Raid 5 space difference

[JohnG86]

I just built a RAID 5 with the Promise S150 SX4 using 4, 250 GB SATA drives. The Controler says that the array size is 749.999 GB but Windows says it is 698 GB. I have the Block Size set to 64 KB. Is something wrong since I was expecting to get around 730 GB of space after formating or is everything correct and I should just start using the RAID?

I am using Windows XP Pro with ServicePack 2.

 

[Bobthelost]

It's not the old problem of defining gigabytes is it?

Windows uses a different definition to some other programs/systems/standards.

 

[V00D00]

4 x 250gb = 1TB = 1,000,000,000,000 bits

1 TB in base 2 = 931.32 GB.

Here's how you figure it:
250 gb drive
250 x 1000 = 25,000 mbytes
250 x 1000 x 1000 = 25,000,000 kbytes
250 x 1000 x 1000 x 1000 = 25,000,000,000 bytes

So that's actual BYTES in base 10.

Real bytes are base 2

25,000,000,000 / 1024 / 1024 / 1024 = 232.8 GB

250 GB drive = 232.83 GB actual space

So that's how much space a 250gb drive is. Multiply by 4 to get 931.32 GB.

931.32 GB in your WHOLE RAID ARRAY.

But remember, RAID 5 uses one drive for parity.

Subtract one drive = 931.3 GB - 232.8 GB = 698.5 GB

Well it seems like it's right on based on my crappy calculations I just did real quick.

 

[JohnG86]

Thank you V00D00 for conferming that I am getting the right space that I should. I was thinking that it was correct but I just couldnt figure out why I would not have more then 700 GB after formating.

 

[V00D00]

You have to divide it all out. You can't just divide once, that would be mixing your units. The problem lies in the fact that hardware manufacturers assume 1,000,000,000 bits per gigabyte. This is incorrect. It is actually 1,073,741,824 bits per gigabyte. It is a way for them to inflate their numbers. Just like how LCD manufacturers show a 12ms responce time but that is just gray to gray, full color changes can be more like 20ms.

 

[AsianriceX]

http://en.wikipedia.org/wiki/Gigabyte

Damned Gibibytes