RAID TECHNOLOGY GUIDE

What is RAID?

The Term R.A.I.D is short for Redundant Array of Independent (or Inexpensive) Disks.  Basically RAID is when two or more hard disk drives are combined in a way to either increase performance, add data protection (fault tolerance), or both.  It is a very useful technology for today’s high-demand storage subsystems, with practical and affordable configurations for everything from home desktop systems to  high-end  workstations or servers.

RAID Levels

  Today there are many choices for RAID configuration, and you must know the abilities of each type of RAID to choose the right one.  It is best to choose the type of RAID level you need first before choosing a RAID product.

  Here is a chart of popular RAID configurations

Raid Level Definition Min. # Drives Advantages Disadvantages Uses
RAID 0 Striped disk array - the data is broken down into blocks and each block is written to a separate drive 2 Faster Performance No Fault Tolerance Video Editing, High Bandwidth I/O Applications
RAID 1 Mirrored disk array - second drive is exact copy of first drive 2 Simple to setup, Fault Tolerance Less Efficient Applications / Servers Requiring High-Availability
RAID 1+0 Two striped arrays that are mirrored with each other 4 Fault Tolerant, High I/O due to striping Limited scalability Workstation, File Server
RAID 5 Striping plus parity - data written across all drives in array 3 Fault Tolerant, fast data reads, good for hot swap Takes longer to rebuild than RAID 1 Database, Web, E-mail, and News servers
 

Hardware or Software RAID?

Most forms of RAID can be implemented in either software or hardware.  Hardware RAID just means you use a PCI RAID controller card or a controller chip built onto your motherboard.  Hardware RAID uses its own BIOS and configuration utility for setup of the arrays - a utility you must access during system boot process.  Software RAID is a function of the disk management of your operating system.

Hardware RAID is the preferred configuration since it performs independent of the operating systems installed and does not impact system performance by using CPU for processing.  Hardware RAID is also more reliable than software RAID and gives you more options.  Software RAID does have a use when you do not want to spend money buying RAID controller card.  You can do software RAID (such as striping or mirroring) using an OS such as NT 4.0 Server or Windows 2000 Server (these features are not built into NT Workstation or 2000 Professional).  To do software RAID you must first load the OS onto one of the hard drives then setup the RAID configuration you want in the software.

IDE or SCSI CHANNEL RAID?

You now have the choice to build a RAID system using either IDE hard drives (PATA or SATA) or SCSI hard drives.  Standard IDE RAID uses any two IDE hard drive to create RAID 0 (striping) or RAID 1 (Mirroring) disk array, or more recently you have the option of RAID 0+1 (uses 4 drives to combine benefits striping and mirroring).

Some IDE RAID controllers can do RAID 5 or handle more than 2 to 4 drives, such as the Promise SuperTrak SX6000.  In general, IDE RAID arrays are easier to set-up than SCSI arrays and just as reliable.  IDE RAID provides a good low cost alternative to SCSI RAID.  Promise www.promise.com and Highpoint Technologies www.highpoint-tech.com are leaders in the field of IDE controllers and IDE RAID cards.  You can even get a motherboard with built-in IDE RAID controller option, giving you two IDE channels separate from the IDE channel already built into the chipset.

Still, in some cases IDE RAID is just not enough.  For Corporate/Enterprise Level or Web Servers, very fast data IO throughput is critical, as is 24/7 reliability. SCSI RAID is the solution as it offers more options (RAID 0, 1, or 5), more drives that can be supported per channel or array, faster input/output (especially using Ultra 160 technology), and hot-swap drive bays, and therefore is best suited to critical server environments where even shutting down the system for 10 minutes is a major problem.

Adaptec www.adaptec.com and Intel www.intel.com are the primary manufacturers for top-quality SCSI RAID cards.  Yes, there are IDE hot swap and some higher-end IDE RAID cards that can do RAID 5 but in most situations SCSI would be your best option fro hot swap.  Some server motherboards now have the option of ZCR (Zero Channel Raid) that allows for a motherboard with onboard SCSI to be upgraded to SCSI RAID at a lower than normal cost by adding in special card in the zero-channel slot.