Clarify between decimal bytes and binary bytes

Curtis Stevens curtis.stevens at
Mon Jul 12 16:03:20 PDT 2010

* From the T10 Reflector (t10 at, posted by:
* "Curtis Stevens" <curtis.stevens at>
IDEMA is about standardizing the number of sectors reported for a given GB
or TB.	The goal is to allow you to have some degree of confidence that when
you purchase a 5TB replacement drive for your raid, that it has the same
number of sectors as the other 5TB drives.  This is not about TiB vs TB, its
about raw numbers of sectors vs what's on the label.
Curtis E. Stevens
Sr. Staff  Engr - Standards & Features Technology
20511 Lake Forest Drive #A 113-F
Lake Forest, California 92630
Phone: 949-672-7933
Cell: 949-307-5050
E-Mail: Curtis.Stevens at
-----Original Message-----
From: owner-t10 at [mailto:owner-t10 at] On Behalf Of Pat LaVarre
Sent: Friday, July 09, 2010 5:52 PM
To: t10 at
Subject: Re: Clarify between decimal bytes and binary bytes
* From the T10 Reflector (t10 at, posted by:
* Pat LaVarre <p.lavarre at>
 > IDEMA ... is the reported capacity ...
The way I hear it, IDEMA tells us how many bytes should be counted by  
SBC READ CAPACITY for any given drive capacity X within the domain of  
their formulae. On the other hand, me googling a little then doesn't  
immediately dig up much confirmation ...
 > overwhelming NEED to use the binary terms ? If not, then no problem
 > ...
 > don't ... have to deal with something that is not used in our  
standards ...
 > exactly what it is you are proposing to un-confuse
 > looks like everything is clearly stated. So where is the problem?
 > ...
 > IDEMA formula ... JEDEC ... not a T10 or T13 problem
 > ...
 > uncompressed capacity rather that the presumed compressed capacity
 > ...
 > SAS-2 ... megabytes ... decimal ...
 > SSC-4 ... megabytes ... decimal ...
 > SPC-4 uses binary units in an EXTENDED COPY command-related	
TRANSFER COUNT field (see pg 324 table 206) ...
 > SES-3 uses binary units to report non-volatile cache sizes (see pg  
100 table 94) ...
 > SBC-3 uses binary units when discussing the addressing limits of  
READ(6) (see pg 99 note 10).  I thought we deleted ...
 > ...
 > See SPC-4 revision 25 section 3.6.4
 > SES-3 revision 2 section 7.3.11 table 95
 > SBC-3 revision 22 has no remaining uses of binary OR decimal  
prefixes, so no longer references IEC 60027.
 > ...
That page works better when read aloud. Mebibyte sounds like  
may be byte, Gibibyte sounds like gibberish.
Nist's best story is the "1.44 MB" unfloppy diskette that in fact held	
1.44 KKiB.
Those IDEMA units are not simple linear translations. I once wrote on  
my cube wall:
1,000,194,048 B = 1 IDEMA GB circa 300 GB
I must have then been under the impression that the IDEMA unit for 1  
Gig has some other value at other capacities, courtesy their mystic  
formulae. It's not a simple relation like the more familiar:
1,000,000,000 B = 1 GB = 1 Gigabyte
1,073,741,824 B = 1 GiB = 1 Gibibyte
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