36.4GB drive formats out to 32.8GB? what am I missing please

D. Goss lists at dylangoss.com
Sun Jun 5 22:10:40 GMT 2005

>> I am adding some drives to an IBM xSeries 345.  I recently picked up
>> (via eBay) an IBM packaged Seagate drive, all with the proper IBM
>> part numbers as:
>> U320 15k
>> 36.4GB formatted capacity
>> (IBM part 06P5776 / 06P5778, with Seagate drive ST336753LC)
>> Looking up both the IBM part numbers and the Seagate part numbers via
>> Google, I consistantly get that the drive's formatted capacity is
>> 36.4GB.
>> When partitioned either in "safe" or "dd" mode (via sysinstall) and
>> set to use the entire disk as one slice, once the drive is mounted I
>> show:
>> # df -m
>> /dev/da1s1      33617    0 30928     0%    /misc
>> # df -h
>> /dev/da1s1      33G    4.0K     30G     0%    /misc
>> I'm seeing approx. 30,600MB (32.8GB?) free -
> I suspect three things are goping on
> 1) disk makers specify GB as 1,000,000,000 bytes but everybody else
> specifies 1,073,741,824 bytes (1024*1024*1024) this will yeild 33.9 GB
> from your 36.4 GB drive

I have always seen drives format out at less but the thing that threw  
me was that 36.4 was listed as "formatted capacity" - by IBM (not  
Seagate).  This helps though.  Somehow this reminds me of the # of  
hotdogs in a pack vs. # of buns.

> 2) formatting itself takes some space for superblocks etc

Figured, but not much, right?

> 3) some psace is reserved (man newfs and tunefs for info)

Read those man pages and that helped a lot.  I have read this before  
and it wasn't retained.  8% is the kind of loss I was looking for.

Thanks for all the quick answers.


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