ARTICLE: Launching A Storage-Area Net
Note: Finally, an intelligent and informative overview of FC SAN products and vendors sans hype. Too much great stuff for mere excerpting, article reprinted in entirety, definitely worth sloggin through imo. Excellent diagrams and selectable vendor/product comparison tables available on sidebar also, sample:
data.com
Article: data.com
Gonna send this lady a love letter!
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Data Communications Magazine
March 21, 1998
By Mary Jander
Copyright c 1998 Data Communications Magazine, The McGraw-Hill Companies, Inc. All Rights Reserved
Launching A Storage-Area Net
High-speed storage-area networks help net managers hurl stored data around quickly-and take the load off the LAN and WAN
Out of sight, out of mind? If that's the philosophy on storage, net managers who haven't taken a look lately might not like what they see now. Client-server networks, a miscellany of protocols, and the rise of the Internet have combined to put a serious strain on those old-school SCSI links between hosts and storage devices. Forget about monolithic arrays humming in tune with the mainframe: These days, dealing with loads of stored data can make for a load of trouble.
Maybe it's time to launch a new storage campaign-and set up a SAN (storage-area network). SANs are dedicated, parallel networks built especially for backing up and archiving the data on corporate nets. They're based on 1-Gbit/s fibre channel links, so there's no need to worry about speed when tossing huge amounts of data around. And having a separate net for storage does more than ease the warehousing hassles; it also helps keep massive storage transfers from clogging the pipes on the main corporate network. What's it all add up to? Just "the most significant development in storage we've seen in 15 years," according to Michael Peterson, president of Strategic Research Corp. (Santa Barbara, Calif.), a consultancy that specializes in storage management.
But make no mistake: Rewriting the story on storage means constructing an entire network from scratch-beginning with the topology. SANs can be built with switches or hubs as the key connectivity devices, and each presents its own set of concerns. Fibre channel switches may offer dedicated 1-Gbit/s links, but products (and standards) are in short supply. Shared-media hubs split that 1 Gbit/s among as many as 126 nodes, but the gear is here, the standards are well defined, and the prices are a lot lower. If switches are chosen, check that they can handle the right mix of service classes and find out how the backplane is set up. And with both switches and hubs, look into capacity, redundancy, and management. After that, consider cabling: Fibre channel distance limitations differ depending on whether copper or fiber is used. Then take a look at the gear used for linking the SAN to the LAN-and keep in mind that getting it all to work together isn't a given. So do some hands-on testing before simply hooking fibre channel connections into legacy equipment.
Still, net managers who follow these SAN steps not only will keep their sanity-they could also reap big rewards. Ask David Grandin, president of Avid Sports LLC (Lowell, Mass.), which builds video networks that professional sports teams use to review their plays and performance. He says SAN technology permits simultaneous access to videos stored on multiple disk arrays, something LAN technology doesn't. That's won Avid more customers-and more profits.
Sanity Check
Not quite sold on SANs? That's OK: Building an entirely new network-and taking on the configuration, management, and maintenance chores that go with it-can be daunting. Net managers are right to wonder whether it's worth the hassle.
But consider the current alternatives. There are two other choices for connecting storage devices to the network. One is to hook RAID arrays, tape storage libraries, and optical jukeboxes directly into the LAN. The other is to use the SCSI (small computer systems interface) standard to funnel data back and forth between hosts and servers. The problem with the first? It kills LAN performance. The problem with the second? SCSI doesn't work beyond 25 meters with copper, and it doesn't work at all with fiber.
Now consider a SAN. It's based predominantly on 1-Gbit/s fibre channel connections and has its own switches, hubs, and gateways. These are linked to the servers and hosts on the corporate intranet, which furnish the point of contact between it and the LAN (see Figure 1). And because fibre channel doesn't suffer the distance limitations of SCSI, the SAN can be extended over the entire site-delivering 1-Gbit/s rates at up to 30 meters over copper, half a kilometer over multimode fiber, and 10 kilometers over single-mode fiber.
What's more, fibre channel frames have a lot more room for data. For one thing, they can be up to 2 kbytes in length (as opposed to the 1.5-kbyte max of gigabit Ethernet). For another thing, they don't have to carry the packet-acknowledgment overhead required in Ethernet LANs running such protocols as TCP.
Switched Storage
But getting up to speed on the SAN itself is just the beginning. The first big decision for net managers concerns topology-and whether to go with switches or hubs.
Right now, switching is where the action is, as indicated by the growing number of vendors. Ancor Communications Inc. (Minnetonka, Minn.), Brocade Communications Systems Inc. (San Jose, Calif.), Gadzoox Networks Inc. (San Jose), McData Corp. (Broomfield, Colo.), and Vixel Switch Operations (formerly Arxcel Technologies Inc., Irvine, Calif.) are the five players now plying the market (see Table 1). But they might soon be joined by Compaq Computer Corp. (Houston), Storagetek Network Systems Group (Brooklyn Park, Minn.), and Sun Microsystems Inc. (Mountain View, Calif.), all of which say they'll release SAN switches within the next year. What's more, most vendors of fibre channel storage hubs are planning to get into switching as well.
But the presence of more vendors doesn't make the decision any easier. There are a lot of issues to consider, starting with the types of network traffic the switch can handle. Like ATM, fibre channel offers specific classes of service. Class 1 creates a direct, acknowledged, connection-oriented fibre channel link. Net managers running apps that require continuous connections, like video clips with synchronized sound, should look for switches with Class 1 capabilities.
Class 2 and Class 3 both cover connectionless links and are the right choice for packetized traffic. The difference between them is that Class 2 also furnishes an acknowledgment that information has been received. Class 3 is similar, but it doesn't issue the acknowledgment. Then again, the value of acknowledging receipt depends on the traffic. "With traffic that's fairly reliable, and with packetized traffic that already contains acknowledgments, you don't buy that much with Class 2 service," says Wayne Rickard, a vice president and general manager with Gadzoox. When should Class 2 be used? When handling online transactions. But for tasks like linking a disk drive and tape backup unit, Class 3 may be just fine.
Two switches-Ancor's Gigworks MKII and Vixel's Rapport 4000-also offer intermix, in which Class 1 and connectionless Class 2 and 3 traffic work together. Here's how intermix could be used: If a set of large files is being transferred, a Class 1 link can handle the transfer itself while a Class 2 or 3 message alerts the host to set up the next transfer, reducing the time of the total transaction.
Another factor to weigh is how the switch shunts data over its backplane, since this can affect capacity. Ancor and Brocade, for instance, say their boxes use switching matrix backplanes. Net managers can get a fix on the capacity available with such an architecture by multiplying the number of connections on the switch's internal matrix by the speed that's offered. Based on these calculations, Ancor's Gigworks MKII and Brocade's Silkworm each offer 16 Gbit/s capacity.
In contrast, switches from Vixel, Gadzoox, and McData feature backplanes in which traffic is arbitrated on and off the switch using time-division multiplexing. To calculate the capacity of these switches, users need to multiply the number of bits the bus is capable of handling at once by the speed at which it operates, in MHz. So the Vixel and Gadzoox switches, which each feature a 32-bit bus running at 33 MHz, offer overall capacity of just over 1 Gbit/s. McData did not disclose the size and speed of its bus.
Net managers interested in switches may also want to look into such added features as multicasting. In other words, can traffic that's coming into the switch be copied and exported to multiple outgoing ports simultaneously? The obvious advantage of this is that it saves time (and trouble) when undertaking massive file transfers or batch-mode backups, which would otherwise have to be done individually.
Every switch but the DS-1000 from McData offers multicasting-but don't think for a minute that multicasting translates into Layer 3 capabilities. "Fibre channel is the highway, the physical medium, we use to transmit any number of protocols," says Jack Rubinson, director of product management at Brocade. "Routing would require snooping packets, and when you start that, you affect speed and defeat the whole purpose of having a storage network."
What net managers may be more interested in are vendor efforts to bring switch prices under control. Right now, for instance, Vixel's 8-port Rapport 4000 costs $2,300 per port, Gadzoox's 3-port Denali $4,200 per port, and McData's 32-port ES-4000 $4,625 per port. The 16-port boxes from Ancor and Brocade cost $1,995 and $1,875 per port.
And per-port prices are only part of the story. Prospective buyers also will most likely need to factor in the cost of GBICs (gigabit interface converters)- small transceivers that plug into a fibre channel device in order to adapt ports to copper, multimode fiber, and single-mode fiber. GBICs fit into the form factors of switches and so are considered more solid and reliable than those external media converters that dangle conspicuously from network devices. They also cost more-typically $150 to $400 per port. Some vendors figure the GBIC price into the per-port cost of the switch (like Ancor), while others charge extra (Brocade).
There's something else to keep in mind about switches. Right now, there's concern that the fibre channel spec is weak on how switches should track network devices. It requires the setup of a database (called a simple name server) listing the IP addresses of each storage device and host attached to the switch. Devices in a SAN perform a routine login against the simple name server to join the network. But most vendors would like to see more detail added to the simple name server, such as location information and the protocols supported by various devices. The Fibre Channel Association, which helps define standards for international approval, is looking into improvements, but no specific goals or deadlines have been set.
A Handle on Hubs
The arrival of more vendors into the SAN switch market will probably help bring prices down, but customers who can't put their SAN plans off until then may want to consider hubs (see Table 2).
Then again, going with hubs means paying a different price. Hubs share available bandwidth. Although most conform to FC-AL topology, which means they can theoretically handle up to 126 nodes, many vendors say response time sags noticeably when just five or six arrays are attached. Of course, a lot depends on the applications being run. Video clips, for instance, require more bandwidth than data files.
In such cases, corporate networkers should identify those spots where an additional hub might be added (cascaded). For example, if there are two or three huge arrays associated with a particular server or cluster of servers, then a second hub might be necessary between the servers and the storage devices. But if it gets to the point where the number of hubs is affecting performance (and by many accounts that number is pretty low-no more than several), then it may be time to think of adding a switch.
There are other hub issues to keep in mind. Like token ring, fibre channel requires all nodes in a logical loop to respond to switch polls. If a node doesn't respond, the entire loop malfunctions. But most hub vendors offer an automatic bypass feature for failed nodes, which means that a nonresponding node won't bring the entire loop down.
Several hub vendors-Gadzoox and Vixel Corp. (Bothell, Wash.) among them-offer a manual bypass feature. This is helpful in pulling a device out of the loop for reconfiguration or maintenance. The node can then be returned to the SAN, all without stopping the rest of the network. All it requires is making a selection on the configuration interface menu.
Still, when it comes to hubs, price is one of the most attractive features. Gadzoox and Vixel offer 12-port hubs that cost roughly $1,000 per port, depending on media and management options. Gadzoox and G2 Networks Inc. (Los Gatos, Calif.) each offer 10-port hubs, for about $1,000 or less per port. At the low end, Sun and Vixel offer unmanaged 7-port hubs for about $350 per port.
But as with switches, GBICs will have to be factored into the overall hub price. The 12-port Gadzoox Gibraltar GS costs roughly $550 per port if users buy $60 copper GBICs for each port. But the price can climb to $950 per port if multimode fiber GBICs are added; they cost $475 apiece.
One more thing about SAN hubs: Not every vendor is convinced that fibre channel is the way to go. Gigalabs Inc. (Sunnyvale, Calif.) runs SCSI over ATM to furnish a fast link between remote sites. "There's no need for users to switch to fibre channel," says Kon Leong, president of Gigalabs. "We've made a killer application of simple backup." Still, the Gigalabs hub delivers speeds of just 640 Mbit/s (as opposed to 1 Gbit/s), and it has to be shared among SAN devices. Still, Leong is clear that resistance to fibre channel migration doesn't mean he rejects the technology out of hand: He says that fibre channel interfaces will be added to the Gigalabs Jigsaw switch this year, at which point users will have the choice of using both SCSI and fibre channel.
Whether switches or hubs are chosen, net managers in large organizations may want to invest in the fibre channel test equipment that's now coming to market. Such gear can be a big help in making configuration decisions. Among the vendors selling test equipment are Ancot Corp. (Menlo Park, Calif.), Finisar Corp. (Mountain View, Calif.), and Xyratex International Ltd. (Hampshire, U.K.).
Of course, building another network also means managing it, and vendors of SAN switches and hubs offer varying degrees of help. Ancor and Brocade, for instance, furnish browser-accessible SNMP agents with their switches, and G2 does the same with its hubs.
Other vendors, however, are less generous. Vixel, for example, charges $1,250 extra for a hardware module that works with the vendor's element management app or with HP Openview. On the plus side, Vixel says one management package is sufficient for as many as 16 hubs.
There and Back
Another SAN subject net managers should look into is the conversion between SCSI and fibre channel devices. Fortunately, several vendors now offer SAN internetworking gear, including Advanced Digital Information Corp. (ADIC, Redmond, Wash.), Atto Technology Inc. (Amherst, N.Y.), and Crossroads Systems Inc. (Austin, Texas) (see Table 3). And most products aren't limited to one or two ports, the way most fibre channel adapters are. Rather, they can link groups of SCSI devices (typically seven per bus) to a fibre channel switch or hub. They work by adding a header or other notation to a SCSI transmission in order to shunt it over the fibre channel network.
How does SAN internetworking gear rate when it comes to cost? The answer depends on the size of the network. High-end adapters that furnish a port-to-port link between one host and one fibre channel storage array are typically priced at $1,995. But if four adapters are needed to link a server to four arrays, the cost is $7,980-and four server slots are taken up. In contrast, Crossroads' Crosspoint 4100, which comes with one fibre channel link and one SCSI bus, can connect as many as seven devices to a server-and it costs only $5,995.
Most internetworking gear is intended for use on LANs only, but that's changing too. The Ultranet Storage Director from Computer Network Technology Corp. (CNT, Maple Grove, Minn.), for instance, can extend fibre channel SANs over fast WAN links, including ATM OC3 (155-Mbit/s) circuits and T3 (45-Mbit/s) leased lines (see "Widening the Warehouse," January 1998; data.com products/ultranet.html).
Right now it's the only product that offers WAN connectivity, but things could change by late this year, when ANSI is expected to approve a WAN tunneling technique for use in fibre channel networks.
The Rest of the Gear
So if net managers have by now decided that SANs are in the plans, they might be wondering whether their current storage devices can work with the fibre channel switches and hubs being introduced. When it comes to disk arrays, at least, the news is good: Most products comply with the fibre channel standard. Leading vendors Fujitsu Ltd. (Tokyo), Quantum Corp. (Milpitas, Calif.), and Seagate Technology Inc. (Scotts Valley, Calif.) offer links to fibre channel controllers on their drives, which means networkers can simply plug in their switches and hubs and get going.
Makers of tape libraries and optical storage jukeboxes-like ADIC and Tandberg Data Inc. (Simi Valley, Calif.)-are reportedly readying fibre channel tape drives, but at press time none had shipped.
Net managers also can assemble their own fibre channel arrays using stacks of disks from different vendors along with third-party controllers. These "just a bunch of disks" (JBOD) configurations are easy to assemble and relatively cheap. An alternative to JBOD arrays are so-called intelligent or all-in-one arrays. These tend to offer better performance because they're optimized for use on fibre channel networks. Among the all-in-one vendors are Megadrive Systems (Chatsworth, Calif.) and Raidtec Corp. (Alpharetta, Ga.).
Specialized servers that are built with SANs in mind also are emerging. These devices combine arrays with software and hardware designed just for storage applications. Artecon Inc. (Carlsbad, Calif.), for instance, packs multiple processors, RAID arrays, an NFS server, and operating system software into a single box. The advantage? According to the vendor, it's cheaper than dealing with multiple arrays and servers, and it offers a boost in performance, as well.
And a handful of vendors sell host adapters that can furnish fibre channel connectivity for all types of servers. These include Adaptec Inc. (Milpitas, Calif.), Emulex Corp. (Costa Mesa, Calif.), Interphase Corp. (Dallas), Jaycor Networks Inc. (San Diego), Strategies/Qlogic Corp. (Costa Mesa, Calif.), and Sun. Prices typically range from $595 to more than $3,000.
Finally, some vendors are addressing efficiency by equipping their standalone servers with file system software. Tricord Systems Inc. (Plymouth, Minn.) is one of them. "We try to reduce the number of backup requests going over the network in order to accommodate SAN configurations," says Joan Wrabetz, vice president and chief technical officer
The vendor is still testing its software but says it can increase performance on SANs, where file I/O isn't slowed by the handling of multiple packets.
data.com
Mary Jander is associate managing editor/special projects for Data Comm. Her e-mail
address is mjander@data.com.
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