RAID 0+1 setup doesn't see full HD capacity

[AndyM]

Anyone have this problem? I setup 4 40 GB IBMs in a 0+1 config on my Abit BE6II mobo but it appears that the 2 striped drives aren't seen in computer management in their combined capacity. I only got 62.98 GB capacity showing as DISC 0. I expected to have the full 80. Am I wrong in that assumption? Before I set up the 0+1 I had two drives in a RAID 0 setup and they showed as a DISC 1 at about 64GB and DISC 2 at about 19GB. What the heck? Thanks, AndyM

[Toaster]

Howdy,
First off, the capacity of HDDs is always somewhat below "stated " specs.
A 40GB drive often is closer to about 36GB in "true" GB.
HDD manufacturers use "1,000,000" as a MB when 1024KB is a "real" MB.
Second, a RAID 1+0 means 2 drives are "striped" and 2 drives mirror the stripes.
To use RAID 1+0, each "logical" drive needs to be the same capacity.
So, the "striped" capacity (RAID 0) is 40GB or slighly below and the second pair would be the same. The first and second "pair" are then "mirrored".
Total capacity of the "array", under RAID 1+0, would be about:
Total capacity of 40GB+40GB. Since the drives don't actually format out to a "true" 40GB, the total capacity would be approx. 73GB.
A more efficient array would be 4-40GB drives in RAID5.
The data rates would be somewhat lower then RAID1+0 but the toal capacity would be about 75% of the combined total of about 110GB. (approx)
The "estimated" throughput might be "about" 75% of a RAID1+0 array.
As the number of "physical" drives increase, the array becomes more efficient.
Keep in mind that most drive makers use "marketting math". Or, the capacity is about 3% below a "true" MB/GB.

[AndyM]

Thanks Toastman,
I see where I might expect somewhat less than 80, and the 73 figure is acceptable, so where's my other 10:)

[Toaster]

Howdy,
You other 10GB never really existed unless the drive maker stated capacities in "true GB". In RAID-5, the capacity is about 75% of the toal of the combined drives. This percentage increases with the "number" of attached drives.
At peak efficiency, array capacities can achieve about 83% dependant on the controller.
I recently was asked to retrieve files from a failing 30.0GB drive of Maxtor manufacture. The "actual" capacity of the drive when set to its perfered settings was 27.4 GB. Its all in the "type" of math a particular maker uses at the time. Most drive makers assume 1,000,000 as a MB where actually its closer to 995KB. DOS/Windows FDISK and format use "real" MB and thats where the confusion begins. So we have a drive maker that uses 1,000,000 as a MB and DOS/Windows (and all other O/Ss for that matter) use 1024KB as a MB.
End result is a drive somewhat smaller then advertised or "marketted".