IBM N Series Hardware Manual page 192

For more information, see the following website:
http://en.wikipedia.org/wiki/Gigabyte
Remember: This document uses decimal values exclusively, so 1 MB = 10^6 bytes.
Raw capacity
Raw capacity is determined by taking the number of disks connected and multiplying by their
capacity. For example, 24 disks (the maximum in the IBM System Storage N series disk
shelves) times 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 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 would go in the example 24 x 2 TB drive system.
Capacity usually gets 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.
The minimum acceptable practice would be to allocate one spare drive, per drive type, per
controller head. In the example, that would be two disks because it is a two-node cluster.
RAID: When a drive fails, it is the RAID information that allows the lost data to be
recovered.
– RAID-4: Protects against a single disk failure in any RAID group, and requires that one
disk is reserved for RAID parity information (not user data).
Because disk capacities have increased greatly over time, with a corresponding
increase in the risk of an error during the RAID rebuild, do not use RAID-4 for
production use.
The remaining 11 drives (per controller), divided into 2 x RAID-4 groups, require two
disks to be reserved for RAID-4 parity, per controller.
– RAID-DP: Protects against a double disk failure in any RAID group, and requires that
two disks be reserved for RAID parity information (not user data).
With the IBM System Storage N series, the maximum protection against loss is
provided by using the RAID-DP facility. RAID-DP has many thousands of times better
availability than traditional RAID-4 (or RAID-5), often for little or no additional capacity.
The remaining 11 drives (per controller), allocated to 1 x RAID-DP group, require two
disks to be reserved for RAID-DP parity, per controller.
The RAID groups are combined to create storage aggregates that then have volumes
(also called
Normal practice would be to treat the nine remaining disks (per controller) as data disks,
thus creating a single large aggregate on each controller.
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IBM System Storage N series Hardware Guide
file systems ) or LUNs allocated on them.