Where does an IP-SAN solution fit? - Special SAN Section

Computer Technology Review, July, 2003 by Will Parker

Recent announcements affirming the ratification of the IP storage protocol from the Internet Engineering Task Force (IETF) and SNIA marks a milestone in the realization of IP-based storage solutions. The time is now right for vendors such as Elipsan to bring the benefits of Storage Area Networks (SANs) to a much larger customer base. The opportunity exists to leverage the benefits of many of the new storage technologies that have emerged over the last few years within the infrastructure of an IP-SAN. These benefits include the ability to build industry standard flexible, scalable cost-effective ways of providing remote mirroring, fault tolerance and high availability using standard switches and components, and easier management of shared storage systems--all at a significantly lower administrative burden.

Efficiencies gained through easier remote management, use of existing products and less wasted resources will all lead to a significantly lower total cost of ownership and much higher return on previous investments. The initial purchase price of systems and the associated cost-per-gigabyte will continue to drop significantly through the use of lower cost technologies, such as Ethernet networking and Serial ATA disk drives.

Background to IP-SAN

Until a few years ago, storage was still seen as an integral part of a file server, directly attached using either SCSI or a proprietary protocol. This is referred to as Direct Attached Storage (DAS) and, today, is still by far the most widely used.

The move towards networked storage started in the '80s with the introduction of Network Attached Storage devices and continued later in the '90s when Fibre Channel (FC) emerged as an alternative method of connecting storage. FC could be used in either a loop technology, with up to 126 devices attached to a loop, or in a fabric with switches connecting multiple file servers and storage devices. This removed the direct connection between file servers and storage, allowing storage to be networked with multiple file servers sharing storage devices on a Storage Area Network (SAN).

A SAN is a network that connects file servers and storage devices. A SAN storage device is a block-structured storage device, serving blocks to a file server. What is important about a SAN is that it separates the storage from servers, allowing the storage to be utilized and managed in much more effective ways than is possible with DAS. This allows storage to be consolidated in storage pools and allocated to file servers, as needed.

Instead of a systems manager having to take a server down to deal with a failed disk or a server that has run out of space, with a storage network servers can be provided with virtual disks with built-in redundancy and the ability to grow as the demand for capacity grows. Unlike a Direct Attached environment, storage can be added to a SAN without server downtime and disruption. Separating storage from servers also enables highly available systems with no single point of failure to be developed. If a file server fails, another can easily take over its storage, and if storage is mirrored on the SAN, a file server can easily connect to a mirror of a volume if the main volume (or the path to the main volume) fails.

SANs allow storage-related features such as snapshot and mirroring to be implemented in the SAN fabric, avoiding overhead on file servers and putting the functionality where it belongs on SAN devices, such as storage controllers.

NAS

In a client-server environment, file servers serve files to clients using a network filing protocol such as CIFS or NFS. A NAS device is simply a specialized file server, which replaces a conventional file server. What makes NAS attractive is the advantages it offers over a conventional file server. NAS devices are appliances, and the most important feature of appliances is their ease of use. NAS appliances are sold as plug-and-play devices, which can be added to a Local Area Network and configured within a few minutes. Configuration is done using simple, user-friendly Web GUIs. When you add a new NAS device to a network, client systems will simply see a new file server with one or more new network shares available.

NAS vendors typically claim that their devices are optimized for file serving. In some cases, particularly for high-end systems, the NAS devices are what are sometimes called "thin servers" running special purpose operating systems designed for optimized file serving. In order to meet the demands of the enterprise, high-end NAS systems have added business continuance features such as RAID storage, enhanced backup options including point-in-time snapshots, local and remote mirroring and high availability. Often, they have a SAN behind them, in which case they may be called NAS heads. However, these features are not specific to NAS, they are really related to the storage devices behind the NAS head. SAN devices can provide them equally well.

The major selling point for low- and mid-range NAS devices has been the fact that they provide an easy way of adding storage to a TCP/IP network. NAS has been very successful as a storage technology, and NAS products have penetrated most sectors of the storage market. At the low-end, very low cost NAS appliances have been sold to small businesses with little technical expertise; mid-range systems with extra functionality such as hardware RAID and mirroring have sold into the middle market; high-end systems with high-performance and business continuance features have been sold into the enterprise sector.