Storage Hierarchy
The range of memory and storage within and attached to a computer system is known as the Storage Hierarchy and to help understand this further can be categorised into 4 segments. As memory and storage devices move down the hierarchy they reduce in performance and cost/MB or GB but tend to rise in capacity through to the last category which includes removable media which in effect has no restriction on capacity a device can store.
Primary Storage is the top level and is made up of CPU registers, CPU cache and memory which are the only components that are directly accessible to the systems CPU. The CPU can continuously read data stored in these areas and execute all instructions as required quickly in a uniform manner.
Secondary Storage differs from primary storage in that it is not directly accessible by thy CPU. A system uses input/output (I/O) channels to connect to the secondary storage which control the data flow through a system when required and on request. Secondary storage is non-volatile so does not lose data when it is powered down so consequently modern computer systems tend to have a more secondary storage than primary storage. All secondary storage today consist of hard disk drives (HDD), usually set up in a RAID configuration, however older installations also included removable media such us magneto optical or MO.
Tertiary Storage is mainly used as back up and archival of data and although based on the slowest devices can be classed as the most important in terms of data protection against a variety of disasters that can affect an IT infrastructure. Most devices in this segment are automated via robotics and software to reduce management costs and risk of human error and consist primarily of disk & tape based back up devices.
Offline Storage is the final category and is where removable types of storage media sit such as tape cartridges and optical disc such as CD and DVD. Offline storage is can be used to transfer data between systems but also allow for data to be secured offsite to ensure companies always have a copy of valuable data in the event of a disaster.
