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Question:
I read your Ask the Expert answer on the comparison between SCSI HDD and IDE HDD. In case of an IDE-based RAID system, I have been wondering whether parity generation explained below has happened or not.
"RAID controllers can also create "parity" information for every write to the raid group. This parity information is used to recreate data if a disk drive n the RAID group fails. This provides data protection and continuous data access in the event of disk hardware failures. Creating RAID sets can make data I/O go much faster in many cases too. That's why high-end servers use SCSI and cheap PC's usually use IDE."
Chris Poelker's response:
SCSI is used in high-end systems due to the advanced feature/functionality of the SCSI command set. The ability to use Tag command queuing and active/active disk ports give SCSI and architectural advantage over IDE (Integrated Drive Electronics) disk solutions. The server can tell the SCSI controller to do a write and those writes can be queued on the controller so the server can go off and do more productive work instead of waiting for the acknowledgment that the write was completed.
Fibre Channel-based disks have advantages over many SCSI disks since both ports on the disk can be active at the same time. This means commands can be queued to both ports making the drives more efficient.
There are IDE-based RAID solutions that can translate SCSI commands to a lower cost string of attached IDE disks or solutions that can attach multiple serial ATA or IDE disks into a RAID set. If you want the advantages of SCSI and the benefit of using low cost drives, then the translating controller is what you want. If all you want to do is make your cheap IDE disks more reliable by using RAID technology than a simple IDE RAID controller will do.
SCSI and Fibre Channel disks are manufactured to much stricter tolerances than IDE disk and even ATA disks. The "duty cycle" for IDE and ATA is "spec'ed" much lower than that of Fibre or SCSI disks. This means the bearings and the actuator arm mechanics used in the higher priced drives are rated for more useful life than both IDE and ATA. SCSI and Fibre disks are built to get hammered by read/write data requests all day long, every day, for years. IDE is built to sit in a PC or laptop and get spun down when you go home.
Just to let you know that even though RAID is available on a much cheaper cost per megabyte using ATA and IDE disks, you get what you pay for.
Chris
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