Tuesday Sep 23, 2008

LDOMs or Containers, that is the question....

An often asked question, do I put my application in a container (zone) or an LDOM ? My question in reply is why the or ? The two technologies are not mutually exclusive, and in practice their combination can yield some very interesting results. So if it is not an or, under what circumstances would I apply each of the technologies ? And does it matter if I substitute LDOMs with VMware, Xen, VirtualBox or Dynamic System Domains ? In this context all virtual machine technologies are similar enough to treat them as a class, so we will generalize to zones vs virtual machines for the rest of this discussion.

First to the question of zones. All applications in Solaris 10 and later should be deployed in zones with the following exceptions
  • The restricted set of privileges in a zone will not allow the application to operate correctly
  • The application interacts with the kernel in an intimate fashion (reads or writes kernel data)
  • The application loads or unloads kernel modules
  • There is a higher level virtualization or abstraction technology in use that would obviate any benefits from deploying the application in a zone
Presented a different way, if the security model allows the application to run and you aren't diminishing the benefits of a zone, deploy in a zone.

Some examples of applications that have difficulty with the restrictive privileges would be security monitoring and auditing, hardware monitoring, storage (volume) management software, specialized file systems, some forms of application monitoring, intrusive debugging and inspection tools that use the kernel facilities such as the DTrace FBT provider. With the introduction of configurable zone privileges in Solaris 10 11/06, the number of applications that fit into this category should be few in number, highly specialized and not the type of application that you would want to deploy in a zone.

For the higher level abstraction exclusion, think of something at the application layer that tries to hide the underlying platform. The best example would be Oracle RAC. RAC abstracts the details of the platform so that it can provide continuously operating database services. It also has the characteristic that it is itself a consolidation platform with some notion of resource controls. Given the complexity associated with RAC, it would not be a good idea to consolidate non-RAC workloads on a RAC cluster. And since zones are all about consolidation, RAC would trump zones in this case.

There are other examples such as load balancers and transaction monitors. These are typically deployed on smaller horizontally scalable servers to provide greater bandwidth or increases service availability. Although they do not provide consolidation services, their sophisticated availability features might not interact well with the nonglobal zone restrictive security model. High availability frameworks such as SunCluster do work well with zones. Zones abstract applications in such a way that service failover configurations can be significantly simplified.

Unless your application falls under one of these exemptions, the application should be deployed in a zone.

What about virtual machines ? This type of abstraction is happening at a much lower level, in this case hardware resources (processors, memory, I/O). In contrast, zones abstract user space objects (processes, network stacks, resource controls). Virtual machines allow greater flexibility in running many types and versions of operating systems and applications but also eliminates many opportunities to share resources efficiently.

Where would I use virtual machines ? Where you need the diversity of multiple operating systems. This can be different types of operating system (Windows, Linux, Solaris) or different versions or patch levels of the same operating system. The challenge here is that large sites can have servers at many different patch and update versions, not by design but as a result of inadequate patching and maintenance tools. Enterprise patch management tools (xVM OpsCenter), patch managers (PCA), or automated provisioning tools (OpsWare) can help reduce the number of software combinations and online maintenance using Live Upgrade can reduce the time and effort required to maintain systems.

It is important to understand that zones are not virtual machines. Their differences and the implications of this are
  • Zones provide application isolation on a shared kernel
  • Zones share resources very efficiently (shared libraries, system caches, storage)
  • Zones have a configurable and restricted set of privileges
  • Zones allow for easy application of resource controls even in a complex dynamic application environment
  • Virtual machines provide relatively complete isolation between operating systems
  • Virtual machines allow consolidation of many types and versions of operating systems
  • Although virtual machines may allow oversubscription of resources, they provide very few opportunities to share critical resources
  • An operating system running in a virtual machine can still isolate applications using zones.
And it is that last point that carries this conversation a bit farther. If the decision between zones and virtual machines isn't an or, under what conditions would it be an and, and what sort of benefit can be expected ?

Consider the case of application consolidation. Suppose you have three applications: A, B and C. If they are consolidated without isolation then system maintenance becomes cumbersome as you can only patch or upgrade when all three application owners agree. Even more challenging is the time pressure to certify the newly patched or upgraded environment due to the fact that you have to test three things instead of one. Clearly isolation is a benefit in this case, and it is a persistent property (once isolated, forever isolated).

Isolation using zones alone will be very efficient but there will be times when the common shared kernel will be inconvenient - approaching the problems of the non-isolated case. Isolation using virtual machines is simple and very flexible but comes with a cost that might be unnecessary.

So why not do both ? Use zones to isolate the applications and use virtual machines for those times when you cannot support all of the applications with a common version of the operating system. In other words the isolation is a persistent property and the need for heterogeneous operating systems is temporary and specific. With some small improvements in the patching and upgrade tools, the time frame when you need heterogeneous operating systems can be reduced.

Using our three applications as an example, A B and C are deployed in separate zones on a single system image, bare metal or in a virtual machine. Everything is operating spectacularly until a new OS upgrade is available which provides some important new functionality for application A. So application owner A wants to upgrade immediately, application B doesn't care one way or the other, and (naturally) application C has just gone into seasonal lock-down and cannot be altered for the rest of the year.

Using zones and virtual machines provides a unique solution. Provision a new virtual machine with the new operating system software, either on the same platform by reassigning resources (CPU, memory) or on a separate platform. Next clone the zone running application A. Detach the newly cloned zone and migrate it to the new virtual machine. A new feature in Solaris 10 10/08 will automatically upgrade the new zone upon attachment to a server running newer software. Leave the original zone alone for some period of time in the event that an adverse regression appears that would force you to revert to the original version. Eventually the original zone can be reclaimed, but at a time when convenient.

Migrate the other two applications at a convenient time using the same procedure. When all of the applications have been migrated and you are comfortable that they have been adequately tested, the old system image can be shut down and any remaining resources can be reclaimed for other purposes. Zones as the sole isolation agent cannot do this and virtual machines by themselves will require more administrative effort and higher resource consumption during the long periods when you don't need different versions of the operating system. Combined you get the best of both features.

A less obvious example is ISV licensing. Consider the case of Oracle. Our friends at Oracle consider the combination of zones and capped resource controls as a hard partition method which allows you to license their software to the size of the resource cap, not the server. If you put Oracle in a zone on a 16 core system with a resource cap of 2 cores, you only pay for 2 cores. They have also made similar considerations for their Xen based Oracle VM product yet have been slow to respond to other virtual machine technologies. Zones to the rescue. If you deploy Oracle in a VM on a 16 core server you pay for all 16 cores. If you put that same application in a zone, in the same VM but cap the zone at 4 cores then you only pay for 4 cores.

Zones are all about isolation and application of resouce controls. Virtual machines are all about heterogeneous operating systems. Use zones to persistently isolate applications. Use virtual machines during the times when a single operating system version is not feasible.

This is only the beginning of the conversation. A new Blueprint based on measured results from some more interesting use cases is clearly needed. Jeff Savit, Jeff Victor and I will be working on this over the next few weeks and I'm sure that we will be blogging with partial results as they become available. As always, questions and suggestions are welcome. <script type="text/javascript"> var sc_project=1193495; var sc_invisible=1; var sc_security="a46f6831"; </script> <script type="text/javascript" src="http://www.statcounter.com/counter/counter.js"></script>



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