topic Re: MSA 1000 disk size in Disk Enclosures

MSA 1000 disk size

Khurram Shahzad — Thu, 26 Jun 2008 03:42:43 GMT

Hi on MSA 1000 from 300GBx3 Disk i have created unit 0 with RAID 5 configuration, the size of this unit after RAID 5 shown is almost 570 GB, but when I see this LUN in Linux it gives me 599 GB.

Can anyone explain why its happening.

Re: MSA 1000 disk size

Uwe Zessin — Thu, 26 Jun 2008 03:59:45 GMT

The values do not 'match' exactly, but it looks like Linux shows the size in Hardware GigaBytes while the MSA1000 shows it in Software GigaBytes:

599/1.024/1.024/1.024 = 557.86

Re: MSA 1000 disk size

Khurram Shahzad — Thu, 26 Jun 2008 04:17:32 GMT

Great answer but can you tell me what is the difference between Hadware Gigabytes and software gigabytes.

what is this formula 599/1.024/1.024/1.024?

what is 1.024

Re: MSA 1000 disk size

Uwe Zessin — Thu, 26 Jun 2008 04:40:37 GMT

Sure.
A disk block is 512 bytes.
Lets assume a '300GB' disk has 585937500 blocks.

A hardware vendor multiplies the number of disk blocks by 512 and use this value to specify the size of a disk.

585937500 * 512 = 300000000000

That value is a bit hard to read so we translate it:

300000000000 Bytes =
300000000 KBytes =
300000 MBytes =
300 GBytes

Or: 300000000000 / 1000 / 1000 / 1000 = 300

And that is what you can see printed on the disk drive, so it is called the "Hardware GigaBytes".

A computer works on 'base 2', not 'base 10'. Many programs do not divide by 1000, which is not a 'nice looking' number inside a computer [=0011.1110.0100(2)], they divide by 1024 (=2^10)
[=0100.0000.0000(2)] :

300000000000 / 1024 = 292968750
292968750 / 1024.0 = 286102.29
286102.29 / 1024.0 = 279.39
(please ignore rounding errors)

So this gives 279.39 Software GigaBytes. Although the number looks smaller, it is the same amount of bytes, just a different base. You can compare it with:
5(10) = 101(2)

Why 1.024? Well, I was too lazy to type:

599000000000 / 1024 / 1024 / 1024 = 557.86