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In computing, a storage area network (SAN) is a network designed to attach computer storage devices such as disk array controllers and tape libraries to servers. As of 2006, SANs are most commonly found in enterprise storage. A SAN allows a machine to connect to remote targets such as disks and tape drives on a network for block level I/O. From the point of view of the class drivers and application software, the devices appear as locally attached devices. There are two variations of SANs:
Defining SAN Storage networks are distinguished from other forms of network storage by the low-level access method that they use. Data traffic on these networks is very similar to those used for internal disk drives, like ATA and SCSI. In a storage network, a server issues a request for specific blocks, or data segments, from specific disk drives. This method is known as block storage. The device acts in a similar fashion to an internal drive, accessing the specified block, and sending the response across the network. In more traditional file storage access methods, like SMB/CIFS or NFS, a server issues a request for an abstract file as a component of a larger file system, managed by an intermediary computer. The intermediary then determines the physical location of the abstract resource, accesses it on one of its internal drives, and sends the complete file across the network. Most storage networks use the SCSI protocol for communication between servers and devices, though they do not use its low-level physical interface. Typical SAN physical interfaces include 1Gbit Fibre Channel, 2Gbit Fibre Channel, 4Gbit Fibre Channel, and (in limited cases) 1Gbit iSCSI. The SCSI protocol information will be carried over the lower level protocol via a mapping layer. For example, most SANs in production today use some form of SCSI over Fibre Channel system, as defined by the "FCP" mapping standard. iSCSI is a similar mapping method designed to carry SCSI information over IP. Benefits Sharing storage usually simplifies storage administration and adds flexibility since cables and storage devices do not have to be physically moved to move storage from one server to another. Note, though, that with the exception of SAN file systems and clustered computing, SAN storage is still a one-to-one relationship. That is, each device (or Logical Unit Number (LUN)) on the SAN is "owned" by a single computer (or initiator). In contrast, Network Attached Storage (NAS) allows many computers to access the same set of files over a network. It is now possible to combine the SAN and NAS using a NAS head. SANs tend to increase storage capacity utilization, since multiple servers can share the same growth reserve. 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 idea being pioneered in newer data centers. There are a number of emerging products designed to facilitate and speed up this process still further. For example, Brocade Communication Systems 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 the enterprise datacenter. SANs also tend to enable more effective disaster recovery processes. A SAN attached storage array can replicate data belonging to many servers to a secondary storage array. This secondary array can be local or, more typically, remote. The goal of disaster recovery is to place copies of data outside the radius of effect of an anticipated threat, and so the long-distance transport capabilities of SAN protocols such as Fibre Channel and FCIP are required to support these solutions. (The physical layer options for the traditional direct-attached SCSI model could only support a few meters of distance: not nearly enough to ensure business continuance in a disaster.) Demand for this SAN application has increased dramatically after the September 11th attacks in the United States, and increased regulatory requirements associated with Sarbanes-Oxley and similar legislation. Newer SANs allow duplication functionality such as "cloning" and "snapshotting," which allows for real-time duplication of LUN, for the purposes of backup, disaster recovery, or system duplication. With higher-end database systems, this can occur without downtime, and is geographically independent, primarily being limited by available bandwidth and storage. Cloning creates a complete replica of the LUN in the background (consuming I/O resources in the process), while snapshotting stores only the original states of any blocks that get changed after the "snapshot" (also known as the delta blocks) from the original LUN, and does not significantly slow the system. In time, however, snapshots can grow to be as large as the original system, and are normally only recommended for temporary storage. The two types of duplication are otherwise identical, and a cloned or snapshotted LUN can be mounted on another system for execution, or backup to tape or other device, or for replication to a distant point. Disk controllers The driving force for the SAN market in the enterprise space is rapid growth of highly transactional data that require high speed block level access to the hard drives (such as data from email servers, databases, and high usage file servers). Historically, enterprises would have "islands" of high performance SCSI storage RAIDs that were locally attached to each application server. These "islands" would be backed up over the network, and when the application data exceeded the maximum amount of data storable by the individual server, the end user would often have to upgrade their server to keep up. The disk controllers used in enterprise SAN environments are designed to provide applications with block level access to high speed, reliable "virtual hard drives" (or LUNs). In addition, modern SANs allow enterprises to intermix FC SATA drives with their FC SCSI drives. SATA drives have lower performance, a higher failure rate, higher capacity, and lower prices than SCSI. This allows enterprises to have multiple tiers of data that will migrate over time to different types of media. For example: many enterprises relegate files that are rarely accessed to FC SATA while keeping their frequently used data in FC SCSI. Another feature of most enterprise disk controllers is a I/O cache. This feature allows higher overall performance for writing to the controller, and in some cases (like for contiguous file access where read ahead is enabled) reading from the controller. SAN types SANs are normally built on an infrastructure specially designed to handle storage communications. Thus, they tend to provide faster and more reliable access than higher level protocols such as NAS. The most common SAN technology by far is Fibre Channel networking with the SCSI command set. A typical Fibre Channel SAN is made up of a number of Fibre Channel switches which are connected together to form a fabric. A fabric is similar in concept to a segment in a local area network. Today, all major SAN equipment vendors also offer some form of Fibre Channel routing solution, and these bring substantial scalability benefits to the SAN architecture by allowing data to cross between different fabrics without merging them. However, most of these offerings use proprietary protocol elements, and the top-level architectures being promoted are radically different. When extending Fibre Channel over long distances for disaster recovery solutions, it can be mapped over other protocols. For example, products exist to map Fibre Channel over IP (FCIP) and over SONET/SDH. It can also be extended natively using signal repeaters, high-power laser media, or multiplexers such as DWDMs. An alternative SAN protocol is iSCSI which uses the same SCSI command set over TCP/IP (and, typically, Ethernet). In this case, the switches would be Ethernet switches. The iSCSI standard was ratified in 2003, so it has not yet had time to gather broad industry support. Fibre Channel has existed in production environments for over a decade and has already been widely deployed as strategic network infrastructure, so it will take iSCSI quite some time to make significant inroads into the installed-base of Fibre Channel SANs. It also underperforms Fibre Channel significantly, and may not be suitable for enterprise deployments. As a result, iSCSI is generally seen as being more of a competitor to NAS protocols such as CIFS and NFS. Another alternative to iSCSI is the ATA-over-Ethernet or AoE protocol which embeds the ATA protocol inside of raw Ethernet frames. While a raw Ethernet protocol like AoE cannot be routed without something else performing the encapsulation, it does provide a simple discovery model with low overhead. Connected to the SAN will be one or more servers (hosts) and one or more disk arrays, tape libraries, or other storage devices. In the case of a Fibre Channel SAN, the servers would use special Fibre Channel host bus adapters (HBAs) and optical fiber. iSCSI SANs would normally use Ethernet network interface cards, and often specialized TOE cards. Storage area networks are of two kinds - centralized storage area networks and distributed storage area networks Compatibility One of the early problems with Fibre Channel SANs was that the switches and other hardware from different manufacturers were not entirely compatible. Although the basic storage protocols (such as FCP) were always quite standard, some of the higher-level functions did not interoperate well. Similarly, many host operating systems would react badly to other OSes sharing the same fabric. Many systems were pushed to the market before standards were finalized and vendors innovated around the standards. The combined efforts of the members of the Storage Networking Industry Association (SNIA) improved the situation during 2002 and 2003. Today most vendor devices, from HBAs to switches and arrays, interoperate nicely, though there are still many high-level functions that do not work between different manufacturers' hardware. While this work is substantially completed for Fibre Channel, the process has only just begun for some other SAN protocols such as iSCSI. Interoperability at the IP layer is not a problem, but higher layer functions still need substantial integration work, and this is likely to take years. SANs at work SANs are primarily used in large scale, high performance enterprise storage operations. It would be unusual to find a Fibre Channel disk drive connected directly to a SAN. Instead, SANs are normally networks of large disk arrays. SAN equipment is relatively expensive, therefore, Fibre Channel host bus adapters are rare in desktop computers. The iSCSI SAN technology is expected to eventually produce cheap SANs, but it is unlikely that this technology will be used outside the enterprise data center environment. Desktop clients are expected to continue using NAS protocols such as CIFS and NFS. The exception to this may be remote replication sites. Remote replication enables the data center environment to exist in multiple locations for disaster recovery and business continuity purposes. The performance issues inherent in iSCSI are likely to limit its deployment to lower-tier applications, with Fibre Channel remaining incumbent for high performance systems. SANs in a Small Office / Home Office (SOHO) With the increasing rise of digital media in all phases of life and its effect on storage needs, it's natural that SANs have begun to enter into the SOHO market. Historically, this market was dominated by NAS systems, but SOHO is poised to become a major market for SAN infrastructure as SOHO performance requirements rise. Systems such as film scanners and video editing applications require performance that cannot be provided by traditional file servers. For example, motion picture film at 2048x1556 requires more than 300MBytes/s for each real-time stream, and several of these streams can be required simultaneously. As a result, several Gigabits per second can be required, which creates a problem for standard NAS technologies. In addition, these systems need to work with the same files collaboratively, so they cannot be distributed through different file servers or DAS connections. Instead of having many computers connected to the network, with each one requiring a low bandwidth and only the server being stressed under heavy traffic, the SOHO "real-time" area only needs to integrate a few systems, but all of them require high bandwidth to access to the same files. These problems are addressed very well by 4Gbit Fibre Channel SAN infrastructures, where the aggregated bandwidth for sequential I/O operations is extremely high. See also | ||||||||
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