Data & System Availability

The dynamics of applications and desktops are making them location, device and time independent. Data and systems have completely different availability requirements. They are typically stored in a datacenter that is not dynamically provisioned, although we may see that change in the near future with the upcoming cloud computing initiatives.

Servers

It's servers that provide users or applications with the services they need. Services can be anything: web services, file and print services, authentication, database services, etc. In a traditional datacenter, these services are mostly executed on physical servers.

These physical servers come with a lot of resources that most services don't need. They either have way too much storage, CPU power and memory, or too little. When there's not enough resources available, adding more usually adds too much of the resource, over-dimensioning them.

Also, physical servers with local storage have a few disadvantages that limit their availability. If a physical server fails, the service is no longer available. A new server has to be setup, data restored and settings reconfigured. All in all a process that could take up to several days.

Storage

To cope with these availability problems, it makes sense to start with centralizing the storage. This makes it easier to allocate the right amount of storage to a service and makes it easier for the service to access it from another location, thus enhancing its availability.

Centralizing storage also has some disadvantages. All storage is now on one system that becomes a new single point of failure. If it fails, the whole infrastructure fails.

So this central storage has to be redundant in every aspect. It needs redundant connections, redundant switches, redundant power, redundant hard disks, redundant everything. This is what makes a Storage Area Network (SAN) more expensive than local storage.

The SAN infrastructure

Connectivity to the SAN can be divided into two main groups: Fiber Channel (FC) and Ethernet.

Where Fiber Channel provides the best performance, it's also the most expensive. A very valid question in designing a storage infrastructure therefore is 'does the customer really need that high end performance?'. The alternatives aren't really that far behind anymore.

Ethernet based infrastructures are less expensive because connectivity takes place over regular Ethernet switches and regular Network Interface Cards (NICs).

Not too long ago, iSCSI was the main storage protocol to be used over Ethernet. It allows LUNs to be presented as full disks to a host. With the upcoming virtualization technology however, NAS is a strong contestant now too. Whether it's NFS or CIFS, a host simply connects to a network share and stores it's data on the file system that the storage provides. This flexibility has some disadvantages though. Hosts are no longer managing the storage and proprietary file systems like VMFS don't work on it. On the other hand, a storage solution with a smart file system like NetApp's Write Anywhere File Layout (WAFL) makes it very easy, with the right toolset, to work with (consistent!) snapshots.