Direct Attached Storage (DAS)


The Storage that is directly attached to PC/ Server/ Workstation is called Direct attached Storage. The main protocols used in DAS are SCSI, SAS, and Fiber Channel. Traditionally, DAS systems enable storage capacity extension for servers, while keeping high data bandwidth and access rate. Most common DAS Systems are made of one or more enclosures holding storage devices such as hard disks, and one or more controllers. The interface with the server or the workstation is made through a HBA (Host Bus Adapter).
 

Network Attached Storage (NAS)


A Storage that is connected directly to the LAN to allow common storage is called Network Attached Storage, where there are multiple users that need to share data across a network. In addition, it facilitates backup of data since the storage is centralized. Where cost is an issue, it is much cheaper to build a robust server with high levels of reliability than to submit that level of reliability and performance to all the network nodes.

Another consideration specific to storage is expandability; how we will cope with increases in storage requirements over time. Some network attached systems are great in the first year, but as needs expand, you basically have to double your initial investment to double your storage, by duplicating your initial purchase. The technology that you bought the first time does nothing for your future expansion, this is something that we tried hard to prepare for.

NAS hardware is similar to the traditional file server equipped with direct attached storage. However it differs considerably on the software side. The operating system and other software on the NAS unit provides only the functionality of data storage, data access and the management of these functionalities. Use of NAS devices for other purposes (like scientific computations or running database engine) is strongly discouraged. Many vendors also purposely make it hard to develop or install any third-party software on their NAS device by using closed source operating systems and protocol implementations. In other words, NAS devices are server appliances.

NAS provides both storage and file system. This is often contrasted with SAN (Storage Area Network), which provides only block-based storage and leaves file system concerns on the "client" side. SAN protocols are SCSI, Fibre Channel, iSCSI, ATA over Ethernet, or Hyperacid.
The boundaries between NAS and SAN systems are also starting to overlap, with some products making the obvious next evolution and offering both file level protocols (NAS) and block level protocols (SAN) from the same system. However a SAN device is usually served through NAS as one large flat file, not as a file system per se. An example of this is Open filer, a free product running on Linux.

Storage Area Network (SAN)

In computing, a Storage Area Network (SAN) is an architecture to attach remote computer storage devices (such as disk arrays, tape libraries and optical jukeboxes) to servers in such a way that, to the operating system, the devices appear as locally attached. Although cost and complexity is dropping, SANs are still uncommon outside larger enterprises.
By contrast to a SAN, Network-Attached Storage (NAS) uses file-based protocols such as NFS or SMB/CIFS where it is clear that the storage is remote, and computers request a portion of an abstract file rather than a disk block.