If you can't beat 'em, join 'em. That's the rallying call embraced by tape library vendors as they respond to the steady encroachment of disk on their core tape business. Front-end software transforms disk into virtual tape drives and policy-driven backup software handles the data migrations from the disk to the tape and back again. This new climate of coexistence between disk and tape gives users a powerful new tool for improved data protection, high availability and information lifecycle management (ILM). There are two compelling reasons to deploy disk in front of tape libraries: better backup and recovery times and lower costs.
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Jumping on the cheaper disk bandwagon, tape library vendors now provide their own ATA disk arrays with front-end I/O controllers running tape virtualization engines that present themselves as virtual tape drives to backup software. The new virtual disk interface gives storage administrators the ability to transparently choose between tape and disk as the primary backup target and to migrate data between the two using policies nestled within their backup software. The integration of disk and tape also frees users to replicate data during off-peak times to off-site disk arrays for disaster recovery purposes, and then transfer the data to tape to satisfy vaulting requirements.
Despite the appealing benefits that closely integrated disk and tape offer, users first need to verify that the vendor's disk array tape virtualization product is compatible with their backup software and storage environment. For example, Spectra Logic Corp.'s disk-based RXT appliance only emulates LTO-2 tape drives, while IBM Corp.'s B20 Virtual Tape Server (VTS) systems only provide a 3490 tape drive target. The limited tape selection could be a compatibility issue in shops where either SDLT or SAIT are the main tape media used. For vendors targeting Unix and Windows environments, the trend increasingly is to support LTO as the standard virtual interface.
IBM/Tivoli targets its disk/tape product for storage environments with mainframe, Unix and Windows operating systems. IBM offers its disk-based B series line of VTS, along with its 3590 tape systems. The VTS and 3590 tape systems can be monitored and configured using Tivoli's Storage Resource Manager (SRM) and Storage Manager (SM) tools, which enable policy-driven data management to move data between the virtual and real tape subsystems.
Storage Technology Corp. (StorageTek) also offers a range of tape libraries and disk subsystems for MVS (mainframe) storage environments. StorageTek's L, SL, PowderHorn and TimberWolf tape libraries work in conjunction with its B, D and V series of disks under the umbrella of the company's Virtual Storage Manager (VSM) software. The VSM software emulates T9840 and T9940 tape drives. In addition to the VSM software, StorageTek also offers Storability Software Inc.'s Global Storage Manager (GSM) that reports on both the tape libraries and disk arrays.
Recently, a number of storage vendors have partnered to fill gaps in their disk/tape offerings. Advanced Digital Information Corp. (ADIC), for example, filled two holes in its product lineup through its recent alliance with EMC Corp. ADIC offers neither disk arrays that compete in the high-end and midrange areas of the market nor storage resource management software that works outside of their product line. EMC fills these gaps with its high-end and midrange arrays and its Control Center SRM software. EMC also provides an enterprise tape library product to round out its ILM strategy.
The influx of ATA disk
While companies such as IBM and StorageTek have offered integrated tape and disk products for the mainframe for years, it's only with the introduction of low-cost ATA disks that this solution becomes affordable to the masses. The quality of disk needed for backups is generally lower than what's required for day-to-day operations, and the price per gigabyte is significantly lower than Fibre Channel (FC) disks.
Despite this allure, users must still verify some basic facts about an ATA array:
- Does it offer tape virtualization software?
- If it offers tape virtualization, what types of tape drive interfaces does it present?
- Will it support the I/O for your environment?
- Does the array support RAID? If so, in what configurations?
- What software is used to integrate the array with tape libraries?