Storage Capacity: TB vs TiB

For years, ion has reported usable capacity of disk and SSD storage in GiB and TiB instead of GB and TB. Why?  And what’s the difference?  Basically, 1 GB = 1,000,000,000 bytes while 1 GiB = 1,073,741,824 bytes. That is about 7%!  Look at ion‘s SR-71mach6 SpeedServer or PS StorageServer for examples of capacity reporting.

You can learn more about the differences in the Wikipedia article on Orders of Magnitude in Data, but the differences are simple.  Back when people cared about kilobytes, the difference between a kilobyte, kB, (103 or 1,000 bytes) and a kibibyte, kiB, (210 or 1,024 bytes) was just 2.4%.  No big deal.  And when we moved to megabytes, it still did not matter much: 1 MB or 106 or 1,000,000 bytes was still less than 5% smaller than 220 or 1048576 bytes, one mebibyte, 1 MiB.  Then it started to matter more.  1 GB (gigabyte), 109, is almost 7% less than 230,1 GiB (gibibyte). And 1TB (terabyte), 1012, is more than 9% smaller than 1 TiB (tebibyte), 240.  People start to notice those differences.

How would they notice that difference?  With RAM, there is no difference. Memory modules are sold in power of 2 sizes, GiB sizes, and everything looks as expected.  Manufacturers of disk drives and SSDs, however, report their size in power of 10 math, while operating systems use power of 2 math.  So, a 8 TB disk in a system gets reported by the operating system as a 7.2 T disk.  Yes, that should be TiB, but you might notice that the OS probably just says G or T, and not GiB or TiB.  And of course, if the size of your volume is multiplied by using RAID over several disks or SSDs, that difference gets even more noticeable.

Some might still say, “Interesting, but who cares?”  Well, imagine that you calculated the storage requirements for your new server based on what the OS in the old one told you was the total storage and the available storage and how much you expected your requirements to increase.  For example, let us imagine that the requirement was 72T.  Now we know that what the OS reported was 72TiB.  But one might try to meet that requirement with a RAID 5 array of (10) 8TB disks.  RAID 5 would yield 9x8TB = 72TB. Perfect. Until the operating system reports that the capacity is 65.5T. 65.5TiB, that is.  Maybe that is fine, but maybe that causes some consternation.

At ion we decided long ago that, to avoid confusion, we would describe drives as their manufacturers do, in GB and TB.   When ion provides a configuration to a customer, however, we report usable RAID capacity in TiB, even when we are talking about a simple RAID1 mirror of a pair of boot drives.  For arrays of SSDs which we have over-provisioned, that usable RAID capacity will be even less.  The bottom line though, is that the operating system will report the same capacity that ion promised.

At ion, we think that is a little more honest and a lot more accurate.

Leave a Reply

Your email address will not be published.