Data Storage Glossary of Terms
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Definitions from Online Capacity Expansion to Redundancy

Online Capacity Expansion (OLCE)
The ability to add additional storage media, in the form of hard drives or external devices to a server, without the need to disconnect or disrupt any user’s current access to their data.
Operating System (OS)
The base software that all other software on a computer / server runs off. It controls access to input and output devices, programs and provides a single method of control. This allows a user to interact with the hardware to access their data and run programs.
Petabyte (PB)
A unit of measure approximately equal to (1,000 terabytes).
Platform
A base of the system comprised of hardware and software, onto which other functions / programs are built or installed.
Point-in-Time Copy
A full copy or a backup of all data, available for restoration on a system from a particular date and time.
Rack-Dense/Rack Density
The amount of space taken up by a server or other device in a server cabinet. Usually measured in U, e.g. a server being 2U high.
RAID – (Redundant Array of Independent Disks)

A method of splitting up data across a set of hard disks, allowing for faster access, more resiliency or a combination of both.

* RAID 0 – Data is written / read across two or more drives simultaneously, effectively allowing it to be accessed much more quickly. It allows all the hard drives in the RAID array to appear as a single device with the total combined storage of all the drives. E.g. two hard drive of 256GB and 512GB, would appear as a single 768GB drive.
This form of RAID is sometimes called striping and does not provide any extra redundancy than storing your data to a single hard drive. If one drive fails and is then replaced, the other can not rebuild the whole data file.

* RAID 1 - Two or more disks are used to store the data as exact images of the other. As data is being written or read from one disk the RAID controller “mirrors” this data to the other drive/s. This provides a high level of redundancy as if one drive fails the other has a full working set of data. However, it means that you would only be able to have as much storage space available as the size of the smallest disk in the RAID array. E.g. two hard drive of 256GB and 512GB, would appear as a single 256GB drive.

* RAID 4 - Data is striped at a block level across several drives, with parity stored on one drive. The parity information allows recovery from the failure of a single drive. The performance of RAID 4 is very good for reads. Writes, however, require that parity data be upgraded each time. This slows down random writes in particular, though large writes or sequential writes are fairly fast.

* RAID 5 - Employs data striping and parity across all drives in the array creating better performance and security. Since parity information is striped across all drives, lost data can be retrieved and rebuilt from the parity.

* RAID 10 - A combination of RAID 0 and RAID 1, data is distributed across multiple drives without parity, and then the entire array is mirrored. Although this delivers good performance, the drive storage overhead is 50% because you are mirroring the data.

Redundancy
The duplication of information or a hardware component to ensure that should the primary resource fail, a secondary resource can take over its function.
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