iSCSI for Storage Networking




In a world where Internet Protocol (IP) dominates local and wide area networks, and data storage requirements grow unabated, it seems inevitable that these two forces converge. The Internet Small Computer Systems Interface (iSCSI) protocol unites storage and IP networking. iSCSI enables the transport of block-level storage traffic over IP networks. It builds on two widely used technologies ó SCSI commands for storage and IP protocols for networking. iSCSI is an end-to-end protocol for transporting storage I/O block data over an IP network. The protocol is used on servers (initiators), storage devices (targets), and protocol transfer gateway devices. iSCSI uses standard Ethernet switches and routers to move the data from server to storage. It also enables IP and Ethernet infrastructure to be used for expanding access to SAN storage and extending SAN connectivity across any distance.

Factors Enabling IP Storage

To date, SAN deployments have been driven by an insatiable demand for storage and the user benefits delivered by networked storage. These benefits include efficient storage utilization through storage consolidation, the ability to manage more storage capacity, rapid deployment of new storage, higher availability, and faster backup and restore operations. With the recent development of the iSCSI protocol and silicon-based TCP/IP offload engines, SANs based on IP networks are now possible. The IP networking infrastructure includes multi-gigabit networks, sophisticated bandwidth allocation and network management tools, and the ubiquitous reach of IP and Ethernet. These factors together enable new IP Storage solutions, using the iSCSI protocol.
Fig1 Factors Enabling IP Storage
With IP network bandwidth expanding, storage requirements growing, and the advent of iSCSI, networking and storage will converge.

Leveraging the Best from Storage AND Networking

iSCSI builds on the two most widely used protocols from the storage and the networking worlds. From the storage side, iSCSI uses the SCSI command set, the core storage commands used throughout all storage configurations.

On the networking side, iSCSI uses IP and Ethernet, which are the basis for most corporate networks, and are increasingly being used for metropolitan and wide area networking as well. With almost 30 years of research, development and integration, IP networks provide the utmost in manageability, interoperability and cost-effectiveness.

Figure 2 shows how SCSI is mapped to TCP/IP through the iSCSI layer, freeing SCSI from its parallel bus structure.
Fig2 Leveraging SCSI & IP for Storage Networking
Figure 3 shows a simplified version of a protocol stack including iSCSI. Use of the standard SCSI command set facilitates interoperability with existing operating systems and applications that lay above that layer. Additionally, use of a standard TCP/IP network provides universal reach to the global IP infrastructure.
Fig3 The iSCSI Protocol Stack

Advantages of IP Storage

  • Familiar network technology and management
  • Reduces training and staff costs
  • Proven transport infrastructure
  • Increases reliability
  • Transition from 1 Gigabit Ethernet to 10-Gigabit Ethernet and Beyond
  • Protects investment with simplified performance upgrades
  • Scalability over long distances
  • Enables remote data replication and disaster recovery
  • Brings Ethernet economics to storage
  • Enables lower total cost of ownership

Elements of IP Storage Networking

IP Storage Networking refers to computer systems and storage elements that are connected via IP. It also refers to the IP infrastructure transporting storage traffic among these targets. Figure 4 shows the various elements of an IP storage network.

The first element, Device I/O, refers to computer systems and storage resources that have native IP interfaces. These could include servers, disk arrays or tape libraries with an iSCSI adapter or iSCSI controller. These interfaces typically are Ethernet and include protocol processing, such as TCP/IP offload engines, to reduce processing loads on the host devices.

The second element of an iSCSI storage area network is the fabric of switches. The advantage of an IP based fabric is that users can create a SAN using standard Ethernet switches and routers to transport data over the SAN fabric. The fabric also may include storage routers and switches that have a combination of iSCSI interfaces and other storage interfaces such as SCSI or Fibre Channel. Storage switches and routers provide multi-protocol connectivity not available in conventional IP and Ethernet switches. They also provide storage specific functionality, such as peer-to-peer copy commands.

The third element of IP Storage networking is the SAN interconnection. Since iSCSI is a native IP-based protocol, SAN interconnection does not require storage-specific functionality and can use a shared or dedicated IP/Ethernet network.

Fig4 Segments of Storage Networking

Understanding End Systems

Given that an iSCSI environment can leverage existing IP networks, a significant portion of the overall solution involves iSCSI end systems. Figure 5 outlines the significant functions of traditional IP Network Interface Cards, traditional Storage Adapters, and new IP Storage Adapters.

Network Interface Cards (NICs)

Traditional NICs (Ethernet adapters in servers and PCs) are designed to transfer packetized file level data among PCs, servers and storage devices, such as NAS appliances. However, NICs do not traditionally transfer block level data, which is handled by a storage host bus adapter, such as Fibre Channel or parallel SCSI. In order for a NIC to process block level data the data needs to be placed into a TCP/IP packet before being sent over the IP network. Through the use of iSCSI drivers on the host or server, a NIC can transmit packets of block level data over an IP network. When using a NIC, the server handles the packet creation of block level data and performs all of the TCP/IP processing. This is extremely CPU intensive and lowers the overall server performance. The TCP/IP processing performance bottleneck has been the driving force behind the development of TCP/IP offload engines (TOE) on adapter cards. A TOE removes the TCP/IP processing from the host CPU and completes TCP/IP processing and packet creation on the HBA. Thus a TCP/IP offload storage NIC operates more like a storage HBA rather than a standard NIC.

Storage Host Bus Adapters (HBAs)

Unlike NICs, Storage HBAs are designed to transmit block level data to and from storage applications. A reference to the entire block is transferred from the application to the adapter, bypassing the need to break the block into smaller frames. However, because of this, the HBA must handle the task of segmenting the block into frames, a process that takes place through a set of specialized chips that enable the HBA to relieve the computer resources of the CPU for this process. When it completes that task, the HBA forwards the frames.

iSCSI Adapters
iSCSI Adapters combine the functions of NICs with the function of a storage HBA. These adapters take the data in block form, handle the segmentation and processing on the adapter card with TCP/IP processing engines, and then send the IP packets across an IP network. The implementation of these functions enables users to create an IP based SAN without lowering the performance of the server. In advance of the introduction of iSCSI adapters, some vendors released software versions of these adapters. These software-enabled adapters accept block level data from applications, but still require CPU cycles for the TCP/IP processing. The advantage of such adapters is that they can work on existing Ethernet NICs. The main disadvantage is that they require heavy CPU utilization for TCP/IP processing.


Deployment Scenarios for iSCSI

Building New Native iSCSI Storage Networks
Data Center Approach
An iSCSI SAN is a perfect choice for a user interested in moving to networked storage. Using the same block level SCSI commands as direct attach storage, iSCSI provides compatibility with user applications such as file systems, databases, and web serving. Similarly, since iSCSI runs on ubiquitous and familiar IP networks, there is no need to learn a new networking infrastructure to realize SAN benefits. To build an iSCSI storage network in a data center, iSCSI host bus adapters can be used in servers, along with iSCSI storage devices and a combination of IP and Ethernet switches. IP Storage switches and routers can be used if required. Figure 6 shows a primary data center iSCSI storage network connecting to remote sites.
Expanding iSCSI Storage Networks to Metro and Wide Area Networks
The rapid adoption and expansion of IP data on the Internet has proven the viability of using IP across long distance wide area networks. Although it is expected that initial deployments of iSCSI will use private networks, the use of IPís security infrastructure (such as IPSec and SSL to provide authentication and privacy) it will be possible to use public networks for wide area iSCSI storage traffic as well.
Fig6 Data Center iSCSI Storage Networks Offer Seamless MAN/WAN Access

Applications for Native iSCSI Storage Networks

Building New Native iSCSI Storage Networks
With a native iSCSI storage network, customers can benefit from the following applications in the data center:
- Server and storage consolidation
With a networked storage infrastructure, customers can link multiple storage devices to multiple servers allowing for better resource utilization, ease of storage management, and simpler expansion of the storage infrastructure.
- Accelerated Backup Operations
Backup operations previously restricted to operating across traditional IP LANs at the file level can now operate across IP Storage networks at the block level. This shift facilitates faster backup times, and provides customers the flexibility to use shared or dedicated IP networks for storage operations.
- Seamless Remote Site Access and Storage Outsourcing
With the storage network based on IP, customers can easily enable remote access to secondary sites across metropolitan or wide area IP networks. The remote sites can be used for off-site backup, clustering or mirroring/replication. Additionally, customers can choose to link to storage service providers for storage outsourcing applications such as storage-on-demand.

Linking SANs with iSCSI

The iSCSI protocol is ideal for linking SANs over MAN and WAN environments since it uses TCP/IP as the transport. This includes extending native iSCSI SANs and Fibre Channel SANs across a wide area network. iSCSI SANs can be connected over a wide area network with standard Ethernet equipment. When connecting to Fibre Channel SANs, an IP Storage Switch is needed to convert the FC protocol to iSCSI.

IP storage routers can bridge FC to iSCSI, while IP storage switches bridge FC to iSCSI and provide added switching functionality. That functionality, when provided in an IP Storage switch, allows users to perform FC-to-FC switching, FC-to-iSCSI switching, or FC-to-Gigabit Ethernet switching and Gigabit Ethernet to Gigabit Ethernet switching. Both IP storage routers and switches allow users to extend the reach of the FC SAN and bridge FC SANs to iSCSI SANs. Figure 7 shows a sample configuration based on IP Storage routers and switches.
Fig7 Linking iSCSI to Fibre Channel SANs

Applications for linking iSCSI to Fibre Channel

IP Access to Storage / Storage Consolidation
Using iSCSI Adapters in servers, customers can now provide access to FC storage resources across an IP network. This greatly expands storage access flexibility, allowing customers to consolidate FC storage without limiting access to servers. Database information, for example, can be directly accessed from servers across an IP network.

Remote Backup for Enterprise Customers
With the ability to use IP networks, the combination of iSCSI and IP Storage routers or switches enables remote backup for enterprise customers. Remote sites can operate independently, yet still benefit from enterprise storage resources for iSCSI server to FC storage backup and recovery. This application allows data center managers to centralize corporate data resources in one location while providing sophisticated enterprise storage management to several remote customers at different sites.

iSCSI will expand the market for networked storage by giving IT managers another alternative to direct attached storage that delivers the advantages of networked storage. IP storage networks take advantage of IP networking knowledge in IT departments and use existing network management and tools for LANs, MANs and WANs today. Riding the IP wave of technology development and enhancements like the introduction of 10 Gigabit Ethernet, iSCSI provides a logical unified infrastructure development path for corporations and service providers alike.

Source: SNIA IP Storage Forum

Back to Article List