DO-IT-YOURSELF RAID

By J.B. MilesSpecial to GCNThe beauty of redundant arrays of independent disks is that they can be scaled as high or as low as you want. Need a few hundred gigabytes of storage? Buy a single external RAID module from a vendor's product line. Want more? String together several modules with high-capacity drives for terabytes of storage.Most people think of RAID as expensive external systems with dozens or hundreds of drives, multiple controllers and redundant power supplies. But there are other kinds.Smaller but still scalable RAID systems for workgroups and small departments can be built around the same fast PC servers already in your office.Server-based RAID arrays act the same as any other type of RAID, making all the available disk space on multiple drives appear as a single logical drive to the server. Server RAID provides plenty of available storage with fast transfer speeds and protection against data loss in case of drive or disk failure. And it generally costs far less to implement than a high-end, scalable RAID system.The key is the controller. Server RAID is built around the same hardware and software as external RAIDs are built. The hardware typically includes a RAID controller, a carrier or tray into which eight or nine 3.5-inch hard drives can be stacked and the drives themselves. The software consists of setup and configuration applications as well as drivers for popular Microsoft Windows, Windows NT and Unix operating systems.Think of a RAID controller as a sort of computer-on-a-card, with a CPU, memory and nonvolatile RAM. This card takes over the job of data processing to and from the RAID drives, freeing the server's own CPU to perform applications processing. Overall data throughput also is increased significantly because RAID controllers read and write data from multiple hard drives simultaneously.It's not difficult to configure a RAID server after the controller is installed. Text-based installation procedures show you a ROM configuration program from which you can select the number of drives and RAID levels to use. Until you make further changes, this configuration will apply every time you boot up the server. The graphical configuration software sold with most new controllers is even easier to use.As for the drives, either one-inch or 1.5-inch drives are used in most cases. The size difference might seem slight but is significant. Storage density is mainly a function of the number of platters included in the drive itself; the extra half-inch often permits double the drive capacity from, say, 9G to 18G. Drives with 36G of capacity are coming on strong in the market, with 50G drives looming on the horizon.A server RAID array with eight drives can easily hold 250G or more of data'plenty for most workgroups and many small departments that don't use imaging, video or other high-bandwidth applications.Most server RAID systems depend on the host server's power supply but provide extra cooling fans and various SCSI or Fibre Channel interfaces for linking the internal drives and the server itself to the network or other servers. An easy route is to buy a name-brand server preloaded with RAID capability from manufacturers such as Compaq Computer Corp., Dell Computer Corp., Hewlett-Packard Co. or IBM Corp. Consider this especially if your technical support team lacks the experience to build its own RAID server.But expect to pay a heavy markup for the controller, drives and necessary hardware; $25,000 to $35,000 is the price range for a fully loaded server with multiple CPUs, plenty of RAM and RAID capability. Save money by buying a RAID controller from a manufacturer listed in this buying guide and insert it into a PC server with an array of JBOD'or just a bunch of disks'drives.A controller suitable for workgroup or small departmental RAID service generally costs $399 to $799, depending on the interfaces used, the number of host channel connections and other variables. Fire it up with the manufacturer's software and you're set.Even an inexpensive PC server can become a full-fledged RAID array for little more than the cost of the controller, a few drives and the base price of the server.RAID controllers from companies such as Adaptec Inc., American Megatrends Inc. (AMI), Distributed Processing Technology (DPT) and Mylex Corp. meet the widest possible spectrum of user requirements, from small workstation and entry-level servers to enterprisewide RAID. Most connect to the host server's PCI bus and use either 40-Mbps Ultra Wide SCSI or the 80-Mbps Ultra2 SCSI LVD interfaces. Several use 100-Mbps Fibre Channel interfaces for linking the server to the network or other servers.The controllers listed in the chart as host-independent power external RAID modules'that is, they don't fit inside a server' are often sold as original equipment manufacturer hardware. But as suitable options they are included in this guide for do-it-yourselfers willing to invest a little time and money in building workgroup to enterprise-level RAID modules.The simplest and least expensive controllers, appropriate for workgroup-level RAID, generally come with only one channel to connect to the host server. Others provide multiple channels for the connection.For example, Adaptec's $499 AAA-131U2 card with an Ultra2 SCSI interface and a single channel is a perfect fit for entry-level servers and workstations. The company's $799 3-channel 799 AAA-133U2 card triples throughput bandwidth from 80 Mbps to 240 Mbps and allows connections for up to 45 devices.Mylex' AcceleRAID 150, 250 and 250, priced from $499 to $799, are similarly scaled for entry-level and midrange servers, with 80-Mbps Ultra2 SCSI LVD interfaces and single channels. If you want to scale up to the large departmental or enterprise level, the company's eXtremeRAID 1100 card, which starts at $2,563, has three channels for 240 Mbps of bandwidth.Other manufacturers scale their controllers differently, depending on their target market. DPT's Millennium and Century controllers, which start at $2,075 and $992, respectively, can be scaled with one, two or three channels for midsize to enterprise RAID purposes. Its $658-to-$992 Decade series can be scaled with one or two channels for smaller, entry-level servers.Which RAID level you buy depends on your needs. RAID Level 0 stripes data across the drives in the array and is very fast but offers no protection against drive failure. Level 1 provides disk mirroring in which the information from one disk is written to another. Level 0+1 combines the speed of Level 0 with the redundancy of Level 1, and it's inexpensive to implement.Level 3 provides data striping, as with Level 0, with a dedicated drive for error recovery. Level 5 stripes blocks of data and formulas for parity checking across multiple drives and thus provides excellent data recovery in case of a single drive failure.For speed and protection against the loss of mission-critical data, the best idea is to select a controller that offers all these RAID levels or a combination of them.Most users are familiar with hot swapping, or the ability to pull out a drive and replace it without shutting down an entire system. Intelligent RAID controllers with levels 3 or 4 monitor for this situation and immediately begin to regenerate the data on the failed drive before it is lost.RAID Level 5 hot spare is similar and even more interesting. An extra, blank drive is set into the Level 5 array and, in case of another drive's failure, is immediately spun into action by the controller, which begins to regenerate data automatically.Controllers such as AMI's MegaRAID series, Compaq's Smart Array series and Mylex' AcceleRAID and eXtremeRAID series also boast self-monitoring and recovery technology. They can automatically recover and protect against data loss in case of problems with the host system or drive array.Fault-tolerant components such as redundant power supplies, cooling fans and multiple host interfaces also help reduce the chances of data loss in cases of cataclysmic events affecting the server or the array. XXXSPLITXXX-