CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [169]
The obvious starting place for RAID is to connect at least two hard drives in some fashion to create a RAID array. For many years, if you wanted to do RAID beyond RAID 0 and RAID 1, the only technology you could use was good old SCSI. SCSI’s chaining of multiple devices to a single controller made it a natural for RAID. SCSI drives make superb RAID arrays, but the high cost of SCSI drives and RAID-capable host adapters kept RAID away from all but the most critical systems—usually big file servers.
In the past few years, substantial leaps in ATA technology have made ATA a viable alternative to SCSI drive technology for RAID arrays. Specialized ATA RAID controller cards support ATA RAID arrays of up to 15 drives—plenty to support even the most complex RAID needs. In addition, the inherent hot-swap capabilities of serial ATA have virtually guaranteed that serial ATA will quickly take over the lower end of the RAID business. Personally, I think the price and performance of serial ATA mean SCSI’s days are numbered.
Once you have a number of hard drives, the next question is whether to use hardware or software to control the array. Let’s look at both options.
Hardware versus Software
All RAID implementations break down into either hardware or software methods. Software is often used when price takes priority over performance. Hardware is used when you need speed along with data redundancy. Software RAID does not require special controllers; you can use the regular ATA, SATA controllers, or SCSI host adapters to make a software RAID array. But you do need “smart” software. The most common software implementation of RAID is the built-in RAID software that comes with Windows 2000, 2003, and 2008 Server. The Disk Management program in these Windows Server versions can configure drives for RAID 0, 1, or 5, and it works with PATA, SATA, and/or SCSI (Figure 11-36). Disk Management in Windows 2000 Professional, Windows XP, and Windows Vista, in contrast, can only do RAID 0.
* * *
NOTE You can use Disk Management in Windows 2000, Windows XP Professional, and Windows Vista to create RAID 1 and RAID 5 arrays, but you can use Disk Management only remotely on a Windows 2000, 2003, or 2008 Server OS. In other words, the capability is there, but Microsoft has limited the OS. If you want to use software RAID in Windows 2000, XP (Home or Professional), or Vista, you need to use a third-party tool to set it up.
Windows Disk Management is not the only software RAID game in town. A number of third-party software programs work with Windows or other operating systems.
Figure 11-36 Disk Management tool of Computer Management in Windows 2003 Server
Software RAID means the operating system is in charge of all RAID functions. It works for small RAID solutions but tends to overwork your operating system easily, creating slowdowns. When you really need to keep going, when you need RAID that doesn’t even let the users know a problem has occurred, hardware RAID is the answer.
Hardware RAID centers around an intelligent controller—either a SCSI host adapter or a PATA/SATA controller that handles all of the RAID functions (Figure 11-37). Unlike a regular PATA/SATA controller or SCSI host adapter, these controllers have chips that have their own processor and memory. This allows the card, instead of the operating system, to handle all of the work of implementing RAID.
Figure 11-37 Serial ATA RAID controller
Most RAID setups in the real world are hardware-based. Almost all of the many hardware RAID solutions provide hot-swapping—the ability to replace a bad drive without disturbing the operating system. Hot-swapping is common in hardware RAID.
Hardware-based RAID is invisible to the operating system and is configured in several ways, depending