Storage prices have dropped precipitously, but RAID systems, the fastest choice available, are still expensive relative to other technologies. One way to keep costs down is to build your own RAID from separate components. This route is not for everyone, but hereıs a primer on just how itıs done.
The first step is to make sure all components use the same version of SCSI. There are more than half a dozen different compatible SCSI standards. SCSI chains are generally slowed to the performance of the slowest component. For example, if you put a narrow SCSI-2 device (10 MB/sec) on an Ultrawide SCSI chain (40 MB/sec), the whole chain could get slowed down to SCSI-2. In my test, every component was Ultra2, a new SCSI standard that supports speeds of up to 80 MB/sec.
The controller supplied by Adaptec (Milpitas, CA) was the AAC-364 RAID controller. Currently available only to OEMs, Adaptec says the controller will be available to resellers by the third quarter. The AAC-364 is a four-channel Ultra2 controller. Itıs also a full-length 64-bit PCI card. Servers with 64-bit PCI slots are currently scarce. However, the card works in 32-bit PCI slots. I tested the card in 32-bit mode in a Hewlett Packard E 50 server with a 300 MHz Pentium II processor.
Installing the card in a Windows NT machine was a multi-step process. Adaptec supplied three sets of floppies with the card. The first set was a firmware upgrade. After inserting the card in a PCI slot, I had to reboot the machine with the firmware upgrade discs to fully update the card. The second and third floppies install the AAC-364ıs Windows NT driver and the RAID management software. Although setting up the system with all these sets of floppies was fairly complicated, the complete installation went off without a hitch.
The AAC-364 comes with Adaptecıs Flexible Array Storage Tool (FAST) RAID controller software. The software gives you the tools necessary to recognize drives connected to the AAC-364 and to combine them into RAID volumes. RAID levels 0, 1, 5 and 0-1 are supported. The software, while comprehensive, is not very intuitive. Many users, including experienced system administrators, may find it difficult to use. FAST is used to set up and manage the discs making up the RAID volumes. A good feature of the software is its ability to manage RAID controllers over a network. This lets a system administrator monitor and repair RAID volumes from a remote location.
FAST presents you with graphical representations of the RAID volumes and hard drives connected to the controller. When you build a RAID, a series of bar graphs let you know the relative performance advantages of each RAID level. For example, RAID Level 1 and 0/1 (mirroring) provide maximum redundancy but the poorest storage space utilization. Mirroring is primarily for mission critical data that must be kept safe at all costs. Most users will opt for RAID Level 5. Level 5 offers space utilization nearly as good as striping while still providing redundancy.
When selecting the drives you want, a screen pops up letting you set various parameters like caching, drive letter, Windows format (None, NTFS or FAT) and chunk size. Whenever you build a RAID array, especially a new system with unformatted discs, set the system up in the late afternoon and start the format operation just before you leave work. Even on a fast server, this operation takes several hours.
One of FASTıs less intuitive features is that the failover drive, which provides an extra layer of redundancy, is configured after the RAID array is set up rather than as part of the same process. When creating any array, be sure to leave one controller-connected drive unused. From FASTıs Container View screen, you can right click on the RAID container and configure the extra drive as a failover drive. If one of the drives in the RAID array fails, the controller will automatically switch in a spare drive to take over.
FAST also demanded a separate setup to format the disks to skip over bad blocks. Most hard disk formatting software does this automatically.
I mounted our test array in a Gigaraid 8000 LVD case from Andataco (San Diego, CA). The case has dual power supplies and a ısplit bus.ı Each of the Ultra2 SCSI buses serves four of the unitıs eight drive bays. Inside, the Gigaraid had Single Connector Attachment (SCA-2) connectors. SCA-2 connectors combine power, SCSI ID and a 68-pin SCSI communication interface into a single connector. SCA-2 is a common connection standard for RAID systems. It reduces the amount of wiring and the number of possible points of failure in a disk array. Gigaraid 8000 array cases are available with rack mount hardware or as freestanding units.
The front of the Gigaraid case has power indicator lights and drive activity displays for each drive and for each power supply. If a power supply goes down an alarm sounds. Each drive bay has a button for deactivating power to the drive for removal.
Hard disc drives are mounted in stamped metal boxes that slide on tracks into the Gigaraid case. There are spring-loaded snaps on the sides of the mounting boxes holding them in place. The rack mounting hardware for the Andataco Gigaraid 8000 includes a metal bar for locking all the drives down. However, the freestanding versions of the case have no lock-down mechanism to prevent the drives from being carelessly pulled out.
Generally, RAID array cases should have some sort of locking mechanism. This weakness cropped up when I managed to corrupt the file system of the RAID volume by removing a drive from the case prematurely. A proper hot-swapping feature prevents you from pulling a drive out until the hard disk is powered down. When the file system became corrupted, it was impossible to bring the system back up without reformatting each hard drive and losing all information stored on the RAID.
Fortunately, replacing drives is not a daily operation, and regular backups (required for RAID arrays as with any other magnetic storage) give far better reliability than any RAID short of Level 1 mirroring. Users with less-than-mission-critical storage needs may even be able to do without hot swappable drives. This could save you hundreds of dollars because you can use an inexpensive SCSI case rather than a RAID case.
During my test, the Gigaraid lost a cooling fan in one of the power supplies. This set off an alarm and a red indicator light. A call to Andataco had a replacement power supply on its way the same day. Other than this minor hiccup, there were no problems with cooling or with the supply of power to the drives. Andataco says the Gigaraid case will cool up to eight 12,000 RPM drives.
I used six 7,200 RPM, 9 GB Viking II Ultra2 SCSI drives supplied by Quantum (Milpitas, CA). The drives had 80-pin SCA-2 connectors, but theyıre available with a more conventional 68-pin interface. The Viking II drives have a 512 byte cache and a five-year warranty. In my experience, Quantum drives are very reliable.
The one pitfall I encountered during setup was that the AAC-364 has four ıvery-high-densityı 68-pin connectors for each of its four channels. These very-high-density connectors arenıt widely used as yet and you may have to order special cables, especially if youıre connecting to an array that uses older Centronics style connectors. The Andataco Gigaraid array case featured the more recent and familiar HD68-pin connectors on its two SCSI ports. Cables with very-high-density connectors had to be special ordered.
I arranged the six Viking II drives in the Andataco Gigaraid into a five-drive, 36-GB Level 5 array with the extra drive used as a hot spare. When I checked the performance of the array, I found it to be about three times faster than a single 9.1 gigabyte drive (35 GB/sec vs. 12 GB/sec). With the card installed in a full 64-bit PCI slot and with additional drives, you should expect even faster performance.
To test hot swapping, I pulled one drive out of the array. The Adaptec controller successfully started up the spare drive and set it up on the fly while issuing an alert for the system administrator. While all this was going on, there were no interruptions in the availability of the RAID volume.
Building a RAID system is a money-saving storage option. Depending on your drive and array case choices, you can shave as much as 50% off the cost of a fully pre-configured system using a PCI RAID controller. The system I put together, for example, would have cost $4,000 to $6,000. A pre-configured system of the same capacity would have cost more than $9,000. The savings are even greater when compared to a RAID array with an external controller solution.
The tradeoff of building it yourself is that you do have to do without the service and features you get when you work with a RAID vendor -- services such as drive certification, service contracts, guaranteed compatibility and on-site tech support. Pre-configured RAID systems also offer features such as redundant RAID controllers and thermal monitoring with specialized software for maintenance and RAID management.
Are these add-on features and the time savings of buying it all in one package worth a 50% price premium? If your knowledge of RAID systems is strong enough or you can rely on your IS support staff to directly maintain an array, building a RAID is an inexpensive and viable option.
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