InformationWeek

These products are examples of the basic divisions of current RAID technology; the StorComp RAID 7 is a very large, multiport RAID server that can be connected to as many as 48 host computers; the LynxArray is a seven-drive SCSI-to-SCSI array; and the RAIDPro is a RAID subsystem that is managed by a dedicated controller in the server. These three systems cover the complete range of what's available in RAID, from the standpoints of performance, size, and price.

The StorComp RAID 7 is a large, rack-mount system that weighs as much as three tons, supports up to 192 drives for 4 terabytes of data, and costs up to $650,000. The LynxArray is a tower about the size of a small server that supports up to nine half-height drives (up to 9.1 Gbytes each), provides up to 80 Gbytes of storage, and costs up to $42,390. The RAIDPro is a desktop model that supports up to six 1-inch d rives (up to 4.3 Gbytes each), provides up to 21.5 Gbytes of storage, and costs $10,500. The StorComp RAID 7 and the LynxArray are usually sold with setup by a systems engineer.

There are six formally defined RAID levels: zero through five. RAID 0 is disk striping, or writing files across several disks, which improves performance but offers no protection-the failure of any disk in the array causes all data to become unavailable.

RAID 1 is disk mirroring, or writing everything on one disk to a second disk. If the first disk fails, the second disk picks up the load. RAID levels 2 through 5 use different schemes for parity, which lets the system re-create all data on a missing drive. Almost all RAID systems use RAID 5. In addition to the best all-around performance, RAID 5 offers a high degree of fault tolerance; if any drive in the array fails, all data will still be available, and when a new drive replaces the failed one, the array rebuilds itself by replicating the data from the failed drive. However , if two drives fail, all data is lost-this is why tape backups are still a necessity.

There are two basic methods for connecting a host computer or server with a RAID subsystem. The first is SCSI-to-SCSI. This means that the array has a built-in SCSI controller and appears to the host to be a single large SCSI drive. The advantage of this type of connection is that the array is very easy to configure and use from any computer that supports the SCSI interface. The disadvantage is that total throughput can only be as fast as the SCSI channel.

The second type of connection is a host controller. This requires a board in the host computer and has the advantage of allowing a connection as fast as the computer's bus supports. The disadvantage is that it can support only those computers for which cards have been made, and also needs a driver specific to the operating system used.

StorComp RAID 7
The StorComp RAID 7 competes with large RAID systems from Digital Equipment, EMC, IBM, and other s. In its basic form, it's a rack-mount chassis on wheels that can support up to 12 SCSI channels and up to 27 3.5-inch drives. As many as four chassis can be connected to get the maximum 48 channels and 192 drives. It includes a built-in server, a 486 or Pentium system that controls the multiple SCSI channels for external connections, and an internal high-speed bus. Storage Computer's RAID 7 allows the use of RAID 0, 3, and 5 within a single array.

All drives are hot-swappable, and a dedicated spare can be designated as well. Failover channels are supported that allow two connections to a server, with the second channel taking over if any part of the first one fails. For its size and complexity, the StorComp RAID 7 is amazingly easy to configure. After booting the on-board computer from a diskette, the RAID array is initialized in the configuration of your choice.

After the RAID is initialized, it can be attached to a host and used as if it were a single 100-Gbyte drive (as was the case with the rev iew unit). The large number of host channels lead to some interesting possibilities. For instance, rather than transferring large data sets from one system to another to perform different operations, the array holding the data can be mounted by one system for processing, then dismounted, then mounted by the next system for the next phase of processing. This is orders of magnitude faster than moving the data.

LynxArray
The LynxArray from Artecon is similar to the StorComp RAID 7 in function. But it is much smaller, with a capacity of nine hot-swappable drives, plus two controllers and a tape drive, and dual redundant hot-swappable power supplies.

It also has a handy LCD on each drive that indicates the SCSI ID of the drive, as well as its temperature. The two controllers allow for fail-over using dual SCSI controllers in the host computer. The system can be configured via buttons on the controllers or with a serial terminal.

RAIDPro
The RAIDPro from Storage Dimensions was the smallest, lightest, and least-expensive unit I tested, but had some of the most interesting features. It uses a PCI RAID controller, available in single- or dual-channel configurations (to provide failover capability), and supports up to six drives. It has three small hot-swappable power supplies-if any one fails, the other two will run the unit.

When configuring the RAIDPro, you have an interesting option; you can start with as few as three drives, and add others later, but add them to the existing volume, rather than having to create new ones. No system shutdown is required; you don't even have to dismount the volume!

This is done by creating a virtual partition that's larger than the actual capacity of the RAID. When more disks are inserted, they automatically become part of the partition and can be added to the original volume on the fly. The sole danger is that you can easily define a volume that's bigger than the maximum capacity, which can crash the server and cause other problems. But this is well-documented and the necessary precautions are spelled out plainly.

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