DAS vs NAS vs SAN
Historically, data centers first created "islands" of SCSI disk arrays as direct-attached storage (DAS), each dedicated to an application, and visible as a number of "virtualhard drives" (i.e. LUNs). Essentially, a SAN consolidates such storage islands together using a high-speed network.
Operating systems maintain their own file systems on them on dedicated, non-shared LUNS, as though they were local to themselves. If multiple systems were simply to attempt to share a LUN, these would interfere with each other and quickly corrupt the data. Any planned sharing ofdata on different computers within a LUN requires advanced solutions, such as SAN file systems or clustered computing.
Despite such issues, SANs help to increase storage capacity utilization, since multiple servers consolidate their private storage space onto the disk arrays.
Common uses of a SAN include provision of transactionally accessed data that require high-speed block-level access tothe hard drives such as email servers, databases, and high usage file servers.
SAN and NAS
In contrast to 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. Recently,[___when?___] the introduction of NAS heads[clarification needed] hasallowed easy conversion of SAN storage to NAS.
[edit*] SAN-NAS *hybrid
Hybrid using DAS, NAS and SAN technologies.
Despite the differences between NAS and SAN, it is possible to create solutions that include both technologies, as shown in the diagram.
Sharing storage usually simplifies storage administration and adds flexibility since cables and storage devices donot have to be physically moved to shift storage from one server to another.
Other benefits include the ability to allow servers to boot from the SAN itself. This allows for a quick and easy replacement of faulty servers since the SAN can be reconfigured so that a replacement server can use the LUN of the faulty server. This process can take as little as half an hour and is a relatively new ideabeing pioneered in newer data centers. There are a number of emerging products designed to facilitate and speed this up still further. Brocade, for example, offers an Application Resource Manager product which automatically provisions servers to boot off a SAN, with typical-case load times measured in minutes. While this area of technology is still new many view it as being the future of theenterprise datacenter .
SANs also tend to enable more effective disaster recovery processes. A SAN could span a distant location containing a secondary storage array. This enables storage replication either implemented by disk array controllers, by server software, or by specialized SAN devices. Since IP WANs are often the least costly method of long-distance transport, the Fibre Channel over IP(FCIP) and iSCSI protocols have been developed to allow SAN extension over IP networks. The traditional physical SCSI layer could only support a few meters of distance - not nearly enough to ensure business continuance in a disaster.
The economic consolidation of disk arrays has accelerated the advancement of several features including I/O caching, snapshotting, and volume cloning (Business...