<p class="MsoNormal" style="margin: 0in 0in 10pt;"><span style="font-size: small;"><span style="font-family: Calibri;">Historically, data centers first created "islands" of SCSI disk arrays. Each island was dedicated to an application and visible as a number of "virtual hard drives" (i.e. LUNs). Essentially, a SAN connects storage islands together using a high-speed network, thus allowing all applications to access all disks.</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 10pt;"><span style="font-size: small;"><span style="font-family: Calibri;"><!--more--></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 10pt;"><span style="font-size: small;"><span style="font-family: Calibri;">Operating systems still view a SAN as a collection of LUNs, and usually maintain their own file systems on them. These local file systems, which cannot be shared among multiple operating systems/hosts, are the most reliable and most widely used. If two independent local file systems resided on a shared LUN, they would be unaware of this fact, would have no means of cache synchronization and eventually would corrupt each other. Thus, sharing data between computers through a SAN requires advanced solutions, such as SAN file systems or clustered computing. Despite such issues, SANs help to increase storage capacity utilization, since multiple servers share the storage space on the disk arrays. The common application of a SAN is for the use of transactional data that requires high-speed block-level access to the hard drives such as email servers, databases, and high usage file servers.</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 10pt;"><span style="font-size: small;"><span style="font-family: Calibri;">Network Attached Storage (NAS) allows many computers to access the same file system over the network and synchronizes their accesses. With the introduction of NAS heads or gateways, this has allowed easy conversion of SAN storage to NAS.</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 10pt;"><span style="font-size: small;"><span style="font-family: Calibri;">Shared storage usually simplifies storage administration and adds flexibility since cables and storage devices do not have to be physically moved to shift storage from one server to another.</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 10pt;"><span style="font-size: small;"><span style="font-family: Calibri;">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.<span style="mso-spacerun: yes;">  </span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 10pt;"><span style="font-size: small; font-family: Calibri;">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. </span></p>