IBM N series Hardware Manual page 209

To add to the confusion, these suffixes often were applied in different ways across different
technologies. For example, network bandwidth is always decimal (100 Mbps = 100 x
10^6 bits). Memory is always binary, but is not usually shown as "GiB" (4 GB = 4 x
2^30 bytes).
Table 14-1 shows a comparison of the two measurements.
Table 14-1 Decimal versus binary measurement
Name (ISO) Suffix (ISO)
Kilobyte kB
Megabyte MB
Gigabyte GB
Terabyte TB
Petabyte PB
Some systems use a third option in which they define 1 GB as 1000 x 1024 x 1024 kilobytes.
This conversion between binary and decimal units causes most of the capacity "lost" when
calculating the correct size of capacity in an N series design. These two methods represent
the same capacity, which is similar to measuring distance in kilometers or miles but then
using the incorrect suffix.
For more information, see this website:
http://en.wikipedia.org/wiki/Gigabyte
Remember: This document uses decimal values exclusively; therefore, 1 MB =
10^6 bytes.
Raw capacity
Raw capacity is determined by taking the number of disks that are connected and multiplying
by their capacity. For example, 24 disks (the maximum in the IBM System Storage N series
disk shelves) x 2 TB per drive is a raw capacity of approximately 48,000 GB, or 48 TB.
Usable capacity
Usable capacity is determined by factoring out the portion of the raw capacity that goes to
support the infrastructure of the storage system. This capacity includes space that is used for
operating system information, disk drive formatting, file system formatting, RAID protection,
spare disk allocation, mirroring, and the Snapshot protection mechanism.
The following example is where the storage goes in the example 24 x 2 TB drive system.
Capacity often is used in the following areas:
Disk ownership: In an N series dual controller (active/active) cluster, the disks are
assigned to one or the other controller.
In the example 24 disk system, the disks are split evenly between the two controllers (12
disks each).
Spare disks: It is good practice to allocate spare disk drives to every system. These drives
are used if a disk drive fails so that the data on the failed drive can automatically be rebuilt
without any operator intervention or downtime.
Value (bytes) Approximate
difference
10^3 2%
10^6 5%
10^9 7%
10^12 9%
10^15 11%
Value (bytes) Suffix (IEC)
2^10 KiB
2^20 MiB
2^30 GiB
2^40 TiB
2^50 PiB
Chapter 14. Designing an N series solution
Name (IEC)
Kibibyte
Mebibyte
Gibibyte
Tebibyte
Pebibyte
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