Disk choices
Using disk as a backup device isn't really a new concept. In the mainframe world, virtual tape systems utilizing disk drives as a cache and staging area for tape devices have been in widespread use for a number of years. In open systems, disk has been involved with backup in at least three ways. First, business continuance mirroring products such as EMC's TimeFinder have played an increasingly important role
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in improving backup performance and offloading production servers. Also, some backup products, such as IBM Tivoli Storage Manager, have utilized the concept of intermediate disk storage pools on backup servers as the target repository for nightly backups, thereby reducing contention for tape drives. Finally, virtually all backup products support backup to a disk device instead of--or in addition to--tape. In all of these cases, however, disk has been used as a temporary repository--the backup data ultimately ends up on tape.
Recently, a number of new products have emerged based on low-cost storage devices, most notably ATA disks. These and other devices are presenting new options that provide some interesting options for designing a backup architecture. These new devices tend to fall into one of three categories: low-cost disk subsystems, virtual tape devices and content-addressable storage systems (see "Disk-based backup options," this page).
Low-cost disk subsystems are probably the most recognizable type of device. These are typically ATA-based systems that share many characteristics with traditional SCSI and Fibre Channel (FC) disk systems. The major differentiating factors between these and their higher-priced siblings are performance and cost. They are positioned in the market as near-online storage devices. Because their cost per megabyte is approaching tape, and they provide some level of RAID protection, it has become feasible to consider their use in backup environments. Additionally, some vendors offer features such as replication and/or snapshot capabilities to further enhance their capabilities.
