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First Look At VMware's vSphere "Cloud OS"
snydeq writes "InfoWorld's Paul Venezia takes VMware's purported 'cloud OS,' vSphere 4, for a test drive. The bottom line: 'VMware vSphere 4.0 touches on almost every aspect of managing a virtual infrastructure, from ESX host provisioning to virtual network management to backup and recovery of virtual machines. Time will tell whether these features are as solid as they need to be in this release, but their presence is a substantial step forward for virtual environments.' Among the features Venezia finds particularly worthwhile is vSphere's Fault Tolerance: 'In a nutshell, this allows you to run the same VM in tandem across two hardware nodes, but with only one instance actually visible to the network. You can think of it as OS-agnostic clustering. Should a hardware failure take out the primary instance, the secondary instance will assume normal operations instantly, without requiring a VMotion.'"
FT only supports a single vCPU from my understanding... Not too many people running single CPU VM's, at least in my experience...
You should be running single vCPU machines by default and only scale CPU's if absolutely necessary and if the app is fully SMP aware and functional.
http://virtualize.wordpress.com/
If your apps are not SMP aware, WTF at you using? Multiple CPUs has been the standard for servers for at least a decade in x86 gear.
Yes, but it doesn't work in VMs the same way, at least not in VMWare. On a loaded system you often find a single-vCPU VM will out perform one with more than one vCPU, in fact if you can spread your app over multiple machines you are generally better off running two single CPU VMs instead of one dual-CPU one. This is true no matter how many physical CPUs/cores you have available.
Why is this? Because a single CPU VM can be scheduled when-ever there are time-slices available on any physical CPU/core (though a good hypervisor will try not bounce VMs between cores too often, as this reduces the potential gains from using the core's L1 cache and (on architectures where L2 cache isn't shared) L2 cache too), but a dual vCPU VM will have to wait until the hypervisor can give it timeslices on two CPUs at the same time. If this is the only VM that is actively doing anything on the physical machine (and the host OS is otherwise quiet too) this makes little difference aside from a small overhead on top of the normal hits for not running on bare metal, but as soon as that VM is competing with other processes for CPU resource it can have a massive negative effect on scheduling latency.
Instantly? Of course not. But the time required is equivalent to vmotion/live migration in bog-standard virtualization. How long? "That depends." To throw numbers at you, 30-100ms -- variance largely dependent upon how quickly your network infrastructure can react to MACs changing locations, whether in-flight TCP streams are broken as a result, etc. To help switches cope, people usually send a gratuitous ARP to jumpstart the process.
Like everyone else pointed out, it's a VM in lockstep with a 'shadow' VM. This is not just 'continuous VMotion'.
If something happens to the VM, the shadow VM goes live instantly (you don't notice a thing if you're doing something on the VM).
Right after that, the system starts bringing up another shadow VM on another host to regain full FT protection.
This can be network intensive, depending on the VM load, and currently only works with 1 vCPU per VM. Think 1-2 FT VMs per ESX host + shadow VMs.
You'll need recent CPUs that support FT and have an VMware HA / DRS Cluster set up.
So if you've got it, use it wisely. It's very cool.
-- Robi