[X4U] RAID Software Feedback

[Philip J Robar]

On Oct 24, 2004, at 12:24 PM, Stephen Lanza wrote:

> On 10/19/04 8:52 PM, " Philip J Robar <philip.robar at myrealbox.com>" 
> wrote:
>
>> Given that you can buy a 160-200 GB drive for well under $100 without 
>> a
>> rebate, why bother? (I've seen 120 GB drives for $40 after rebate
>> recently.)
>>
>> Raid 5 requires drives of similar size and all of the space on one of
>> the drives in the array is lost to check summing. Given the three
>> drives you mention above you'd end up with a 40 GB array. Of course
>> RAID 5 does have the advantage of reliability in the face of a drive
>> failure.
>>
>> Note that several recent reviews have shown conclusively that two 
>> drive
>> RAID 0 setups are not significantly faster than a single drive in
>> single user systems. Quoting from AnandTech 
>> (http://tinyurl.com/5myyl):
>>
>> "If you haven't gotten the hint by now, we'll spell it out for you:
>> there is no place, and no need for a RAID-0 array on a desktop
>> computer. The real world performance increases are negligible at best
>> and the reduction in reliability, thanks to a halving of the mean time
>> between failure, makes RAID-0 far from worth it on the desktop."
>
> Actually, one reason that RAID-0 is useful is to take a number of 
> smaller,
> odd sized drives and make a larger drive out of them. At least that is 
> my
> understanding of what RAID-0 can do. Am I wrong on this?
>
> Stephen Lanza
> Software Complement


You're wrong. :-)

The original RAID levels all had some level of data redundancy, i.e. 
protection against single drive failure. RAID 0 came later and provides 
no protection against drive failure. It stripes data across several 
drives, but size of the array is limited by the size of the smallest 
drive:

	array size = numberOfDrives x sizeOfSmallestDrive

Combining several drives of various sizes into a single image is called 
concatenation. Since concatenation doesn't have redundancy it also 
isn't RAID, but like "RAID 0" it is commonly included in RAID software.

BTW, I made a mistake in my posting above. "all of the space on one of 
the drives in the array is lost to check summing" could lead one to 
believe that RAID 5 uses a dedicated parity drive when it actually 
spreads the error correction data over all of the drives. (RAID 3 and 4 
use dedicated parity drives.)


Phil