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Raid Arrays


Gorgon

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Can someone explain to me what the great wonder of a raid array is. Does it take several HDDs and treat them as one, scattering file segments all over the place. I have heard some reports of a critical error meaning data loss on all of the drives in an array.

 

Part of the reason I have more than one drive is to have some protection against mechanical failure. Is there any real gain, considering that HDDs are hopelessly slow no matter what.

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Short answer: Yes

 

Long answer: You have different raid configurations for different needs.

 

If you want speed, you go for a raid that adds "striping" (RAID 0), i.e. it splits data between multiple disks, treating them as a single disk as far as you application sees it. It can theoretically double your disk speed (although the gains are typically around a 50% speed increase). Another raid configuration is "mirroring" (RAID 1) which gives you redundancy because, if one your disks crashes, you have identical data on the other. Ideally you would have both (RAID 1+0) which has two striped sets, mirrored (or mirrored stripe sets). That means you get the speed increase and the redundancy (because if you lose a disk in a striped set, you do indeed lose the entire content of all disks). There is also something called RAID 5, striping with parity, but that is an ugly thing best forgotten and hopefully never seen in a production environment. A poor mans raid so to speak, giving you redundancy at the cost of (random write) speed.

 

The better you want it, the more HDD's you need.

“He who joyfully marches to music in rank and file has already earned my contempt. He has been given a large brain by mistake, since for him the spinal cord would surely suffice.” - Albert Einstein

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The "wasted" space is different for different raids:

 

It isn't really wasted, since it is what gives redundancy, but lets just call it "waste" since you can't use it directly.

 

RAID 0 = No wasted space

RAID 1 = half your disk capacity

RAID 5 = 1/number of disks

RAID 1+0 = half your disk capacity

 

Speed gains:

 

RAID 0 = Lots

RAID 1 = Insignificant/None

RAID 5 = Loss

RAID 1+0 Lots

 

Redundancy:

 

RAID 0 = None, on the contrary...

RAID 1 = Lots

RAID 5 = Lots

RAID 1+0 = Lots

 

(Lots = can survive the complete crash of a single disk)

 

 

Lets say you put together a couple of RAID 0's like I used to have in my old pc:

 

RAID 0 set 1

 

HDD1 + HDD2 striped

 

RAID 0 set 2

 

HDD3 + HDD4 striped

 

Now, what windows sees is just "Physical Drive 1" and "Physical Drive 2", which can be treated like any other single drives. The RAID hardware is what handles all the disks and tells Windows, Hey, I got one disk for you here.

 

My RAID 0 set 1, I had split up in 3 partitions, two smaller (E: and F: and a larger G: with the remaining disk space)

 

Set 2 was likewise split up in three logical drives, with primary partition and extended partions like set 1

 

Makes sense? :shrugz:

“He who joyfully marches to music in rank and file has already earned my contempt. He has been given a large brain by mistake, since for him the spinal cord would surely suffice.” - Albert Einstein

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Real men use RAID 0 and don't take backups. It's better to burn out than fade away ;)

 

As I said, it really depends on what you need. Cheap storage where speed doesn't matter, you could go for RAID 5. Gives you redundancy at the cost of 1 HDD, no matter how many more disks you add to the set.

 

If you need speed (not just read, but write speed), you need something with a 0 in it. RAID 0, RAID 1+0 or RAID 0+1.

 

If you need safety you can get RAID 5 or 1 (if you don't need the speed) or RAID 1+0 or RAID 0+1

 

Both the 1+0 and 0+1 requires 4HDD's as a minimum, giving you the capacity of 2 HDD's

 

I am currently looking to buy two 500 Gb disks to put in a RAID 0 to keep my 1TB disk (no RAID) company. But then, I *do* back up essential stuff every now and then to either DVD or Blu-Ray disks.

“He who joyfully marches to music in rank and file has already earned my contempt. He has been given a large brain by mistake, since for him the spinal cord would surely suffice.” - Albert Einstein

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  • 2 weeks later...

Would it work if I had one of those plug in HDD? Or do I have to put it in the case for the sake of reliability?

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             -Elwood Blues

 

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It depends on what your RAID controller supports.

 

I think for most home desktops, they would need to be all SATA drives (all IDE on older motherboards). Having similar disks also prevents you from wasting the better drive, as you get the "lowest common denominator" regarding size and speed.

“He who joyfully marches to music in rank and file has already earned my contempt. He has been given a large brain by mistake, since for him the spinal cord would surely suffice.” - Albert Einstein

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Or you could get on of them fancy new Intel X25 SSD drives, they're so fast its ridicilous.

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Both the 1+0 and 0+1 requires 4HDD's as a minimum, giving you the capacity of 2 HDD's

 

I'm curious. Since disk 3 and 4 are being used as a backup, would it be possible to have the +1 portion covered by a single drive of equivalent capacity to disk 1 + disk 2? Say you have two 320GB drives in RAID 0, could you use a single 640GB drive to bring that to RAID 0+1?

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Both the 1+0 and 0+1 requires 4HDD's as a minimum, giving you the capacity of 2 HDD's

 

I'm curious. Since disk 3 and 4 are being used as a backup, would it be possible to have the +1 portion covered by a single drive of equivalent capacity to disk 1 + disk 2? Say you have two 320GB drives in RAID 0, could you use a single 640GB drive to bring that to RAID 0+1?

Not really :)

 

In a RAID 1+0, you create the stripes between the mirrors

 

Mirror1 -Stripe- Mirror2 ... etc.

 

In a RAID 0+1, you mirror the stripes

 

Stripe1

|

mirror

|

Stripe2

 

You end up needing an even number of disks when mirroring.

 

There are some non-standard RAID setups that deviates from that though, like the Linux MD RAID10 which can do what you suggest. It is just not commonly available.

“He who joyfully marches to music in rank and file has already earned my contempt. He has been given a large brain by mistake, since for him the spinal cord would surely suffice.” - Albert Einstein

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