How much actual disk space after formatting?

Posted on 2004-03-30
Last Modified: 2008-03-17
I am trying to figure out how much disk space is available after formatting, is there a set formula I can use to figure this out.  

example:  In Windows although I have a 250GB hard drive, Windows only reports 232GB

How much actual usable space will be on a hard drive array of 1.7TB?  
(using a scsi drive cage 14 drives 146.8GB each -  what will I have as usable disk space?  How do I calculate to include the hot swappable spare and parity of the raid5?
Question by:srusk
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Accepted Solution

Bartender_1 earned 500 total points
ID: 10719559
Hi srusk,
The difficulty is that the drive manufacturer measures the space using 1MB = 1000 KB, 1GB = 1000 MB etc.

You computer reads it the "real" way. (1 KB = 1024 bytes, 1MB = 1024 KB, 1GB = 1024 MB etc.)

The actual space that is used up by the file system is usually not as noticable as the difference caused by the difference in measuring a "MB"

Hope this helps!


LVL 22

Expert Comment

ID: 10719581
See here for actual performance differences etc, of RAID (all different levels are described.)

Hope this helps!


LVL 67

Expert Comment

ID: 10719609
This may help explain it as well.

Has to do with how the drive is prepped for use...
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Author Comment

ID: 10719801
reply back to bartender 1.
On your 4:20pm comment, is there a link/url that I am missing?  I am definitely interested in how the RAID comes into play with the sizing and if there is any limitations to Microsoft windows in sizing drives.

If I add up what I should have I come up with 1.7 TB, but I am looking for a formula that will give me how much of that 1.7 TB I will actual be able to use.

HP said I should allow 1 - 1.5 GB per drive for the RIS overhead with no OS on these drives (the OS is on mirrored drives separate from this drive cage).  If OS is on the drive that is reported as unusable space.

They also said the more drives I have in the array the more drive space I'll lose but no one seems to know what formula or calculation that is based on.
LVL 22

Expert Comment

ID: 10719842
Sorry about that. Here's the "missing link"... (~LOL~ Let's see how much fun the googlers have with that one.....)

Hope this helps!


LVL 22

Expert Comment

ID: 10719948
Also, here is a link about different file systems.

This will provide a better understanding on common file systems.

Hope this helps!



Author Comment

ID: 10720516
Bartender 1 and sirbounty -

I thank you both for responding.  I have also found out some other information...I've included it to help wrap this up.

Posted by timothy on Wed Oct 08, '03 12:26 AM
from the fudging-the-numbers dept.
mrklin writes "James Wiebe of has written a clear example of how hard drive capacity is calculated  (PDF file) by hard drive manufacturers (base 10) and OS (base 2). He failed to name how the capacity should be described (, though."
A hard drive maker will market his drive as 200 GB, but it is not *true* GB because in his definition, 200 GB = 200 x 10^9 Bytes = 200,000,000,000 Bytes. He has used Base10.

But computers read space using Base2 (2^10 = 1024).
200 GB = (200 x 1024 x 1024 x 1024) Bytes = 214,748,364,800 Bytes

The difference between using Base2 (the computer's method) and Base10 (the marketer's method) is:
214,748,364,800 - 200,000,000,000 = 14,748,364,800 Bytes

Convert Bytes back to GB, and you get:
14,748,364,800 Bytes / 1024 = 14,402,700 KB = 14065 MB = 13.74 GB,

So you are LOSING 13.74 GB of space just because of DRIVE MARKETING (yes they think they can cheat you by using base 10 instead of 2, thinking that no one will know the difference).

So - if I understood all the reading I've done my 146.8 GB drive should come out as:  

146.8 = 146,800,000,000 bytes
Unfortunatley, in order to figoure out how many REAL gigabytes that its, you must first find out how many kilobytes that is.
Divide 146.8 billion by 1024 (NOT 1000!!!) and you get 143359375kilobytes
Divide that by 1024 and you have 139999.3896484375
Divide that number by 1024 again to find out how many gigs there are: 136.71815395355224609375



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