|
US Computer Makers Work on Networked Storage
While storage device manufacturers continue to announce their own open systems to connect HDDs to a variety of platforms, a SAN industry organization plans to establish a standard protocol.
Since the middle of 1997, all storage device manufacturers have been trumpeting "open" designs. IBM Corp, Sun Microsystems, Inc and Hewlett-Packard Co (HP), all of the US, have announced their open systems concepts (Table 1).
While the names may be different, they definitely have the same aim: networking storage devices and servers that can be shared by a variety of platforms, including personal computer (PC) servers, workstations and mainframes. The target area is the local area network (LAN), and as a result many of the manufacturers involved refer to the new system as the storage area network (SAN).
In 1998 the first full-scale SAN products will be shipped, and while the specifications are currently user-specific, standardization has already started.
Common Storage Network
Until recently, storage devices have been little more than server extensions. Mainframes, Unix workstations and PC servers use different coding for their files, so each type required its own storage device. Most of these devices were connected to their hosts directly through the small computer system interface (SCSI) on a 1:1 basis (Fig 1a).
Through SANs, device manufacturers hope to network storage devices, creating an environment that can be accessed from any computer. LANs, which SANs are modeled on, support communication between any computers on the network, as long as they all have a common communication protocol. The goal of the SAN is to use the same type of network to link servers and storage devices, eliminating server dependency (Fig 1b).
It is important to note, however, that, "SAN is interpreted in a variety of different ways by different storage device manufacturers," said Rodney van Meter of Quantum Corp of the US. In addition to standing for "storage area network," it is also used for "system area network" or "server area network," to represent a cluster network linking multiple servers. In this article, the term SAN indicates the network environment linking a storage device and servers.
Three Major Advantages
The 1:1 relationship that has existed between a computer and its storage devices offered the advantage that the file format was optimized for each operating system (OS) and application software. However, manufacturers have now found new benefits in SANs that are significant enough to discard the advantage of the current system. The three major benefits are cost, system loading and management.
SANs slash costs because multiple computers can utilize a single storage device. If multiple servers are networked to a storage device through a SAN, only one storage device is needed.
They also eliminate some of the system loading problems during data backup, or mirroring. Mirroring is the technique used to make it possible to utilize identical data on different platforms. For example, if data is copied allowing a Unix server and a mainframe to use the same data, the data must pass from the mainframe to the server via the LAN (Fig 2). During copying, loading occurs on both the mainframe and the server, affecting other data processing. If a dedicated storage device network (in other words, a SAN) is used, this system loading disappears.
By concentrating the storage systems of multiple computers in one place, management is also simplified.
First Products Hit the Market
As yet there are few differences between SAN products from different vendors. Conversion software is generally provided to allow the storage device to manage the platform differences, and most systems are configured with Fibre Channel running at 100 Mbytes/s. These products are first-stage releases, aimed primarily at getting connected, but future plans call for support for simultaneous access by an increased number of connected computers, and enhanced management functions.
The StorEdge Series of hard disk drives (HDD) announced by Sun in March 1998 is one such example. The high-end configuration, Sun StorEdge A7000 Intelligent Storage Server, provides connection to not only Sun Unix servers, but also HP and IBM Unix servers, PC servers running Windows NT, and IBM mainframes and compatibles.
Add-on software called StorEdge DataShare provides functions for converting file formats for various servers, making it possible for servers running different OS to share a single physical drive as storage media.
IBM planned to ship HDDs in the second quarter of 1998 with functions for sharing a given file between Unix and PC servers. "We will integrate the S/390 mainframe in the next stage," says Tadahisa Wakita, director, Storage Systems Products, Management & Marketing, IBM Japan Ltd.
Functions are already available, however, to partition a single physical drive for use by both Unix servers and mainframes.
Fibre Channel Leads Pack
The practical implementation of the SAN will require a physical path linking the servers to a storage device, or between multiple storage devices. Fibre Channel is presently the most common solution, although some manufacturers have chosen serial storage architecture (SSA) or the widespread SCSI (Table 2).
The advantages of Fibre Channel, which has attracted the attention of many manufacturers, are high speed, expansibility, and a high degree of freedom which allows designers to choose from a number of different topologies. Fibre Channel can be used not only in a point-to-point configuration linking the server to the storage device, but also in star and loop configurations.
Not many manufacturers consider SCSI a viable choice for SANs, because SCSI, as it stands today, "cannot bring out the full potential of the hard disk drive," according to Mikinori Furuya, program manager, Product Marketing Department, Marketing Division, Nihon Sun Microsystems KK. The internal data transfer rate, when reading data from the disk, is quite high, but the peak rate of SCSI input/output (I/O) is just too low when multiple datacomm streams are used simultaneously on the same transmission path.
There has been a steady improvement in recording density and disk speed, the factors which determine the internal data transfer rate. For example, the maximum recording density available on HDD from IBM is 800 Mbits/inch2, and the fastest disks hit 7,200 revolutions per minute (rpm). This means that the top data transfer rate currently is 18.6 Mbytes/s.
The fast wide SCSI used on high-end servers, however, has a top speed of 20 Mbytes/s, but the slim margin of safety is sure to be lost as recording density and disk speed continue to rise. This is why Fibre Channel, currently the speed winner, is emerging as the main choice.
In addition, SCSI limitations on distance cannot be overlooked. Avoiding system faults demands that data be backed up to physically remote locations, and SCSI's 25m maximum transmission distance is far too short.
IBM Opts for SSA
IBM plans to stick with SSA, which it developed for storage devices. Logically speaking, SSA connects terminals in two rings, which according to IBM's Wakita provides "superior redundancy and expansibility." At present the transfer rate is 80 Mbytes/s, but the firm plans to double this to 160 Mbytes/s, and believes that it offers more expansibility for speed than Fibre Channel.
While IBM seems to believe that SSA is the best choice for the storage device, they are more flexible when it comes to the server side, offering Fibre Channel as an option for both servers and mainframes. Wakita comments that the firm gave consideration to connecting servers from other vendors.
Other manufacturers offer a SCSI/Fibre Channel protocol converter to allow widely-used SCSI devices to be utilized within a Fibre Channel SAN environment, but there are very few vendors who feel that SCSI will remain a key part of the system for long.
Some firms have already revealed concrete strategies to implement SAN over wide area networks (WAN). Sun, for example, plans to provide asynchronous transfer mode (ATM) as a method for mirroring between files on local and remote storage devices. The release date is expected to be summer or fall 1998.
SAN Standard Starts Up
While manufacturers advance their own "open" strategies, an industry body has appeared to promote SANs. The Storage Networking Industry Association (SNIA), was launched formally in January 1998 (Fig 3).
Most of 54 founding members are manufacturers of storage devices, servers and LAN equipment. At the annual meeting, held in Colorado Springs, Colorado, the US in March 1998, there were about 150 names on the list.
One of the objectives of the SNIA is to develop standard specifications. Toward that end, six working groups for specific sectors were established (Table 3).
SNIA's standardization activity has only just begun. Quantum's van Meter, the area director for the Storage Networking Protocols Area (one of the working groups and core to SAN's implementation), says "The very first thing we have to do is figure what we need to make decisions on."
The same network problems are faced by all users at present, so the group will investigate whether or not existing communications technology such as Fibre Channel and transmission control protocol/Internet protocol (TCP/IP) can be used.
The working group also announced its charter, which states it will (1) develop a framework to allow functions and data from the storage device to be sent through the network, (2) clarify the standard protocol for SAN (e.g. IP), (3) clarify probable requirements, such as remote mirror, path control and reliability, (4) maintain communication and interaction with other SNIA working groups, (5) provide methods to encapsulate existing protocols, and (6) assure constructive coexistence with other standards bodies such as the Institute of Electrical and Electronics Engineers, Inc (IEEE), the Internet Engineering Task Force (IETF), and the American National Standards Institute (ANSI).
At present, it seems likely that the communication functions will be implemented in a 7-layer hierarchy design, with an existing protocol suitable for SAN applications selected for each layer of the open systems interconnection (OSI) reference model, and the whole rounded out as the SAN protocol.
by Takahiro Kikuchi
- References:
- HP
- IBM
- Quantum
- SNIA
- Sun Microsystems
