Data storage is usually categorized as primary storage or secondary storage with primary storage being available to the computer or server's processor and secondary storage is usually not available by the CPU and is used to store everything from the operating system and the Application to Personal User Data application. It is a nonvolatile memory and maintains its state even after the power.
The main storage area in modern computers available from the CPU is usually called RAM or Random Access Memory which is a volatile memory used to store instructions from the CPU. This storage type often called Memory is volatile and cleans every time the computer is turned off. Most computers and servers now use a form of RAM called DRAM Dynamic RAM where the computer uses input output channels to access memory.
Another storage space in a computer is known as ROM Read Only Memory which is primarily used to store a bootstrap file commonly referred to as the Basic Input Output System BIOS used to boot the computer.
The rest of this article will focus on storing operating systems and user applications and data under the Direct Attached Storage DAS NAS Network Attached Storage NAS and SAN Storage Area Network with Fiber Channel.
Direct Attached Storage refers to a storage medium available from the computer or server or from a peripheral computer such as a keyboard. Data passes through the CPU memory and then a bus on the storage medium e.g. a hard drive. A RAID controller can be used to organize data on multiple hard disk drives and provide redundancy and robustness. Files sent to secondary storage systems must be organized to facilitate reading and writing so a formal file system is required to do this. Data is organized into folders and files to make the location of data easy. Further storage can be provided in the form of flash drives magnetic tape or ZIP devices.
Networkconnected storage or NAS as known is a method of storing data over a data network where the server communicates with the NAS server located remotely within a TCP IP network. The server that can be Windows Unix or Linux based is designed to host common file sharing protocols such as NFS or CIFS. A later development is the use of Appliance servers that have a very basic operating system designed to support file sharing protocols.
SCSI commands that are block level commands must be converted to file protocol level to be transmitted over TCP IP networks and reconverted to block level SCSI commands again when received by the NAS server.
The server only needs a Network Interface Card NIC to communicate with the NAS server via the TCP IP network which also needs an NIC. NAS is a very common storage option used by small businesses and organizations as well as larger corporate networks. One of the main problems with NAS is the big deal at all when it comes to protocol conversions that must happen when data is sent and received. Fast network connections are required if data is to be transmitted within acceptable time limits and Gigabit Ethernet or even 10 Gigabit Ethernet provides the performance needed for reliable fast reading and data writing.
An alternative to standard Network Connected Storage is iSCSI which is a method for encapsulating standard SCSI commands within TCP IP protocol eliminating the need for file protocol conversion. iSCSI was the precursor of genuine storage networks by expanding the local bus through a network connection. In order for iSCSI to work efficiently it is advisable to run it over a dedicated network or subnet.
True modern storage space networks use a dedicated network running Fiber Channel technology. Fiber Channel Networks are designed specifically for transfer and recovery of data from storage servers. Resistant power can be provided by means of redundant paths to and from the servers ensuring that there is no single point in error scenario. The main benefits of Fiber Channel are:
The data transfer rate which is 2 Gigabit with Fiber Channel.
The number of devices that can be connected within the network is 127 which is significantly more than SCSI.
The distance between the servers and storage spaces can be as much as 10km which provides a lot of flexibility and capabilities when deciding where to place the servers and storage facilities.
Fiber Channel switch allows hundreds of disks to be connected in multiple arrays or clusters.