While a single computer can hold data in electromagnetic form via random access memory (RAM), and other built-in devices, IT professionals are mainly concerned with larger capacity external storage devices and data retention methods often called secondary storage.

There are a variety of types of secondary storage systems. Rather than using electronic circuits, these systems typically use magnetic or optical means to retain data for some period of time. They have higher capacity and are slower. Mutable storage is the term for data that can be overwritten at any time. Immutable storage cannot be overwritten, but can be changed.

There are several kinds of data access employed. Block access, for instance, means that a disk is divided into fairly similarly sized chunks of data that are accessed at random. File access means there is a system of particular files and directories, which can be used to refer to storage content.

In larger companies, the storage architecture is often composed of several, linked types of storage hardware. These are typically classified as Direct Attached Storage (DAS), Network Attached Storage (NAS), or Storage Area Networks (SANs).

DAS

These more basic secondary storage devices are directly connected to a host computer or server. For instance, disk drives for disk backups, RAID arrays, and tape libraries for tape backups are DAS systems, usually connected by standard protocols like small computer system interface (SCSI). The numerous variations of SCSI developed by vendors create numerous component-driven storage standards. Data retrieval is at the block level. DAS systems are used for local file sharing.

RAID is an acronym for Redundant Arrays of Inexpensive Disks, coined in an era of Single Large Expensive Drives (SLED) to basically mean "Don't put your eggs in one basket." (These days, with change and perhaps inflation, it typically is known as a Redundant Array of Independent Disks.) There are several types of RAID, but the concept is the same: an array of disks composed of various sized stripes of usually redundant data. This increases speed and fault tolerance.

NAS

NAS is composed of both hard disks and management software, and is completely dedicated to serving files from a company network running a Gigabit Ethernet. It is based on standard network protocols such as TCP/IP, FC, and CIFS. NAS systems typically consist of RAID systems and software for configuring and mapping file locations to a network-attached device. Storage is shared across multiple servers.

SAN

A storage area network, or SAN, is a highly scalable, dedicated, high-speed storage network of devices for transferring large blocks of data securely among servers, networking components, and storage devices. It is separate from the corporate local area network (LAN).

In a SAN infrastructure, storage devices such as NAS, DAS, RAID arrays, or others are connected to servers using highly reliable interconnect technology called Fibre Channel or, in the case of IP SANs, Internet Protocol. Serial ATA and Serial Attached SCSI interfaces are also making headway with SANs.

DAS, NAS, and SAN all offer benefits, but each is best suited for a particular environment.

For more information on choosing the right solution for your company, please read our Storage First Steps.

Go to Bitpipe Research Guide: Storage.