open in firewall.
Common characteristics and perfomance:
- Host must be certified by OEM company as FT-compatible. List of such servers is available at HCL.
- Primary and secondary ESX hosts must be in HA cluster.
- VmWare recommends to use minimum 3 clusters: if one server goes down, it will allow to run VM on another host.
- Overhead depends on load and can be 5-20%.
- All hosts must use ESX\ESXi of the same version and one set of patches. You cannot use mixed ESX/ESXi environment.
- 4-8 VMs are recommended to run on a single post (either active or passive).
- Check host certificate check (on by default).
- Disabling the virtual machine restart priority setting for a fault tolerant virtual machine causes the Turn Off Fault Tolerance operation to fail. In addition, fault tolerant virtual machines with the virtual machine restart priority setting disabled cannot be deleted.
- Every VM in FT runs double capacity, and this should be built into configuration while using VMware DRS or VMware HA.
Requirements for VMs:
- NPIV (N-Port ID Virtualization) is not supported by VMware FT.
- Hot plugging devices cannot be done with VMware FT.
- USB and sound devices must be turned off since they are not supported by FT, either all non-replayable devices. You can`t also use VMDirectPath for such VMs.
- VMXNET3 (kb 1013757), paravirtual SCSI adapters, and some legacy NICare also not supported. VMXNET2 is supported, but in some OSes you will need to install VmTools, to get access to it instead of vLance.
- Extended Page Tables (EPT)/Rapid Virtualization Indexing (RVI) will be automatically disabled.
- VM must use not more than 1 vCPU.
- FT requires virtual hardware 7.
- VMs must not use paravirtualization, moreover VMI will not be available in future products.
- VMs should have no snapshots, or be a template.
- FT VM requires thick disk. If disk is thin or sparsely allocated, it will be automatically converted to thick one, while VM will be powered off.
- Datastore must use virtual RDM. Physical is not supported.
- VmWare recommends to run maximum 16 disks per VM.
- Storage vMotion is not supported for FT VMs.
- MSCS clustering must be removed from VM before protecting it with FT.
- HA must be on.
- VMCI stream socket connections can`t be established while FT is on.
- HotPlug will be turned off. To use it you must first turn off FT.
- Software virtualization is not supported.
- Combination of the virtual machine’s guest operating system and processor must be supported by Fault Tolerance.
Common:
- Actual release of AppSpeed can`t monitor VMs protected by FT, or will generate empty reports. KB 1013896