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Consistency of backups. Fuzzy backups--backups that are not from a single point-in-time--are sometimes used in sophisticated environments that have numerous application interdependencies. While this topic is usually more relevant to DR, it's still important to ensure the data being backed up is useable and in a coherent state. Disk-based backup doesn't address this issue. If a disk-based backup now takes one hour instead of four, it still isn't from a single point in time. Approaches such as snapshots and split mirrors address this specifically, and still need to be used regardless of the ultimate backup approach. Ironically, if snapshots or split mirrors are deployed in a disk-based backup environment that still uses tape for long-term archival and off-site rotation, the backup environment now has a total of three tiers of data protection storage (online disk, backup disk and tape).

Capacity planning. In a tape-based environment (see "The case for tape"), there are several important considerations when determining the required number of tape drives, amount of tape media and slot capacity of the tape library(s). Research shows that for every gigabyte stored online, there are between 10 and 15 copies of that data stored offline. Capacity planning for backup is an inexact science that's further complicated when online storage growth isn't accurately forecasted.

These issues take on a new twist in a disk-based backup environment.

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Instead of tape media capacity, you need to consider the file system and LUN sizes of your backup environment. Instead of tape library slots, you need to look at overall array capacity. Rather than estimating the compression ratio you may achieve during your tape backups, you need to investigate the size (including RAID level, amount of cache and disk drive) of the disk-based backup hardware, as well as how long the data being backed up will remain on spinning disk. Adding more tapes to a tape library for additional backup capacity is fairly straightforward; adding storage to a disk-based backup appliance requires considerably more effort and preparation. Also, don't forget that buying disk to handle a last-minute capacity crunch is much more difficult than buying some additional tapes.

Networking issues. Numerous approaches are used with today's popular tape-based backup environments. Some back up over the network, with Gigabit Ethernet quickly becoming the norm. The backup servers are sometimes dedicated, sometimes shared and are rarely sized appropriately specific to CPU, memory and backplane. Other backup environments use LAN-free backups, which have the application server backing up directly to tape devices on a storage network (e.g., Fibre Channel). And a few are even investigating server-free backups, which don't involve the application servers whatsoever.

The case for tape
Adding disk doesn't necessarily mean eliminating tape.

This was first published in September 2004

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