6.3.18 Hard Disk Storage Systems  Hard Disk Storage Systems are centralised systems that are used to maximise disk storage utilisation and to provide large capacities and/or high performance. These systems are used in conjunction with server computers so that server have only small amount of internal hard disk storage or do not have it at all. These kind of systems are often used in mid and large size environments as storage for an archiving system. Alternatively an archiving system can share a centralised storage system with a number of other computer systems. The size of a system can vary from 1 terabyte to several petabytes. It should be taken into consideration that performance characteristics of a storage system can vary notably according to its chosen configuration and it is essential that the actual needs for a system are carefully planned beforehand and a qualified professional is used to configure the storage structure and interfaces of a system to produce the best value for ones investment.  Centralised disk storage systems are designed to provide better error resilience than independent hard disk drives. These systems provide several alternative levels of RAID protection, their components can be redundant in order to avoid single point of failures, and systems can be locally or geographically distributed to protect valuable assets from different kind of failures and disasters.  The connection between the storage system and the computers it serves play important role regarding performance of a system. Generally speaking, two methods used are NAS (Network Attached Storage) and SAN (Storage Area Network).While NAS utilises regular IT network like Ethernet to move data between computer and storage system SAN uses switched Fibre Channel connections. NAS systems can operate at 100 Mbit/s, 1 Gbit/s and 10 Gbit/s speeds while SANs operate at 2 Gbit/s or 4 Gbit/s. Both technologies have clear road map to the future and their performance can be expected to grow in the future. SAN technology is usually chosen for more demanding environments since it gives better performance due to specific design. For example, the in/out (I/O) block size can be controlled more effectively in SAN environments while networking protocols tend to force NAS systems to use quite small I/O blocks. From economical point of view NAS technology is cheaper than SAN technology.