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RAID

Posted on 1999-06-22
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Last Modified: 2013-11-14
What is RAID ?

What is the differents between RAID and just a SCSI controller ? Which one have higher performance and stability if I connect serval hard disk on a computer ?

Andrew
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Question by:andrewyu
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siabod earned 40 total points
ID: 1157668
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Expert Comment

by:mark2150
ID: 1157669
RAID is Redundant Array of Inexpensive Disk drives.

It comes in several levels, each providing more redundency. RAID 0 is effectively just your normal disk drive. Raid 2 is disk mirroring, Raid 5 is five disks configured to "stripe" the data. This allows any single drive to fail and still have *all* data available. You can get "hot swap RAID5" chassis that allows you to pull out a failed drive and plug in a replacement without shutting down the system or losing any data. Is way cool! There is also Raid 10 and other intermediate levels, but those listed above are the most common ones.

SCSI is how the drives are physically connected to the controller. This is independent of the RAID technology. SCSI is used in RAID more often than IDE, but the two are not synonyms.

M

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Expert Comment

by:kschang
ID: 1157670
mark's right. You CAN have an IDE RAID array, but since IDE only supports two devices per channel, and SCSI supports 7, SCSI is used more often as RAID "bus". SCSI and RAID is not really related in any way.

Also, RAID makes all the drives connected appear as a SINGLE device, which means all the HD's used MUST BE IDENTICAL (same model number), so the controller must be quite robust.

RAID arrays usually cost THOUSANDS of dollars (certain more than most computers), which is why only servers use them, where reliability is more important than speed.
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Author Comment

by:andrewyu
ID: 1157671
Thank you very much !

Andrew
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Author Comment

by:andrewyu
ID: 1157672
Does it support by FreeBSD 3.2 ?

Andrew
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by:mark2150
ID: 1157673
RAID is mostly implemented in HARDWARE. It's really transparent to the OS. You install a RAID5 controller and a bank of disks and let 'er rip!

You can get OS level support for RAID, but it is *so* much easier to let hardware handle it.

M

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Author Comment

by:andrewyu
ID: 1157674
So, I need to buy a SCSI controller support RAID is ok ?

Andrew
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Expert Comment

by:mark2150
ID: 1157675
You need a RAID controller (SCSI preferred) and the appropriate number of drives. If you want "hot swap" this will usually require a special chassis as most PC chassis don't have 5 bays, nor can the drives be removed without a screwdriver and powering down.

M

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Author Comment

by:andrewyu
ID: 1157676
Thank you very much !

Andrew
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