So how big is that disk?

I’m doing a fair bit of SAN-based work at the moment, migrating a bunch of Oracle databases from EMC to HP (posts to follow regarding Orion testing and other related topics). One thing that annoys me is the way SAN Manufacturers have changed the meaning of technical terms over time to suit themselves. Firstly, they changed the meaning of RAID from “Redundant Array of Inexpensive Disk” to “Redundant Array of Independent Disk”. Apparently it’s hard to put a $1,000,000 price tag on “inexpensive” disk.

Also, the industry changed what a Megabyte is. This really annoys me as it can lead to unexpected shortfalls in the LUN allocations if your traditional Oracle Megabyte is different to the SAN Admin’s Megabyte.

In our industry, the terms “kilo”, “mega”, “giga“, “tera”, “peta”, and “exa” are commonly used prefixes for computing performance and capacity. SAN manufacturers use the terms defined in “powers of ten” measurement units:

  • A kilobyte (KB) is equal to 1,000 (103) bytes.
  • A megabyte (MB) is equal to 1,000,000 (106) bytes.
  • A gigabyte (GB) is equal to 1,000,000,000 (109) bytes.
  • A terabyte (TB) is equal to 1,000,000,000,000 (1012) bytes.
  • A petabyte (PB) is equal to 1,000,000,000,000,000 (1015) bytes
  • An exabyte (EB) is equal to 1,000,000,000,000,000,000 (1018) bytes

Most operating systems components and (importantly for us) Oracle use “powers of two” measurement units rather than “power of ten” units. They are defined as:

  • A kibibyte (KiB) is equal to 1,024 (210) bytes.
  • A mebibyte (MiB) is equal to 1,048,576 (220) bytes.
  • A gibibyte (GiB) is equal to 1,073,741,824 (230) bytes.
  • A tebibyte (TiB) is equal to 1,099,511,627,776 (240) bytes.
  • A pebibyte (PiB) is equal to 1,125,899,906,842,624 (250) bytes.
  • An exbibyte (EiB) is equal to 1,152,921,504,606,846,976 (260) bytes.

That means that 1 terabyte is about 9% smaller than a tebibyte. Thanks, SAN manufacturers, for making our lives just a tiny bit more difficult than it needed to be so you can market drives as being seemingly larger than they really are (in my head anyway.)

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