While tape is the traditional technology for backing up your data, it is increasingly being replaced and supplemented by disk. While speed and flexibility are driving this shift, tape still has a number of important advantages, including cost and
Disk is faster than tape
Disk's big advantage is speed. It is considerably faster than tape, even when configured as a virtual tape library (VTL) (which requires extra overhead compared to a straight disk-to-disk operation). Typically, disk will be three to 10 times faster than tape on backup operations and even faster speeds aren't uncommon. How much faster depends on a host of factors, from the configuration of the network to the specific hardware and software to the details of the data being backed up.
This is especially true where data has been backed up incrementally, deduplicated or otherwise processed to save time and space. The nonlinear nature of disk makes it easier to reconstruct the data for restoration.
Disk is reliable
Unlike tape, which depends on humans to swap tapes, or remote backup, which depends on the speed and quality of the connection, disk backup is self contained. There are fewer chances for human error and less dependence on factors outside the system's control.
While disks can still fail, the redundant nature of RAID arrays provide protection. The big limit on disk reliability is the lifespan of the disks in the array. You can figure that the disks in a RAID array will last approximately five years. Properly stored tape will last for 25 years or 30 years. However, this is much more a concern for archival applications than backup.
Disk is easy to automate
Because disk backup doesn't depend on outside intervention, it can be a set-and-forget operation. Because of its speed and nonlinear nature, it's easier to perform more elaborate file operations on data on disk when saving and restoring. Data deduplication, mirroring, snapshots and VTLs are all much harder to perform on tape.
Data on disk is easy to manipulate
As features such as deduplication and continuous data protection (CDP) become more important, disk becomes a larger part of the backup picture. Having disk in the system makes it easier to implement these features and still maintain performance.
However, disk backup does have its drawbacks.
Removable disk systems
Unlike tape, which is designed to be removed and stored off the system, or remote backup over a network, disks are usually a fixed part of the system. If you want to move a copy of your data off site, you have to use another medium, such as replicating a copy of the data over the network.
There are removable disks that can be taken out of the system and shipped offsite like tape, from vendors such as Idealstor , Hewlett Packard (HP) Co., Tandberg Data's RDX, Imation Corp's Odyssey system, ProStor Systems Inc.'s InfiniVault and Dell's PowerVault RD1000.
Removal disk drive units are either sealed containers or caddies that can accept standard disk drives. While the containers and caddies are designed to be transported, they may not be able to stand up to as much handling abuse as tape cartridges because of the weight of disk drives and the types of components they house. For example, an HP data sheet says one of its removable hard drives will continue to work if dropped from a 1 meter height onto a tile covered concrete floor. That's not bad, but nowhere nearly as rugged as a tape cartridge.
Disk is (usually) more expensive
It used to be that tape was always cheaper than disk backups. That's no longer true, but in general it still costs less to back up to tape on a per-gigabyte basis. This is especially true as the amount of data grows. With very large amounts of data backing up to disk or over the network becomes economically prohibitive.
For most SMBs, the decision isn't nearly as clear cut. You have to compare the actual costs, including the cost of tapes, maintenance, etc., as well as the initial costs, for your amount of data. Still, in most cases the cost of the tape system may be cheaper.
About the author: Rick Cook specializes in writing about issues related to storage and storage management.
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This was first published in January 2009