Hot Technologies in Storage

Continued from page: 4

Tuesday, July 03, 2007

Data Replication over WANs

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Not only you need to have a secure first copy, you also need to be having an available second copy of data to restore the normal course of business, in case disaster strikes.

You never know the value of a backup until a disaster strikes and wipes out the principal copy of your data. Many of us have realized the value of investment in creating backups at remote locations, only after seeing the few mishaps which took place in recent times and the devastation that they caused to the unprepared. The traditional method of data protection has been to back it up on tape, and then physically move it to a safe location, preferably far away from the main office. However, with proliferation of a wide range of disk-based automated backup systems the platform is all set for technologies for data replication over WANs to get mature. On one hand we are seeing advances in networking technologies. To name a few-optical networks now support storage networking protocols, inclusion of flow control mechanisms, and efficient transport capabilities. On the other hand in the storage world, we have data de-duplication technologies coming up, which significantly reduce the volume of the data to be backed up.

Networking technologies
To send data to geographically dispersed storage, you need a resilient storage networking infrastructure. For inter-datacenter data replication, you need a network with low latency, so there's minimal packet loss. Plus, the bandwidth should also be scalable for such a network. With these sensitivities in mind, there are two options to build a network to support for data replication-Coarse / DenseWavelength Division Multiplexing (C/DWDM) and Synchronous Optical NETwork / Synchronous Digital Hierarchy (SONET/SDH).

C/DWDM is a technology that maps data from different sources and protocols together on an optical fiber with each signal carried on its own separate and private light wavelength. It can be used to interconnect data centers via a variety of storage protocols such as Fibre Channel, FICON, and ESCON. It has been verified to support data replication over distances up to several hundred kilometers. C/DWDM provides bandwidth from one to several hundreds of gigabits/second (Gbps).

SONET/SDH technology is based on Time Division Multiplexing (TDM). With this technology, enterprise data centers can be interconnected over thousands of kilometers for data replication and other storage-networking needs. Storage over SONET/SDH is a reliable and readily available networking option.

Data de-duplication
Though data de-duplication technologies have been around for years, there is a renewed focus on them recently as they are being utilized by products in the disk-based backup market. Data reduction enables disks to be a feasible long-term retention backup media-making it the same or lower cost than tape-based systems.

Moreover, data de-duplication addresses the issue of data replication for disaster recovery. With the reduced amount of data after de-duplication, the network bandwidth required for replication reduces significantly. This makes replication even possible for smaller companies with lower budgets.
There are two primary methods of data reduction found in disk-based backup systems: One is, byte-level delta data reduction that compares versions of data over time and stores only the differences at the byte level and the other is, block level data de-duplication in which blocks of data are read from the written data and only the unique blocks are stored.

A byte level delta data reduction outperforms data de-duplication in a disk-based backup system, as it scales to larger amounts of data. It avoids hash table and restore fragmentation issues. It also processes the backup data after it's been written to disk and on top of all, it is content aware and optimized for your specific backup application. Therefore, it knows how each backup application operates, and understands file content and boundaries. All in all, it helps in optimizing the de-duplication process.

Flavors of replication
There are several products available in the market for data replication over WANs. There are four flavors of replication to choose from. You can do it at the application level, host level, in the storage arrays, or with a storage networking appliance. The advantage of having it at application level is that the application is fully aware of such replication. DBAs have confidence in data integrity. It supports both synchronous as well as asynchronous replication. And, it is also not hardware dependent. The disadvantages of such an approach are that application owners themselves are responsible for recovery. It is specific to a particular application. Often it does not protect application files.

If you have it done at host level the failover can be automated, while if you have it at application level, then the DBAs themselves need to pull up the data in case of any failure. It supports disparate hardware and many-to-one replication can be facilitated. As cons, you can count its dependence on OS and that, it requires additional resources at host and the replication has to be explicitly integrated with applications. In case you plan to replicate at storage array level, then you must know that it is unlikely to support dissimilar hardware. Secondary copies are only usable with point-in-time copies. It also requires integration with applications. You also need to work on fabric extension and there is added complexity of keeping everything in sync. But having it at storage array makes it agnostic to applications and OS. It does not use any host resources, replicates all kinds of data and is also easier to manage.

Despite being a little costlier as a separate appliance and additional hardware is required, having replication in the fabric provides modern functionalities like CDP. In this approach, no array or host resources are required. Understandably, it is also agnostic to applications and OS. Besides it is highly scalable.

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