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Stupid SCSI Raid Question

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I rarely mess with RAID or SCSI, so forgive me if this is a "well, duh" type question.

I'm setting up a workstation for a user that needs a RAID 5 array (it has 3 x 300GB 10K Fujitsu's.) The SCSI BIOS utility doesn't have RAID 5 as an option (AIC-7901X.) I configured it as a two-drive RAID 0 and then added the third drive as a spare. Did this accomplish the same thing?

Thanks,

Ron

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I rarely mess with RAID or SCSI, so forgive me if this is a "well, duh" type question.

I'm setting up a workstation for a user that needs a RAID 5 array (it has 3 x 300GB 10K Fujitsu's.)  The SCSI BIOS utility doesn't have RAID 5 as an option (AIC-7901X.)  I configured it as a two-drive RAID 0 and then added the third drive as a spare.  Did this accomplish the same thing?

Thanks,

Ron

201797[/snapback]

Absolutely not. Now if one hard disk fails (of those two in raid 0), you'll lose all the data and there's no need for spare. In raid 5 you can lose any one of (in this case) three disks and you won't lose data. There's no redundancy in raid 0. (You can find lots of information about different raid levels in the FAQ section of this site.)

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There's no redundancy in raid 0.
Yeah, it should really be called AID 0, which coincidentally is an accurate description of how much help you'd get when a disk fails.

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I read somewhere that if you're running XP, you can make very minor changes to a few system files and XP will support RAID 5 in software (slower than hardware implementation).

I don't know the legality of this; you can google around to found out more.

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XP Pro supports RAID-5 in the OS, for non-boot volumes only.

If I recall, W2K did as well.

A RAID-5 is fault tolerant because it saves your data plus additional "checksum" data (which is why RAID-5 capacity is one-disk-less than the sum of the of disks in the array. It needs that extra disk for the checksum). The checksum data is actually the XOR of the data sent to all the other drives in the array. When a disk fails in a RAID-5, the data can still be recovered by simply XORing the data from the remaining drives. What you get when you do that is the original data from the now-missing HDD.

RAID-0 is not fault tolerant. In fact, RAID-0 reliability goes down in proportion to the number of drives in the array. In RAID-0, the data is simply uniformly distributed across the drives in the array. Each drive has a totally independent piece of the data, and if one drive fails the pieces of data that drive contained are lost and unrecoverable.

When you put a hot spare in a RAID-5, when a drive fails the controller will "rebuild" the array's "extra" data on to the spare to restore the array to fault tolerance, using the same technique to recover the data lost on the failed drive as it would use to give your application the data.

When you put a hot spare on a RAID-0, when a drive fails the data from the array is lost. The controller will rebuild the array using the spare, but it has no way to recover the original data so it will simply initialize the array to all zeros. You get restored to functionality automatically, but without any of the original information.

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XP Pro supports RAID-5 in the OS, for non-boot volumes only.
You sure? I thought it was only the Server variants of Windoze that were capable of RAID 5.

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[You sure? I thought it was only the Server variants of Windoze that were capable of RAID 5.

Well... no. But I'm typing this on an installation of XP Pro and I'm pretty certain when I set this sytem up I looked at building a RAID-5 and it let me. I think the limitation was that for the W2K OSs, only W2K Server supported software RAID. For XP (Pro), I think that limitation was eliminated.

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[You sure? I thought it was only the Server variants of Windoze that were capable of RAID 5.

Well... no. But I'm typing this on an installation of XP Pro and I'm pretty certain when I set this sytem up I looked at building a RAID-5 and it let me. I think the limitation was that for the W2K OSs, only W2K Server supported software RAID. For XP (Pro), I think that limitation was eliminated.

201863[/snapback]

I wish that XP allowed you to create mirrored or RAID-5 volumes. From the MS knowledgebase:

You cannot create mirrored volumes or RAID-5 volumes on Windows XP Home Edition, Windows XP Professional, or Windows XP 64-Bit Edition-based computers. However, you can use a Windows XP Professional-based computer to create a mirrored or RAID-5 volume on remote computers that are running Windows 2000 Server, Windows 2000 Advanced Server, or Windows 2000 Datacenter Server. You must have administrative privileges on the remote computer to do this

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I'd wager that Windoze Update may fubar such tweaks, so I'd be careful none the less :)

201872[/snapback]

Yeah, but Windoze Update fubars non-tweaked installations as well.

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