IT Benefits from SANs
Storage-area networks can be particularly advantageous in applications such as backup and restore, archiving and retrieval, data migration, database mirroring, and data sharing.
By Larry Kallhof
Remember the Year of the LAN? In a sense, every year in the last decade has been a year of the LAN. Well, 1998 may well be the Year of the SAN, and perhaps the first of many such years.
Here`s why. Enterprise networks today remain broadly heterogeneous, with Windows NT, Unix, OS/390, and proprietary midrange systems. Each kind of server typically has its own storage systems associated with it, creating in effect islands of storage. These islands are becoming more and more densely populated, since the storage capacity of these systems is growing rapidly. At the same time, some islands are not being managed as well as others, particularly when it comes to backup and recovery, because many networks were not originally designed for mission-critical applications.
But these days, almost all network-stored data can be considered mission-critical. Compounding the problem is the fact that the size of IT staffs has either remained the same or has shrunk in recent years.
In this environment, simply deploying more and faster storage devices is not enough; a new kind of infrastructure is needed--one that provides network availability, data accessibility, and system manageability. One approach, network-attached storage (NAS), uses an integrated storage system that communicates with network protocols like TCP/IP. A network device is connected to the storage systems and functions as a server. The device processes file I/O protocols such as NFS, thereby managing data transfers between itself and clients. But there is a drawback: The protocols limit performance.
The storage-area network (SAN) meets the challenge in a different way. A SAN is a high-speed "subnet" between heterogeneous storage resources and servers.
The SAN liberates the storage device so that it is no longer on any one particular server bus, but attached directly to the network via a network processor device. In other words, storage is externalized and functionally distributed to the organization. A SAN architecture makes all storage devices accessible to all servers.
The SAN concept received early support from the Fibre Channel community, particularly those working on Fibre Channel-Arbitrated Loop (FC-AL) and switched Fibre Channel, which are well suited to SAN applications. But just as a LAN can use diverse technologies--such as Ethernet, Token Ring, or FDDI--a SAN can employ a wide range of local- and wide-area technologies, including Fibre Channel, ESCON, IBM`s Serial Storage Architecture (SSA), DS-3, ATM, and SONET.
Currently, the focus is on local and campus-wide SANs, but with the deployment of high-bandwidth, high-reliabil- ity wide-area technologies, distributing a SAN across much wider distances becomes possible. Whichever technologies are used, the objective is a high-performance subnet with fault tolerance and multiple paths. In effect, a SAN combines the high performance of an I/O channel with the connectivity of a network.
SCSI technology uses a point-to-point architecture that does not lend itself to multiple pathing. In addition, SCSI has limitations in distance, connectivity, and performance. At the same time, it is possible to connect SCSI storage devices to a SAN through a network processor that handles SCSI as well as other channel protocols.
Applications and Advantages
With the SAN architecture, data becomes more accessible to users anywhere on the network, and different kinds of storage resources can be shared among different applications and servers. The following applications benefit significantly from SANs:
- Backup and restore
- Archiving and retrieval
- Data migration
- Database mirroring
- Shared data
Backup and Restore
The sophistication of backup systems and procedures is driven by the amount of data involved, the available time window, and the restore requirements of the business (e.g., when the data is needed to continue operations). SANs provide new levels of performance and flexibility in backup and restore, making it possible to back up data from different servers to the same automated tape library, for example. High performance is key to reducing recovery times, and SANs make it possible to better use available bandwidth.
Archiving and Retrieval
Archiving data to less expensive, less immediately accessible storage is typically a function of the age of the data and the relative need to access it. Archival applications include storing check images and point-in-time records such as customer billing statements. Typically, archive networks are configured either within a company`s multiple locations or in conjunction with a business recovery vendor such as IBM, Comdisco, or SunGard. SAN solutions support effective and efficient archiving from different kinds of servers to the same storage system. A SAN can provide both local- and wide-area connections and necessary gateway and conversion functions.
In moving data from one storage system or data center to another, an IT organization confronts several issues: the available time window, the effect of moving data on online resources, and the reliability of the data after the move. For example, some data may need to remain online 24x7 during the move, while other data can be unavailable for days without causing any problems. SANs can also use high-speed connections and software products such as EMC`s SRDF, IBM`s XRC, Hitachi Data Systems` H-XRC and Online Data Mover, and Amdahl`s TDMF.
Daily backups are often not enough, particularly when the effects of losing data between backups are substantial or when nonstop data access is required. That`s when disk mirroring is desirable. With SANs, organizations can perform database mirroring over unlimited distances, synchronizing the data according to application requirements. The network provides high availability and fault tolerance, as well as high performance and manageability.
Beyond shared storage, the ultimate goal of information management is sharing data--the extraction, movement, or loading of data between environments. Think, for instance, of data collected using one platform but needed for an application running on another. There are multiple levels of data sharing today: network transfer, typically involving TCP/IP; channel transfer with, say, ESCON; and controller-based shared-storage transfer. This last method can be very fast (disk to disk), and it allows the use of redundant disk resources. Storage controllers now permit one copy of data to be shared across SANs by heterogeneous computers.
Centralized Systems and Management
SANs enable organizations to use existing storage capacity more efficiently, including centralizing servers and storage resources that are now widely dispersed. The acquisition of new storage devices can also be done more efficiently, since the bytes of capacity in different storage systems are interchangeable. In addition, SANs enable organizations to use only one set of network management tools. IT managers don`t have to undertake costly development efforts to replicate tools for different operating system environments, which is the usual situation today.
To realize these advantages, organizations need to consider the special requirements for implementing SANs. For one, SANs need to be implemented as separate subnets, so the critical I/O traffic between server and storage is not blocked or delayed by other kinds of traffic. But building separate physical networks is prohibitive. In fact, many organizations have spent recent years merging separate networks for IP/IPX traffic, SNA traffic, and channel extensions. The solution is to provide different classes of service on a shared physical network. Fibre Channel has this capability, as does ATM. And emerging technologies like IP v6 will further enhance this prioritizing capability.
Second, attaching heterogeneous storage devices directly to a network requires a special kind of networking device, one that can handle the classes of service that are established and both channel and network protocols. These devices should also provide:
- Fault-tolerant architecture
- Guaranteed data delivery and integrity
- Load leveling
- Data compression
- Alternate path routing
Third, security needs to be addressed differently in a SAN environment, since storage devices are not protected behind servers as they are in traditional architectures. There is no magic bullet when it comes to SAN security. Each installation has to be examined, and security has to be designed to fit the situation. Third-party security, encryption, and firewall technologies will play a critical role. And networking, storage, database, network management, and systems management providers will need to cooperate more closely than they have in the past.
As computing becomes ubiquitous and network connections become faster and less expensive, IT organizations will face relentless demands for network up-time, data accessibility, and system management--all at a lower cost. A SAN addresses these needs by taking advantage of today`s network and channel technologies. As a result, SANs deliver compelling benefits, including greater productivity and reduced costs, for a broad range of storage applications.
A SAN bypasses traditional network bottlenecks and supports direct, high-speed transfer in three ways: server-to-storage, server-to-server, and storage-to-storage.
With a SAN architecture, data becomes more accessible to users anywhere on the network, and different kinds of storage resources can be shared among different applications and servers.
The major benefits of SANs come from external storage, centralized storage, and remote clustering.
Larry Kallhof is director of product marketing at Computer Network Technology Corp. (CNT) in Minneapolis, MN.