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RAID 0+1 vs RAID 1+0 vs RAID 5


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#1 Shark

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Posted 06 March 2002 - 01:09 PM

Say you have 4 drives....

When comparing whether to stripe mirrors, mirror stripes, or stripe with parity:
Which is fastest?
Which is most fault tolerant?

Regarding RAID 0+1 vs RAID 1+0, which is which? This is hard for me to articulate.

One must be striped mirrors, and the other must be mirrored stripes, right? Is one any faster than the other?

Just curious.

#2 P5-133XL

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Posted 06 March 2002 - 01:49 PM

raid 1+0 and raid 0+1 run at the same speed when all drives are working. You are correct that 1+0 is a raid-0 array of mirrored drives and raid 0+1 is a mirror of a raid-0 array. The difference is what happens when a drive fails.

With raid 1+0 and a drive fails, then the mirrored drive replaces the failed drive and everything works as it did before. Also, if a second drive fails from the other mirror, everything still runs as before. So you can potentiall lose two drives with no effect on speed.

With raid 0+1 then the raid-0 array is mirrored. when a single drive fails, the mirror becomes invalid and everything will still work as before with no redundency. When the second drive fails, you lose everything because nothing is mirrored anymore.

#3 leokor

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Posted 06 March 2002 - 04:49 PM

Both RAID 1+0 and 0+1 have more redundancy that RAID 5. With RAID 5, if you lose two drives, you're toast. But with 1+0 and 0+1, you might lose two drives and still be functional, depending on which drives fail.

RAID 5 is much slower at writes. However, it does provide more storage. You lose only one drive's worth with RAID 5, while you lose half the storage with RAID 1+0 and 0+1.

Leo

#4 Defiler

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Posted 06 March 2002 - 05:22 PM

With only four drives, RAID 1+0 and RAID 0+1 are logically identical.
It's only when you have 6 or more that RAID 1+0 becomes a better choice than 0+1.

#5 jehh

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Posted 06 March 2002 - 05:27 PM

Why on earth would you want a 6 or 8 drive raid 1+0 array?

Yikes!

Other than a waste of drive space, transfer rates improve with RAID 5 at about the 6 drive point. STR speed grows as each drive is added (assuming enough DSP processing power is there to handle the new drives)

If you need to have 8 drives and secure data, I personally think RAID 5 is a much better choice than RAID 1+0.

But maybe that is just me. :D

Jason
There are two theories to arguing with women. Neither works.

#6 Defiler

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Posted 06 March 2002 - 05:32 PM

Some systems need high write performance. RAID 5 doesn't offer that.
These days, 8 fast IDE drives and an 8-port controller is a cheap combo (in terms of typical server hardware)
There's a situation where an 8 drive RAID 1+0 array would be practical. (and kickass)
I hit 155MB/sec with just four WD 1000JBs. Imagine 8 1200JBs.

#7 Trinary

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Posted 06 March 2002 - 05:35 PM

Okay, let's reiterate this one more time, just to make sure everyone gets the point.

RAID 1+0 is superior to RAID 0+1 in fault tolerance.

If you have a RAID 1+0 array, you are striping the mirrors. This means that you can have 1 drive from each mirror pair fail and still have a functional array.

That means that as you add more drives to the array, you can tolerate more simultaneous failures, provided that you do not lose more than 1 drive from any given mirror pair.

With RAID 0+1, you are mirroring the stripe. Since RAID 0 cannot tolerate a drive failure and remain functional, you can, at most, tolerate a single drive failure while remaining functional, regardless of how many drives are in the array.

Now, is anyone going to debate that half a loaf is better than none? Do we all get the point?
Trinary

#8 Defiler

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Posted 06 March 2002 - 05:37 PM

Too bad the forums were purged.. I made a really good "0+1 vs. 1+0" post a while back, with diagrams and everything. Heh.
Summary: 1+0 is the most reliable RAID level supported by standard controllers.

#9 jchung

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Posted 07 March 2002 - 07:22 AM

With only four drives, RAID 1+0 and RAID 0+1 are logically identical.
It's only when you have 6 or more that RAID 1+0 becomes a better choice than 0+1.


Actually, with 4 drives RAID 1+0 is slightly better than RAID 0+1 in terms of availability. With RAID 1+0 (striped mirrors), if one drive fails, then either of the drives in the other mirror can fail and you will still be fine. With RAID 0+1 (mirrored stripes), if one drive fails then only the other drive in the same stripe can fail for you to be ok.

Joo

#10 Defiler

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Posted 07 March 2002 - 08:25 AM

Damn. You're right. I wonder why I never realized that?

#11 Shark

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Posted 07 March 2002 - 09:57 AM

Too bad the forums were purged.. I made a really good "0+1 vs. 1+0" post a while back, with diagrams and everything. Heh.
Summary: 1+0 is the most reliable RAID level supported by standard controllers.


A moment of silence for the purged forums.







Better than RAID 5?

I think we've established that striped mirrors are "better" than mirrored stripes.

I'm no RAID expert, so I'm just wanting some confirmation on my grasp of things...

Let's say we have 6 drives. Now, obviously, RAID 5 will yield more capacity. But what of fault tolerance?

If we did a RAID 1+0, don't we have two options? We could:
A) stripe three 2-drive mirrors
or
B) stripe two 3-drive mirrors

Option A should yield 50% more capacity and shouldn't it be faster? It could tolerate at most 1 failed drive from each mirror (3 total failures, selectively chosen). Two selectively chosen drive failures could bring down the array, though?
Option B would give higher fault tolerance, able to tolerate up to 4 selectively chosen drives. It would take 3 selectively chosen failures to bring down the array.

Is that correct? What performance difference are we talking here? How does RAID 5 factor in?



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