Introduction to SANs: Technology, Benefits, and Applications - Page 5


SANs link targets (storage devices) to initiators (servers or clients that initiate data traffic). Aside from the storage devices themselves, the most important devices that live on SANs are the networking infrastructure components; the hubs, switches, bridges, and routers that connect the storage devices and servers together on the SAN. Network infrastructure components are usually purchased through a SAN vendor and/or integrator as part of a complete SAN solution, and not as individual components directly by the end user.

Switches and Hubs
Switches allow the devices connected to them to communicate over a deterministic medium. Fibre Channel switches allow devices to communicate at the maximum possible speed since traffic is directed towards the proper target by the switch hardware. This is in contrast to a hub, where all the devices are connected to a shared bus, and the medium is contention based.

In a hub environment, a device broadcasts its data to all the ports on the hub simultaneously, and as a result only one device can use the hub at a time. The bandwidth on a hub is the roughly the same as that of a single switch connection. On a hub only one full bandwidth connection is shared among all the devices, while on a switch, every device gets the full bandwidth at the same time. Hubs are gradually being phased out in favor of switches, but IT managers should expect to still seem them around since the earliest Fibre Channel SANs from about 3 years ago were built on Fibre Channel hubs.

Brocade Communications Systems is the leading vendor in the Fibre Channel switch space, but comparable switches can be purchased from Vixel Corporation and Gadzoox Networks, Inc.

Interoperability between switches is still questionable, but with the emergence of the E port spec (which will allow fibre channel systems from different vendors to uplink to each other), IT managers will eventually be able to mix and match switches.

Host Bus Adapters (HBAs)
Fibre Channel Host Bus Adapters are installed into servers and they act as the gateway to SANs. On a physical level, fibre channel HBAs look quite similar to gigabit Ethernet HBAs. Fibre Channel HBAs usually slide easily into the 64-bit PCI slots of a server/workstation motherboard, and they can be installed in a matter of minutes.

From a software point of view, the device driver of a Fibre Channel HBA starts the storage virtualization process by fooling the servers operating system into thinking that it is a SCSI adapter. At first it may seem strange to make a networking component like an HBA (which connects to switches and hubs via fiber-optic cabling) look like a storage controller, but this virtualization is acceptable and works fine with the OS.

A logical unit number (LUN) is a unique identifier used on a SCSI bus that enables it to differentiate between up to eight separate devices (each of which is a logical unit). LUNs can be individual disks, groups of disks, or individual parts of multiple disks defined by a RAID controller or other intelligent storage controller. LUNs are sometimes also called logical disks, partitions, or virtual disks.

When a server boots up and the Fibre Channel HBA becomes initialized, the HBA sends out a broadcast message on the Fibre Channel network. When the available LUNs report back to the Fibre Channel HBA, the HBA reports the presence of these LUNs to the operating system as storage devices that it is managing. Once LUNs are recognized by the operating system, they can be managed by the operating systems disk and volume management systems, and can be carved into partitions, and eventually formatted in the file system of that operating system.

Storage virtualization is a tricky concept, which to this day is still evolving in the SAN industry. In future articles we will cover the different methods by which storage virtualization is achieved. For the purpose of this article, storage virtualization is the process by which a server operating system is fooled into thinking that storage resources on a SAN (which can physically be several kilometers away from the server) are actually locally attached to the server through the Fibre Channel HBA (which is masquerading as a SCSI adapter).

Fibre Channel HBAs are fairly pricey, with new state of the art 2Gbps fibre channel cards running over $2000 and lower end 1Gbps cards usually selling for over $1000 a card. For this reason, we recommend that workgroups that dont really need Fibre Channel for their day-to-day needs should definitely avoid rolling out this technology to low and midrange machines.

Emulex Corp. and Qlogic Corp. are major manufacturer of Fibre Channel HBAs. Hewlett Packard and Adaptec also produce Fibre Channel HBAs.

Fibre Channel/SCSI Routers
As the name implies Fibre Channel/SCSI routers allow the movement of data from the SCSI world to the world of Fibre Channel SANs. Fibre Channel/SCSI routers have ports for both SCSI devices and Fibre Channel, and they allow SCSI devices to present themselves to the SAN.

Early SAN implementations (back in the days when Fibre Channel-based hard drives were hard to acquire and costly) were built out of SCSI hard drives with Fibre Channel/SCSI routers built into the array enclosure. Today, most SAN hard disk storage offerings have full Fibre Channel designs for performance and implementation simplification purposes, but Fibre Channel/SCSI based routers are still prevalent, since the near-line and offline storage vendors have yet to catch up to their hard disk counterparts.

Currently the vast majority of tape drives and libraries are SCSI-based, a market fact that necessitates the use of Fibre Channel/SCSI routers in networks. For the most part, the majority of Fibre Channel-enabled Tape Autoloaders and Libraries entering the market from companies like ADIC are really nothing more than SCSI-based storage units with integrated Fibre Channel/SCSI routers that have gigabit connectors allowing them to hook directly into SANs.

Considering the vast market acceptance of SCSI-based autoloaders, libraries (tape and optical), and standalone tape drives, we expect SCSI storage products to co-exist with SANs for many years, a reality that will allow Fibre Channel/SCSI routers to be important SAN components for years to come.

Crossroads Systems, Inc. and Chaparral Network Storage Inc. are important providers of Fibre Channel/SCSI routers.

Fibre Channel Raid
Fibre Channel RAID units are the core storage devices in SAN environments. Fibre Channel RAID units consist of Fibre Channel hard drives (from vendors like Seagate and IBM), RAID controllers (from vendors like Mylex, an IBM company), and a physical enclosure, which provides power and hard drive connectivity.

Fibre Channel RAID units arent exceedingly different compared to SCSI-built RAIDs in terms of feature sets. Like SCSI units, high-end Fibre Channel units feature dual controllers for both load balancing performance and for redundancy. In a SAN scenario, it is important to have reliable cache coherency built into the RAID controllers in the event of a hardware failure. At the high end, we also expect to see additions like a large RAM cache to improve the performance of disk I/O requests, and a support for hot-swappable hard drives.

The EMC Clariion line of Fibre Channel RAID units is popular for midrange SAN implementations.

SAN Software Managers and Tools
SAN management software is still an emerging product category, despite the fact that it is crucial for long term SAN management. The biggest problem with trying to manage a SAN, is that companies not only have to deal with typical network issues like connectivity and device management, but they also need to deal with traditional storage management issues like volumes, LUN management, and quotas. Future articles will cover the different management offerings and how each fits into a corporate IT network.

Veritas Software is considered the market leader in the space of storage management, and thanks to their comprehensive line of backup/recovery, clustering and replication, and file and volume management products, Veritas is the only major software vendor that can even claim to provide an end-to-end storage management solution for their customers.

Some of the challenges that lay ahead for SANs are obstacles that have already been solved in IP based networks. Over the next few years, we should expect to see device management comparable to the SNMP management solutions that are already available for IP networks. For this to happen, Fibre Channel switch vendors will have to settle on a standardized management interface from which all of their products can be managed.

Another major concern that will develop as we move on towards IP SANs will be the need for a new, scalable naming service to replace the simple naming service currently used in SANs. iSNS (Internet Storage Name Service) is currently making the rounds in the IETF, and it is expected to eventually become the new naming standard for SANs, and may provide the scalability and reliability of its IP-equivalent DNS.

Like IP networks, the day-to-day maintenance and troubleshooting of SANs requires powerful analysis tools that can sniff the wire and find transmission irregularities. Finisar Corp. is the leading developer of Fibre Channel analysis tools. The Finisar GT and GTX Analyzer is used to design, debug, and service SAN systems.

Besides offering protocol analyzers, Finisar also offers software products that analyze complicated SCSI I/O traffic over SANs, Bit Error Rate measurement tools, and system-level fault injection for SAN system testing and troubleshooting. The Finisar tools are used in many interoperability labs and are very useful in trying to diagnose Fibre Channel problems.