Originally posted by Photo Tramp Hi Alan.
You had me thinking, (thats dangerous BTW) I pulled out my class A certification book and I'm sitting here looking at RAID 5 disk striping with distributed Parity. Unlike a dedicated Data and parity drive (like the RAID 1) RAID 5 distributes the data and parity information evenly across ALL DRIVE. You would need at least 3drives but the storage would be limited to only the size of one drive. Say if you have 3 200gb drives your total storage capacity would be 400gb (not the 600gb You would have in a Raid 1 array.)(If you had 4 200gb drives you would have 600gb storage.)
Now to answer the question about a drive failing. If one drive fails it's portion of the striped data can be re-created from the remaining data and parity bits, as long as only one of the drives fail. In RAID 5 it is rare that more then one drive would fail at a time.
Hope this helped.
Good Shooting.
Cheers: David
PS The information and more can be found in the All In One A+ certification Exam guide fifth edition by Meyers/Jernigan Pages 384-385 and page 429
Unfortunately, this information is not 100% correct (I've taken A+ courses, and I have despaired at the information in some of the books- I've also taken MCSE courses and not been overly impressed with some stuff in there too).
In RAID 5, you are not limited to the capacity of ONE drive. If you have X drives in an array, you have X - 1 drives' worth of capacity. For example, with 3 500GB drives, you will have 1TB worth of capacity. 3 drives (1.5TB), with the equivalent of one (500GB) being used for parity. (Wait... looking back over your explanation, you have it right, but typed "one" instead of "two".)
A little information on some more popular RAID levels:
If your data (in chunks of D1, D2, D3, D4, etc.) and parity (P1, P2, P3, etc.) are written to the arrays, you end up with the following:
RAID 0- data striped across all drives:
Drive 1: D1, D3, D5
Drive 2: D2, D4, D6
Lose one drive, you lose it ALL. This is pure speed, no redundancy or fault-tolerance.
RAID 1- No parity, just mirroring:
Drive 1: D1, D2, D3, D4, D5, D6
Drive 2: D1, D2, D3, D4, D5, D6
Lose one drive, you lose nothing. This is just redundancy, no increase in speed (actually a slight hit).
RAID 4: One drive of the array (3+ drives) is used for mirroring, data striped across the rest:
Drive 1: D1, D3, D5
Drive 2: D2, D4, D6
Drive 3: P1, P2, P3
Lose one drive, you can still survive.
RAID 5: Like RAID 4, but everything is striped:
Drive 1: D1, D3, P3
Drive 2: D2, P2, D5
Drive 3: P1, D4, D6
Again, lose one, and you're still in business. This is more fault tolerant than the others.
Generally, unless you REALLY needed a huge single drive array, I would recommend against RAID 5 for most people. The complexity and cost is rather high; better to invest in a next-gen DVD drive (like Blu-Ray or HD-DVD) or a mega capacity backup solution.
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