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IP storage refers to technology for transporting block-mode data across IP/Ethernet networks. Block-mode is the raw mechanism used by SCSI and other disk drivers to directly access data on disks. Most applications go through higher-level file access protocols such as NFS, CIFS, and FTP to access disk information. However, file-mode access is slow and requires many operations compared to block-mode access. Block mode is usually performed between a computer and its directly attached storage devices and is the preferred access method for database applications.

The goal of IP storage is to run block-mode data calls over networks. Doing so can reduce complex file-mode data access, improve disk performance over existing networks, and reduce the need for secondary storage networks such as SANs (storage area networks). Many NAS (network attached storage) devices also work at the file level using protocols such as NFS and CIFS. The performance of these devices improves with support for block-mode data calls over existing networks.

The IETF IP Storage (ips) Working Group is developing standards that will enable block data calls over existing IP networks. The technology encompasses disk, tape, and optical storage devices. The logical terminology for IP storage is "storage over IP," but this terminology and its acronym "SoIP" are a trademark of Nishan Systems. CNT Corporation refers to its IP storage technology as "SAN over IP." Consequently, the IETF refers to the technology as "IP Storage."

Currently, SANs implement Fibre Channel to interconnect storage devices in a separate LAN that is accessible to users, typically in an enterprise environment. However, Fibre Channel SAN solutions are difficult to install and manage, and organizations with high-speed networks see no reason to build a secondary network for storage if the existing network has the bandwidth to support high-performance network-attached storage. IP storage outlines how to build inexpensive IP-based storage networks over existing infrastructure, preferably Gigabit Ethernet networks.

Gigabit Ethernet and 10-Gigabit Ethernet are now being used in metropolitan area access networks, which means that IP storage even becomes practical in the MAN and WAN in some cases. This allows remote disk access, synchronous and asynchronous remote mirroring, and remote backup and restore (tape vaulting). The protocols will ensure data reliability, cope with network congestion, automatically adapt retransmission strategies to WAN delays, and attempt to avoid the need for protocol conversion.

Proposals submitted to the IETF IP Storage Working Group fall into two categories:

• Fibre Channel Tunneling    Provides a way to interconnect two Fibre Channel SANs across an IP network by tunneling Fibre Channel controls and data across the IP network.


• iSCSI    This approach encapsulated SCSI controls and data inside IP packets, enabling the construction of inexpensive network-based SANs.

Adaptec has developed SCSI over IP solutions called EtherStorage. The company notes that IP storage technologies work well over high-bandwidth, low-latency switched Ethernet networks in computer room, LAN, and campus environments, but not over WANs, where latency is unpredictable. The company notes that "some applications will not be able to work efficiently with the unpredictable performance and latency of WAN storage configurations, but other applications (such as remote backups) will be able to use IP networks across WANs to provide valuable storage services."

A related technology is DAFS (Direct Access File System), which supports block-level data transfers between a client and a storage device over VI Architecture networks. With DAFS, a client can gain direct access to a disk and transfer data from it directly into local memory without the need to copy data to or from intermediate buffers or to interrupt the operating system during file transfers. See "DAFS (Direct Access File System)" and "VI Architecture."

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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