fault tolerance

what is fault tolerance in vmware.
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Dawid FusekVirtualization Expert, Sr B&R, Storage SpecialistCommented:
if You asks about VMware Fault Tolerance, in simple worlds it's a mirroring a VMs between 2 ESXi servers, but not synchronizing a HDD data but synchronizing a CPU commands (via network between 2 ESXi servers) so both VMs are in fact identical and working as a mirror VMs, so if one of ESXi server's that service that VMs will fail, the second VM will take actions without any loosing of data (you will loose only TCP/IP sessions, but not any data, for applications it's invisible), so all data send to that VMs are synchronized between them.

It's a very nice feature for company critical servers like MS ADDC, DHCP, DNS, small SQL DB, etc, but not heavily loaded VM's (because it's hard to synchronize CPU operations via a network latency speed limits).

So there are some requirements and also limitations of such technology.
check this links, it's for vSphere v4.x but in 90% it's the same in VMware 5.x:
http://www.vmwarearena.com/2012/08/esx-host-infrastrcuture-requirements.html
and
http://www.vmwarearena.com/2012/08/virtual-machine-requirements-for-vmware.html

p.s. - VM FT will be good improved in vSphere v6.x
p.s. v2 - also remember that VM FT (and any other vendors simmilar technology) will not protect you from software failed in apps and kernel panic in linux or BSOD in Windows, but it protects you from almost any kind of HW fail of one ESXi server that your VM is untouched and still running.

best regards
NTShad0w
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
If you mean VMware FT (Fault Tolerance), it's a feature providing continuous availability of a virtual machine.

If a virtual machine fails, the "paired" virtual machines, takes over.

see here

http://www.vmware.com/files/pdf/VMware-Fault-Tolerance-FT-DS-EN.pdf
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GG VPCommented:
To add on to Andrew's answer, kindly refer below URL's for deeper understanding,

http://www.rodmach.com/p/VMwareFT.html?PHPSESSID=4bea3efef8f410a4285cce0e05e65114

http://theithollow.com/2013/10/vmware-fault-tolerance-ft/

Hope this helps!
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