By user12611829 on Jun 11, 2007
Wikipedia defines virtualization as a technique for hiding the physical characteristics of computing resources from the way in which other systems, applications, or end users interact with those resources. While Wikipedia isn't the final authority, this definition is quite good and we will use it to start our exploration.
So what is true virtualization ? Anything that (potentially) hides architectural details from running objects (programs, services, operating systems, data). No more, no less - end of discussion.
Clearly VMware's virtualization products (ESX, Workstation) do that. They provide virtual machines that emulate the Intel x86 Instruction Set Architecture (ISA) so that operating systems think they are running on real hardware when in fact they are not. This type of virtualization would be classified as an abstraction type of virtual machines. But so is Xen, albeit with an interesting twist. In the case of Xen, a synthetic ISA based on the x86 is emulated removing some of the instructions that are difficult to virtualize. This makes porting a rather simple task - none of the user space code needs to be modified and the privileged code is generally limited to parts of the kernel that actually touch the hardware (virtual memory management, device drivers). In some respects, Xen is less of an abstraction as it does allow the virtual machines to see the architectural details thus permitting specific optimizations to occur that would be prohibited in the VMware case. And our good friends at Intel and AMD are adding new features to their processors to make virtualization less complicated and higher performance so the differences in approach between the VMware and Xen hypervisors may well blur over time.
But is this true virtualization ? No, it is just one of many types of virtualization.
How about the Java Virtual Machine (JVM) ? It is a run time executive that provides a virtualized environment for a completely synthetic ISA (although real pcode implementations have been done, they are largely for embedded systems). This is the magic behind write once and run anywhere and in general the approach works very well. So this is another example of virtualization - and also an abstraction type. And given the number of JVMs running around out there - if anyone is going to claim true virtualization, it would be the Java folks. Fortunately their understanding of the computer industry is broad and they are not arrogant - thus they would never suggest such folly.
Sun4v Logical Domains (LDOMs) are a thin hypervisor based partitioning of a radically multithreaded SPARC processor. The guest domains (virtual machines) run on real hardware but generally have no I/O devices. These guest domains get their I/O over a private channel from a service domain (a special type of domain that owns devices and contains the real device drivers). So I/O is virtualized but all other operations are executed on real hardware. The hypervisor provides resource (CPU and memory) allocation and management and the private channels for I/O (including networking). This too is virtualization, but not like Xen or VMware. This is an example of partitioning. Another example is IBM (Power) LPARS albeit with a slightly different approach.
Are there other types of virtualization ? Of course there are.
Solaris zones are an interesting type of virtualization called OS Virtualization. In this case we interpose the virtualization layer between the privileged kernel layer the non-privileged user space. The benefit here is that all user space objects (name space, processes, address spaces) are completely abstracted and isolated. Unlike the methods previously discussed, the kernel and underlying hardware resources are not artificially limited, so the full heavy lifting capability of the kernel is available to all zones (subject to other resource management policies). The trade-off for this capability is that all zones share a common kernel. This has some availability and flexibility limitations that should be considered in a system design using zones. Non-native (Branded) zones offers some interesting flexibilities that we are just now beginning to exploit, so the future of this approach is very bright indeed. And if I read my competitors announcements correctly, even our good friends at IBM are embracing this approach with future releases of AIX. So clearly there is something to this thing called OS Virtualization.
And there are other approaches as well - hybrids of the types we have been discussing. Special purpose libraries that either replace or interpose between common system libraries can provide some very nice virtualization capabilities - some of these transparent to applications, some not. The open source project Wine is a good example of this. User mode Linux and it's descendants offer some abilities to run an operating system as user mode program, albeit not particularly efficiently.
QEMU is an interesting general purpose ISA simulator/translator that can be used to host non-native operating systems (such as Windows while running Solaris or Linux). The interesting thing about QEMU is that you can strip out the translation features with a special kernel module (kqemu) and the result is very efficient and nicely performing OS hosting (essentially simulating x86 running on x86). Kernel-based Virtual Machines (KVM) extends the QEMU capability to add yet another style of virtualization to Linux. It is not entirely clear at present whether KVM is really a better idea or just another not invented here (NIH) Linux project. Time will tell, but it would have been nice for the Linux kernel maintainers to take a page from OpenSolaris and embrace an already existing project that had some non-Linux vendor participation (\*BSD, Solaris, Plan 9, plus some mainstream Linux distributions). At the very least it is confusing as most experienced IT professionals will associate KVM with Keyboard Video and Mouse switching products. There are other commercial products such as QuickTransit that use a similar approach (ISA translation).
And there are many many more.
So clearly the phrase "true virtualization" has no common or useful meaning. Questioning the application or definition of the phrase will likely uncover a predisposition or bias that might be a good starting point to carry on an interesting dialog. And that's always a good idea.
I leave you with one last thought. It is probably human nature to seek out the one uniform solution to all of our problems, the Grand Unification Theory being a great example. But in general, be skeptical of one size fits all approaches - while they may in fact fit all situations, they are generally neither efficient nor flattering. What does this have to do with virtualization ? Combining various techniques quite often will yield spectacular results. In other words, don't think VMware vs Zones - think VMware and Zones. In fact if you think Solaris, don't even think about zones, just do zones. If you need the additional abstraction to provide flexibility (heterogeneous or multiple version OS support) then use VMware or LDOMs. And zones.
Next time we'll take a look at abstraction style virtualization techniques and see if we can develop a method of predicting the overhead that each technique might impose on a system. Since a good apples to apples benchmark is not likely to ever see the light of day, perhaps some good old fashioned reasoning can help us make sense of what information we can find.
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