For larger applications, the Big 6 vendors offer scalable D2D systems, though practically any NAS, SAN or storage server can be used. Just ensure that throughput is available at each level of the D2D process, and consider client transfer rates, host processing capabilities, target disk array performance and bandwidth to the backup target itself.
The biggest trend in the D2D market is the increasing use of removable, external disk drives for backup. Disk makers like Maxtor, Seagate Technology and Western Digital, as well as dozens of third-party vendors, offer single-drive enclosures that connect via USB or FireWire and include simplified software for one-touch backup. These devices cost about $100 more than raw disk and handle up to 750 GB of data. We're concerned about the long-term stability of this data, however, for those who back up the data and then simply unplug the enclosure when it's not being used, and we find it disturbing that hard-disk manufacturers won't release statistics about the long-term stability of data on non-spinning disk. This becomes less of an issue in midsize applications in which the system remains powered. The trade-off is that you must then ensure that the target disk subsystem is capable of mirrored or parity-protected RAID and can be scaled up enough to handle the full capacity required for all iterations of your backup data. We tested inexpensive NAS products from Buffalo Technologies, Infrant Technologies, Iomega, LaCie and others, as well as the big names in storage, that offer both protection and security for network-based D2D applications (see "NAS for the Masses" ).
Ultimately, your data is only as secure as your disk systems, and without an external media like tape, you're vulnerable to failure or corruption. We've also experienced issues with D2D systems based on conventional file systems. Most file systems are not particularly well-suited for backup purposes because they're susceptible to fragmentation over time and have OS-imposed limitations to the size, capacity and quantity of files supported on their partitions.
In the end, a purely D2D backup environment provides the shortest possible RTO because the data remains stored on a fast disk system, and offers much better RPO timeframes than tape for the same reason. D2D may be a perfect answer for a number of environments where backups are being ignored because of time, cost or complexity, but care must be taken to ensure the long-term reliability.
 What is the physical distance between original data and protected copies in your strategy? Click to enlarge in another window |
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