Fibre Channel and SCSI RAID Compared

By Lowell Rapaport

For every system need, there is (or should be) a storage solution equipped to meet that need. Databases and Web servers that handles high volumes of simple transactions require storage systems designed to support high transaction rates. Environments that include document management software call upon the network infrastructure to serve data in large blocks while also serving large numbers of simultaneous users.

RAID lends itself to many types of storage demands. There are RAID systems designed to serve large blocks of data very fast and systems designed to serve large numbers of file requests. To explore these differences — and in particular the advantages of SCSIvs. fibre channel in RAIDstorage — we looked at two RAID systems: the FlashDisk from Winchester Systems (www.winchestersystems.com), which has Ultra2 SCSI connections both to the host and to the drives; and the 7000 Series from Ciprico (www.ciprico.com), which uses fibre channel connectivity.

We were fascinated by the differences between SCSI and fibre channel technologies. SCSI has demonstrated tremendous resilience as it has been pushed to ever-higher levels of performance. Fibre channel is the latest technology, and it redresses some of the criticisms of SCSI.

SCSI cables are short and thick. They have to be. A SCSI cable has as many as 68 wires sending a 16-bit wide stream of data. The cables have to be kept short because the signals in all those cables must be synchronized.

Fibre channel, on the other hand, is carried by a thin cable with just four wires. A fibre channel cable is scarcely thicker than a Cat-5 ethernet cable. And fibre channel runs are long. Our cable was about 50 feet long. Fibre channel can even use fibre optics for longer runs.

7000 Series
Ciprico
Plymouth, MN
612-551-4000
Description:External RAID Subsystem with Single RAID controllers.
Cache: None
Interfaces to Host: 1 x Fibre Channel
Interface to Drives: 1 x Wide SCSI-2
Enclosure: 9 drive bays, 2 hot swappable power supplies, dual cooling systems in a tower case or tabletop case, rack mountable.
Performance: Cache test: 293 commands per second
73.4 Mbytes per second
Transaction rate: ~92 per second
Throughput: 64.2 Mbytes per second

Advantages: High throughput. Optical fibre channel interface available.
Disadvantages: Low transaction rate.

FlashDisk
Winchester Systems
Woburn, MA
781-933-8500
Description:External RAID Subsystem with a single RAID controller. Dual Active-Active controllers are available.
Cache: 128 Mbytes
Interfaces to Host: 4 x Ultra2 SCSI
Interface to Drives: 2 x Ultra2 SCSI LVD
Enclosure: 12 drive bays, 2 hot swappable power supplies, dual cooling systems in a tower case
Performance: Cache test: 104 commands per second 26.1 Mbytes per second
Transaction rate: ~2,600 to 2,700 per second
Throughput: ~48 to 50 Mbytes per second
Advantages: Terminal based set-up. High transaction rate and high throughput.
Disadvantages: Lower throughput than that available using fibre channel.

In our test lab, we used an ExpressPCI Fibre Channel adapter from Atto (www.attotech.com). This is a 64-bit PCI card that nevertheless works fine in the more common 32-bit PCI slots.

The Winchester FlashDisk is a 12-drive array with a single RAID controller. As tested, it came with 18 gigabyte, 10,000 rpm IBM drives. When we tested it with 2 kilobyte and 10 kilobyte files (the kind found in many document management environments) the FlashDisk was able to process more than 2,600 and 1,900 transactions per second, respectively, delivering a peak of 50 Mbytes per second.

The Ciprico 7000 Series RAID system is designed for throughput. It employed nine SCSI-2 drives arranged in a RAID-3 array. When we tested it with large files of two, three and ten Mbytes, it turn in high speeds of up to 73 Mbytes per second.

These two RAIDsystems aren’t really comparable since the Winchester is designed for high transaction rates while the Ciprico is designed for maximum throughput. Both performed well for their intended use. You would use the Winchester for Web servers, databases and large enterprises. If you have an application that deals with large files or massive amounts of data, such as medical imaging or backup, a RAID system as specialized as the Ciprico would be an excellent choice.

Fibre channel can support as many as 255 devices on a single loop or network. The fibre channel interface we used was the simplest one possible: a dual-loop connection direct to the host with no hubs or switches. The speed of the Ciprico was much faster than the real-world performance of Ultra2 SCSI. Fibre channel supports speeds up to 100 Mbytes per second, while Ultra2 SCSI tops out at 80 Mbytes per second.

Fibre channel technology is just starting out. As a result, there are still interoperability issues between components from different manufacturers, but SCSI had the same issues in its early days. Fibre channel hardware is also two to three times more expensive than SCSI hardware, but it should get cheaper as it becomes more mainstream.

SCSI is under constant development, too. Ultra3 SCSI is now available, and it doubles SCSI’s speed to 160 Mbytes per second. SCSI is familiar technology. SCSI equipment is relatively inexpensive, most all the interoperability issues have been ironed out, and SCSI software has been thoroughly debugged. It’s a proven technology.

Whether you should choose fibre channel or SCSI depends on what attributes you need. For maximum speed from just a handful of devices, Ultra3 SCSI is a good choice. If you expect your storage needs to grow, then fibre channel, with its greater scalability and simplified cabling, will be a better long-term choice.

–Lowell Rapaport