Remote Synchronous Disk Mirroring Attainable by Mid-Tier

Over the past few years, there have been more than 400 separate pieces of legislation put in place across various industries in Europe and the US requiring remote archival and protection of data. Many companies have responded by implementing business continuance strategies using storage extension solutions to hold copies of business critical data at multiple sites. This has coincided with a period where business costs have been under close scrutiny, driving a major trend to consolidate assets. As a result it is not uncommon to see organisations moving down from 10 distributed data centres to 2 or 3 larger facilities. Not only does this reduce costs for these companies, they benefit from improved uptime, better asset utilisation and more streamlined management. These trends are combining together to drive the need to interconnect SANs and other data centre protocols over considerable distances.

A Storage Area Network (SAN) inter-connects application servers to a pool of shared storage devices such as disk arrays and tape drives. This segmentation delivers great efficiency savings and performance improvements when compared to directly attached storage. It also brings a number of data management benefits such as the ability to make regular back-ups directly between storage devices without interrupting the activity of the servers (disk replication) and even the ability to record every transaction on 2 separate disk arrays simultaneously (disk mirroring). The SAN can also be used to cluster servers so they can share computing power and provide redundancy. These building blocks provide the infrastructure to implement various levels of protection up to and including the ability to continue operations without impairment following the total loss of a primary device. These are well understood techniques in the storage community but the difficulty comes when the organisation needs to protect against a catastrophic failure affecting the whole site. In this case the backup servers and disk arrays must be distributed between a minimum of two data centres.

Data Centres use a wide range of protocols, and the interconnect needs to be able to support these to ensure the most efficient and flexible use of this important network resource. Typical requirements include multiple channels each of ESCON, Fibre Channel/FICON and Ethernet at various speeds. Other supporting protocols are also often required such as ETR/CLO timing channels. Finding a solution to meet these criteria has been a huge dilemma for medium sized enterprises as traditionally this connectivity or transmission portion represents up to 75% of the total solution costs.

One of the first things a company should do when developing a business continuance strategy is to look at the applications they have deployed and assess the business impact of downtime related to each. Applications that tend to be fairly tolerant to downtime like e-learning and file services can usually be dealt with by offsite tape backup. E-mail and CRM are much less tolerant and require asynchronous remote replication. Of course applications like ERP and e-commerce have the greatest impact on the business and are extremely sensitive to downtime of any sort. They need the fastest possible recovery time and that means remote synchronous disk mirroring.

Remote disk replication and remote synchronous disk mirroring both involve very large volumes of data being transferred between the remote sites. This means that the MAN or WAN needs to perform to the same level as the in-building SAN. Often a business requires the alternative data centre to be located 50km or more away from the main operational site which in turn requires that the vehicle for SAN extension needs to meet certain metrics for delivery, latency, and recovery from link failures. IP-based SAN extension such as FCIP, and to some extent iSCSI, is more than adequate for many smaller organisations but if you need synchronous disk mirroring, or even if you have a high volume of asynchronous replication, an optical solution is the only viable option. Optical Storage Extension involves establishing high capacity, highly resilient fibre-optic links between the main sites and backup centres. These must handle a wide range of protocols over distances of up to 100km or more.
Wavelength Division Multiplexing (WDM) has proven to be the best technical solution due to its ability to carry many multi-Gigabit channels over a single fibre pair but traditional Metro Dense WDM (DWDM) platforms have been costly, with architectures and components derived from transcontinental telecom line systems. These platforms are excessively over-engineered for the vast majority of metro / regional storage extension applications and as a result remain the preserve of Fortune 500 companies. Generally speaking, SAN extension does not require the extreme capacity that such systems can deliver. Vendors such as Transmode of Sweden recognised this over engineering and pioneered the concept of CWDM where the channel spacing is an order of magnitude wider with the consequential robustness improvements and reduction in costs.

Some CWDM systems such as Transmodes use a modular approach to provide various options for handling each client type in the most efficient manner. The highest speed protocols such as 2Gbit/S Fibre Channel are carried directly on wavelengths but it is usually more efficient to pack multiple lower speed signals, up to 8 in fact, onto a single wavelength using aggregator modules. These can handle protocols such as ESCON, Fibre Channel/ FICON, FDDI, Fast Ethernet, Gigabit Ethernet, STM-1 and even the ETR/CLO timing signals which can be grouped with multiple ESCONs on a single wavelength.

Enterprises are no longer caught between esoteric DWDM and the low bandwidth, high latency IP approach. A range of SAN extension solutions with realistic performance and affordable pricing are now widely available as private build, leased and managed service options.

Transmode are exhibiting at Storage Expo the UK's largest and most important event dedicated to data storage, now in its 5th year, the show features a comprehensive FREE education programme, and over 90 exhibitors at the National Hall, Olympia, London from 12 - 13 October 2005

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