Moving from direct storage

Increasing volumes of data being stored by organisations is driving a need to develop and implement more efficient storage infrastructures. What are the pros and cons of Network Attached Storage?

It has been estimated that storage requirements are growing by 100% per year in the average organisation - a level of growth that Butler Group believes to be unsustainable without a serious rethink about the way in which the data is stored.

The traditional form of storage is for a server to have storage resources - hard disks - directly attached to it. Although this architecture has worked well in the past (and, in Butler Group's opinion, is still the best solution for some companies), many organisations are now looking at alternative infrastructures to support the growing mountain of data. The two major storage architectures that have emerged are Network Attached Storage (NAS) and Storage Area Networks (SANs).

SAN is the architecture that is creating most excitement in the market place at present. However, its complexity and cost makes it an infrastructure that cannot be implemented speedily, and Butler Group believes that it would require board level approval in many cases before a decision to move to a SAN could be taken.

A less expensive alternative to SANs is NAS, regarded by many as a halfway house between a SAN and direct attached storage. However, in Butler Group's opinion, it may not necessarily be feasible to implement NAS as opposed to a SAN, but it nevertheless believes that NAS has an important role to play in a modern storage infrastructure.

The NAS alternative
NAS is being hailed by some as a cheaper and less disruptive
"Because NAS appliances are directly connected to the network they are susceptible to network bottlenecks"
Source: Butler Group
alternative to SAN that, like SAN, addresses some of the weaknesses of direct attached storage. NAS offers true storage sharing on a network. A definition of a NAS device is that it is a server dedicated to file sharing, performing no other application function. Other names for it include "a file server", "filer", or "NAS appliance".

A NAS device works as if it was an attached appliance, yet it provides platform independence. It normally uses Ethernet-based networks - Local Area Network (LAN)
"It would require board level approval in many cases before a decision to move to a SAN could be taken"
Source: Butler Group
or Wide Area Network (WAN) - and supports a number of protocols to communicate with servers.

These are Network File System (NFS) for UNIX systems, Common Internet File System for Windows platforms, File Transfer Protocol (FTP), and HyperText Transfer Protocol (HTTP) for remote file service. Transmission Control Protocol/Internet Protocol (TCP/IP) is used for data transfer and Ethernet and Gigabit Ethernet for media access. Bandwidth ranges from 10Mbps to 1Gbps.

As appliances NAS devices are generally dedicated single-purpose machines or components. They comprise their own operating systems, which tracks where files are re-located on disk, and integrated hardware and software. Best optimised as standalone devices, they serve specific storage requirements, and are often deployed to supplement other storage systems.

One of the advantages of NAS appliances is that they are optimised for easy management and file sharing and are simple to install. Many are shipped pre-configured ready to plug-and-play. This means that a device can be up and running in a few minutes.

NAS shares many of the attributes of SANs including: centralised data storage, with the ability to serve a number of hosts; storage consolidation with support for multiple operating systems; and efficient file access. Both types of storage also provide support for high availability features, such as full redundancy, hot swappable components, redundant disk drives, and Redundant Array of Independent Disks (RAID).

However, NAS differs from SAN in that it is directly attached to the network, which means that routine maintenance tasks will inevitably impact on the network. Despite this, NAS does have a number of advantages. It is much cheaper and less disruptive to implement that a SAN. In fact many NAS devices will more or less plug and play, and some vendors claim to be able to have a NAS device up and running within 15 minutes.

NAS can work in tandem with direct attached storage and SANs, which is why it has come to be regarded as a "halfway house" by many companies. The major advantage of NAS over SAN is in its file sharing abilities across multiple heterogeneous hosts. This makes it suitable for serving small workgroups of clients with shared data, where it can eliminate bottlenecks encountered by users accessing data from a general server.

The integrity of files in this shared environment is provided through concurrency controls, such as locking, which is necessary where reading and writing by multiple users is allowed. Storage capacity is automatically assigned to users on demand. Most NAS appliances include a snapshot facility, from which to take back-up copies to tape, minimising downtime.

Limitations of NAS
The major disadvantage of NAS is that it is directly connected to the network. This means that it is confined to network speeds. Changing business requirements and the need for richer and more complex data structures has resulted in some NAS-based applications approaching their bandwidth limits.

Because NAS appliances are directly connected to the network they are susceptible to network bottlenecks.

NAS devices have a fixed capacity, which can be as little as 120Gbytes - 250Gbytes for an entry-level NAS devices to in excess of 800Gbytes for a high-end appliance. This does not make them very scalable as far as capacity is concerned.

Storage capacity within a NAS device can be pooled because of the way it is configured as one or more file systems. Each file system resides on a set of disk volumes and, as users access a file system, they are assigned space within it. Although this is a vast improvement on the direct attached model where each user would typically have its own storage resource - a user could be an application or a person - it does not have the pooling benefits of a SAN. Unlike a SAN each appliance is basically a standalone device and there is little, if any, sharing of resources across multiple appliances.

Virtualisation is, in Butler Group's opinion, the most talked about aspect of a SAN at present, as most of the major storage vendors introduce or announce virtualisation tools. Virtualisation enables storage resources to be assigned across logical rather than physical disks. This provides the ability to allocate disk space to an application based on the actual resource required rather than the total capacity of a specific disk, so two applications could share a single disk. This also makes disks operating system independent, allowing UNIX and Windows NT-based data to be stored on the same physical disk. NAS offers limited virtualisation as the storage capacity within a single device is pooled and can be allocated to specific applications. However, this falls far short of the virtualisation capabilities of a SAN.

Applications for NAS
The greatest selling point of NAS, in Butler Group's opinion, is its file sharing abilities, which makes it an ideal solution for small workgroups or departments, where access to shared data is required. A logical extension of the department approach is to deploy NAS in a branch office or remote location where it could be a complementary solution to SANs deployed in larger offices.

For many organisations NAS is the first step to implementing storage networking, hence its perception as a compromise between direct attached storage and SANs. Because of its high initial cost and complexity, a SAN will never be a viable option for some companies. In this situation NAS may provide a suitable alternative to SAN, although Butler Group believes that it will probably be used in conjunction with direct attached storage, rather than as an entire storage network in its own right.

However, that having been said, there will be companies for which NAS provides a single or at least the predominant part of a storage solution. The major downside to this is the manageability of NAS, which shares similar management attributes of direct attached storage where each device is managed as a separate entity. Despite this, NAS management requires a lower level of IT skills than the management of a SANs, and for this reason may be a more viable option in organisations where SAN skills are in short supply.

The size of the potential market for NAS is reflected in the fact that so many of the major storage vendors include such appliances in their product portfolios. Especially significant in Butler Group's opinion is the entry into this market sector of EMC, the leading storage vendor and a company that previously has focused on the provision of high-end storage products. Butler Group believes that this move is a direct response to the success of Network Appliance in this area.

Dell is another company that has entered the NAS space, although it is targeting the low-end of the market with its entry-level PowerVault 705N appliance, which originally provided 120Gbytes of network storage capacity with plans to move to a 240Gbyte version. Compaq, Sun, and IBM are examples of other storage vendors that provide NAS solutions.

In Butler Group's opinion, one of the key selling points of NAS is that it can be regarded as a first step for organisations that wish to consolidate their storage resources. It provides true data sharing, which addresses the issue of directly attached storage, and supports concurrent multiple operating systems. It also allows users to access data without server intervention, which can improve performance.

However, its major drawback is the fact that the storage connects directly to the network, typically a LAN, which means that all data traffic has to travel over the LAN, which can create performance issues. This is the reason why Butler Group believes that NAS is best-suited to small businesses and organisations that have a requirement for local data sharing, perhaps within a department or small workgroup, or the branch office of a larger company. In these types of scenario a NAS can provide rapid response times over the network.

Butler Group acknowledges that a NAS system is not suitable for all situations and for many organisations the fact that it transmits files over the network may have bandwidth implications, especially in large enterprises where any increase in traffic may impact on performance. For this type of company a SAN may currently be a better solution. However, although it does not believe that there will ever be a total convergence between SAN and NAS, Butler Group does perceive a time in the not too distant future when the two technologies will reside happily side-by-side in a totally integrated storage infrastructure.

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This was first published in August 2001


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