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Partitioning in network, storage systems (Part 2)

Posted: 29 Sep 2014     Print Version  Bookmark and Share

Keywords:virtualisation  network  storage  partitioning  stateful server 

In Part 1, we focused on the dramatic shift that virtualisation has caused across the network and storage industries and how partitioning-based solutions have been utilised to address some of the key challenges introduced by virtualisation.

I closed out Part 1 with a comment suggesting that these same partitioning solutions can also be used to benefit the deployment of both hypervisors and native operating systems.

In this part of the article, I am going to explore the value that these partitioning technologies can bring to bare metal environments. In order to appreciate that value, it is important to understand the typical architecture of today's servers and some of the inherent challenges that come with it.

The most significant set of challenges are related to the stateful nature of traditional servers. A stateful server is any server that contains data persistently stored on the server itself or has its identity information known by systems or devices external to the server. What makes a server stateful?
 • A local disc that contains the operating system used to boot the server and run applications
 • A network interface whose unique name may be configured in external systems (like a DHCP server), discovered and stored by application software, or discovered and stored by external Ethernet switches
 • A host bus adapter that is used to connect the server to shared storage in order to access application data. A host bus adapter has a unique name that is provided to the storage administrator to grant the server access to the assigned disks within the shared storage environment.
 • A CD drive with media inserted

You might look at this list and wonder what's the downside to any of those items—every server needs an operating system, so why not put it on the local disc? Many servers need to be connected to a shared storage environment – what's wrong with connecting using the identity of the host bus adapter that is on the server?

The short answer is lock-in. Any one or combination of the items above fundamentally limits the flexibility of the compute resources of that server. Let's take a simple example of a server that has a Windows operating system (along with a set of applications) installed on its local disc and has a host bus adapter, which provides connectivity to external storage that contains the application data for the server's applications.

Now let's look at what happens if there is a major failure of that server (e.g. CPU) or if you want to move that application onto a next-generation server. With a stateful design, the move can be complex and require non-trivial time and energy from IT staff.

The first problem is that the new server needs to get the operating system and applications from the failed server. Since the failed server has the root filesystem embedded in it, you have to make a choice – you can either re-install the OS and applications by hand on the new server, or you can try to move the disc to the new server (if it is hot swappable) or copy the contents. If you move the disc or copy it, you have some serious potholes to avoid.

The largest issue is the operating system. Windows, for example, is particular about the underlying hardware and the devices that it learned about during the installation of that OS. Changing the hardware will cause Windows to complain about the new devices, including the boot device. Issues with the boot device will actually prevent the server from booting. And, even if you hack the registry enough to convince Windows to boot, your next hurdle will be the changes to the network and storage adapters. New adaptors can provoke all sorts of problems.

On the network front, you will possibly have connectivity problems—maybe because your DHCP server configuration is based off of the old network adaptor's identity and now you cannot get an IP address.

To further complicate matters, you might end up having application start-up problems. Some applications actually tie their licenses to the unique name of the Ethernet adapter. When that name changes, the application will stop working because it considers itself not properly licensed. Additionally, when you connect your new server to the shared storage environment to access the same disks, you are going to have to get your storage administrator involved because the new host bus adaptor has its own unique name and that name has to be programmed into the storage array and the switching fabric to make the appropriate disks visible to this new device. If you have ever had to manage a shared storage device, you know all the things that can go wrong in that process. It's a real headache. In the end, the only sensible solution is to re-install and re-set up the OS and applications. Not the way you want to spend your IT hours.

Now you might be asking, doesn't virtualisation already solve this problem – it allows my OS and applications to be independent of the underlying hardware so I can avoid these lock-in issues.

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