Simplifying storage: combining the iSCSI standard with SAN functionality.
Shared, networked storage is frequently touted as a way to address cost, growth, management and availability issues associated with storage by facilitating:
* Server and storage consolidation
* Increased utilization of available storage
* Increased performance
* More efficient backup and data protection processes
* High availability of data
Users have had Fibre Channel storage area networks (SANs) and network-attached storage (NAS) available to them for years--in theory, offering most or all of the benefits outlined above. So why is direct-attached storage (DAS) still the dominant storage architecture? With connection costs high (i.e., $1000 HBAs and $1000 per port switches) Fibre Channel SANs have proven too expensive for the masses. Additionally, Fibre Channel's complexity discourages many from implementing the technology. NAS has proven to be difficult to scale and addresses only file storage requirements. As we move into 2004, iSCSI is a new standard protocol being touted as the answer to the call for affordable, scalable, easy-to-manage storage. Is that realistic? Can a protocol do all this?
The History of a Standard
First, let's look back at the development of iSCSI. iSCSI was derived from SCSI, a protocol used for accessing and writing block-level data on locally attached disk drives. Access to block-level data is a fundamental requirement of file systems, databases, and many applications. In addition to being a protocol, SCSI is also a physical bus with distance limitations. As systems were architected that moved the physical storage outside of the computer, other protocols were developed that incorporated the SCSI protocol capabilities, but extended the physical connection distance. Fibre Channel is just such a protocol, as well as a physical network. Fibre Channel found a home in large IT shops, where the budget and expertise to install and manage this complex network was available.
It would seem natural that storage would be networked using similar network technologies as those used in the corporate computing network. The commodity nature of Ethernet networks made them affordable, and expertise around these technologies was almost ubiquitous. Unfortunately, speed (10 Mb/s) and lack of an appropriate protocol initially inhibited the use of IP/Ethernet networks for storage.
Over the ensuing years, Ethernet networks gained the speed (100 Mb/s and 1Gb/s) necessary to support storage traffic, leading vendors to continue the quest to network storage over Ethernet. NAS implementations made use of TCP/IP-based Ethernet networks by moving the file system to the storage device and using file-based protocols (CIFS and NFS). NAS delivered on the promise of easy to use storage at an affordable price. However, unlike Fibre Channel SANs, this architecture didn't support delivery of block data to the databases and applications (i.e., Microsoft SQL Server or Exchange) that required block-level access.
To address this inability to support many database and application networked storage requirements, LeftHand Networks developed a blockbased protocol for TCP/IP networks in 2001. This precursor to iSCSI, called Advanced Ethernet Block Storage (AEBS), sparked the delivery of the first, native IP-based SAN.
Simultaneously, vendors worked for several years through the Internet Engineering Task Force (IETF) to define a standard for a block-based storage protocol specifically designed for native TCP/IP networks. The standard, iSCSI, was ratified in June 2003.
Some offerings make use of iSCSI to connect IP-based servers into Fibre Channel SANs and call this an IP SAN. This article considers the use of iSCSI in constructing complete, or native, IP-based SANs.
The Benefits of a Standard
With iSCSI and TCP/IP-based storage networks, users are assured of the benefits that come with standards:
* Compatibility with a wide range of components: Ethernet has been a standard for many years, and components are truly plug-and-play. Users own and are familiar with a variety of brands of Ethernet switches and other IP-based networking equipment. IP-based storage works with standard network interface cards (NICs), eliminating the need for expensive host-based adapters (HBAs) required with Fibre Channel.
* A large base of management software and tools: Native IP-based storage allows IT managers to handle storage traffic in much the same manner in which they handle other network traffic. Existing software applications and tools for bandwidth provisioning, traffic management, security, and overall network management can be used with the storage. Adapting storage traffic to IP offers a unified view of all traffic and eliminates the need for separate management systems (i.e., storage SAN and corporate LAN).
As networked storage becomes more prevalent, the intelligent services required to effectively assign, secure, and manage these network resources become all the more critical. Layer 2 and Layer 3 services such as virtual LANs (VLANs), quality of service (QoS), high availability, routing, management, and access control lists (ACLs) are key services that IP and Ethernet technologies are able to provide to a storage networking infrastructure today that Fibre Channel does not.
* Greater access to necessary technical skills: IT workers with Ethernet and IP skills are far more abundant and affordable than Fibre Channel specialists.
* Lower total solution costs: In addition to storage prices that are typically considerably lower than Fibre Channel storage, there's no investment in expensive Fibre Channel HBAs and switches, and no investment in acquiring Fibre Channel expertise. Ethernet costs are known and predictable as with any mature technology.
* Lower cost of ownership: Instead of developing storage networks with unfamiliar storage connection technologies, network managers can use the IP/Ethernet skills of existing staff, shortening the time and expense to deliver complete solutions. While it is typically suggested that users deploy a dedicated network (the IP SAN) for shared storage, the deployment of gigabit Ethernet enables network managers to benefit from immediate interoperability, network management, best practices, and analysis tools.
Many sites also need to establish backups and disaster recovery support for critical data, frequently at remote facilities. Because Fibre Channel is limited to 10km, conversion to the IP protocol is necessary for replication over a long distance. This introduces significant cost to the Fibre Channel infrastructure. Utilizing iSCSI in an IP SAN allows users to move data over long distances much more affordably, without any protocol conversion.
These benefits of a standard will encourage the development of more iSCSI-based storage devices and the construction of more IP SANs. In fact, the IT industry is ripe with the buzz of iSCSI or IP SANs. Realistically, however, iSCSI is no more than a protocol, enabling block-based connectivity to a server over a standards-based network. Connectivity alone does not lower costs or increase management efficiency.
The Benefits of a SAN
To realize the potential of IP SANs--cost savings combined with full network storage functionality--users need to realize the benefits now available with Fibre Channel SANs:
The consistent growth of data and the demand that applications be available around the clock mandates that storage scale without downtime. However, dynamic scalability is only a partial fix. Ideally, a networked storage solution automates the process of re-allocating disk usage across the network as capacity is added. With both DAS and NAS solutions, all data movement associated with adding additional captive storage or a new NAS box requires the administrator to manually move and balance data amongst the servers. This is a very labor-intensive process. Some IP SAN solutions automate this capability.
Increased Asset Utilization
Sites using DAS commonly utilize only 40-50% of the storage capacity they've purchased and spend resources to manage. With a SAN, users are frequently able to achieve utilization numbers in the 80% range due to the ability to pool the storage, and then carve out volumes as it makes sense. Utilization increases are seen when administrators can dynamically provision and reallocate resources to the appropriate server and applications as necessary. The flexibility to grow and shrink volumes on the fly also allows users to achieve higher utilization rates, manage storage during normal business hours, and achieve a notable reduction in total cost of ownership.
Increased Data Availability
High availability features allow sites to meet their application uptime requirements. Hardware redundancy, failover to hot spare storage modules, pathing, RAID, and replication are all options offered by SAN solutions. Additionally, all SAN solutions should have the potential to radically change the backup and restore processes by offering features such as snapshot. This can remove the burden of backup processing from application servers and the corporate computing network, greatly simplifying the process for IT managers and eliminating any impact on users.
System Performance Increases
SANs take advantage of highspeed interconnects to large numbers of disk drives to increase storage performance. With iSCSI, users leverage gigabit Ethernet (1 Gb/s), compared to 1 or 2 Gb/s for Fibre Channel. Many IP SAN solutions offer NIC aggregation to enhance network performance. Some modular IP SAN solutions scale performance as capacity scales. This is accomplished by adding processors and network connections with the addition of incremental disk drives. This addresses the potential bottleneck that arises when more and more disk drives are added behind fixed storage controllers and network connections.
Without a SAN, the choices for provisioning and geographical distribution of the physical storage are limited. With a SAN, virtualization capabilities can add flexibility to the provisioning process, as well as facilitate scaling storage on the network. SANs based on IP come with an additional benefit. As a protocol, IP can be extended across long-distance networking infrastructures. This enables an IP-based SAN solution to defy geographic boundaries.
A Powerful Combination
By combining the benefits of an Ethernet-based standard with the strength of a storage area network, IP SANs offer users a compelling new storage alternative. iSCSI plays a role in the architecture as the protocol. Users benefit from strong vendor support and the compatibility, existing tools, existing knowledge, and lower cost that come from being standards- and Ethernet-based. Additionally, users stand to benefit from the innovation and progress that inevitability surrounds a standard. As examples, RDMA and 10 Gb/s Ethernet are on the horizon and hold the promise of significantly expanding the adoption and implementation of IP SANs. However, as users evaluate storage solutions, they must keep in mind that support for any particular protocol, or network (in this case iSCSI and Ethernet) is only a small piece of the equation. iSCSI or IP SANs must be evaluated on their ability to provide SAN services, including dynamic scalability, increased asset utilization, data availability, increased performance, and flexibility. In other words, as in Fibre Channel SANs, the necessary management software is key to realizing the benefits of the IP SAN. By evaluating the ability of a solution to deliver those benefits in conjunction with affordability, users will end up with a shared network storage solution that combines the best that a standard protocol has to offer with the power of a storage area network.
Tom Major is vice president of marketing at LeftHand Networks (Boulder, CO)
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|Title Annotation:||Connectivity; storage area networks; SCSI protocol over TCP/IP|
|Publication:||Computer Technology Review|
|Date:||May 1, 2004|
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