Post your comments and questions for AGW #29: RAID Crazed

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ActiveTuning - Partner & Director Of Sales & Marketing

i'd say 80 gigs total capacity for raid 6+6 using 16 x 20gb drives
tolerance... thats nasty to explain but i'll try.

Top Level:
4 - 80 gig raid 6 (which drops to 40 due to parity) - can lose two of these as a whole

any One of the 80gb raid 6 you can lose two of the individual drives in the raid.

so you could lose up to 8 drives (2 per individual raid 6) and still be able to restore your data on each of the 4 raid 6 pieces. BUT you're also allowed to lose two of the individual raid 6's so you could tack another 4 drives onto the total taking out two of your raid 6's. Grand Total by my strange calculations = 12 down'd drives

but you'd have to be doing something crazy to nuke 12 drives in a go!

did i get it right? if so, gimme another one!

(note: i'm at work and extremely bored.)

You're dead on my friend!

Only wish I had a prize for you.

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Hey. What kind of party is this? There's no booze and only one hooker. -- Bender (Futurama)

another brain bending question would be a good prize! (:

OK then... same questions... same drives....

The array: RAID 5+6.

Here's another one for you... What's the minimum number of drives for you to have in the RAID 6+6 configuration mentioned in the column that would not leave you with half of your drives going towards parity.

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Hey. What kind of party is this? There's no booze and only one hooker. -- Bender (Futurama)

how bout we go with the how many drives to get less than 50% used by parity: 20 drives. 4 sets of 5 drives = 40% of the space used by parity. Capacity for this config = 120 gbs.

as for you the raid 5+6 question...
Capacity: 80 gb
Min Drives: 12
Max Fault Tolerance: 8 drives down at one time, thats two complete raid 5's and one drive from each of the other raid 5's.

am i right? can i have more please? (:

He he he.... We may have to let others give this a try as well.

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Hey. What kind of party is this? There's no booze and only one hooker. -- Bender (Futurama)

alright (:

when are they goin to try and post?

Ummm, regarding Vendetta'soriginalanswer: Your total of 12 possible drives to be lost neglected to take into account this following case...

If you lost 2 drives in each of the individual RAID 6 arrays, that gives you 8. Now, you could technically lose 4 more drives as you said before, however, each pair of these would have to be located together within exactly 2 of the RAID 6 arrays. If, for example, you lost one more drive in each of the RAID 6 arrays for a total of 3 drives per RAID 6 array, your RAID 6+6 array would lose all of it's RAID 6 arrays and cease to function. Similarly, if you lost 4 drives from one RAID 6 array, 2 from another, and 3 from each of the remaining 2 RAID 6 arrays, then 3 of the RAID 6 arrays would fail causing the RAID 6+6 array to lose its Parity drives and one of it's main drives cauing it to fail again.

Therefore, if you're still following, the maximum number of drivesthat can be lost SAFELY would be 10: 2 from each RAID 6, and 2 more either both in one RAID 6 array (losing one part of the RAID 6+6) or 1 in each of 2 of the RAID 6 arrays (losing 2 RAID 6 arrays from the RAID 6+6 but still leaving 2 for functionality).

So, to make a long story short, if you lose more than 10 arrays, you may possibly compromise your entire array but with 10 or less you will never lose your RAID 6+6 array.

Good luck deciphering that one!

AAAAHHH but i did take that into account (:

so you could lose up to 8 drives (2 per individual raid 6) and still be able to restore your data on each of the 4 raid 6 pieces. BUT you're also allowed to lose two of the individual raid 6's so you could tack another 4 drives onto the total taking out two of your raid 6's. Grand Total by my strange calculations = 12 down'd drives

meaning that those extra four drives would all be contained in just two of the 4 raid 6 pieces. so two sets of four drives completely toast and then two out of each of the other two sets totals 12 drives toast.

still. i'd be absolutely amazed to find any self respecting IT professional who fried 12 drives at once.

i still stand by my 12 drives given a perfectly frying world.

doh, can't edit.

i concede that losing more than 10 drives does give possibility of losing the whole thing.

like my post says tho. if everything fries as specified you wont lose everything (:

I agree with the 12 drive solution to the 6+6 problem, but that is only if the drives fail where you want them to, a theoretical maximum. Simply put, you cannot afford to have 3 drives fail in each of the RAID 6 subsets. You CAN afford to lose 3+ in two of those subsets, but only two in each of the other two subsets.
Therefore, we are back to a practical maximum of 10 drive failures.

With 10 drive failures, you can guarantee that two of the RAID subsets will have two or less drive failures, thus preserving your data.

but the question was Maximum fault tolerane (:

Right... keep in mind that I said MAXIMUM number of drives. Typically, I don't see losing more than ONE drive so in my eyes, even just a regular RAID 6 is pointless.

Just give me RAID 5 and hot swappability and I'm happy.

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Hey. What kind of party is this? There's no booze and only one hooker. -- Bender (Futurama)

Allright then, what's the next wacky RAID math problem.