[CentOS] How to test RAID-5

Nick list at everywhereinternet.com
Thu Mar 9 12:12:32 UTC 2006

Fajar Priyanto wrote:
> On Thursday 09 March 2006 06:42 pm, Nick wrote:
>> It uses parity information that it stores across all three drives. In
>> your setup 1/3 of each drive is used for the parity information.
>> Basically any two drives then have enough parity information to recreate
>> the date on the third drive. With raid 5 the usable storage is C * (N
>> -1) where C is the capacity of the drives and N is the number of them.
>> So if you have 3 * 250GB disks the usable storage is 500GB. If two disks
>> die at the same time kiss all your data good bye (although this is a
>> pretty rare scenario)
> What I don't understand is that when it's running with only 2 disks out of 3, 
> it seems that it's still running OK.
> So:
> 1. What is the consequences when RAID-5 array runs with 1 lost disk? Will it 
> damage the data?
> 2. What will happen if I copy a large data (say 30MB) into the running array 
> with 1 lost disk?
Running with 1 disk down the array will function at 100% as all the data 
is still available because of the parity info stored on the two good 
disks. The problem is with one disk down you have absolutely no 
redundancy. If one of the other disks failed the data would be lost for 
ever. If you replace the dead disk you can then start rebuilding the 
array so that you get your redundancy back.

this page explains it a lot better than i can 

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