What you will learn from this tip: Methods that can enable
Simultaneous support for mainframes and open systems is difficult, but users can achieve some ATL sharing between Windows and Unix platforms. But even this basic sharing is a function of backup software or an intermediary device. The backup software (for example, Veritas Software Corp.'s Shared Storage Option) or device (for example, IBM's Virtual Tape Server [VTS] or StorageTek's Virtual Storage Manager [VSM]) controls access to the robot and associated drives, acting as a traffic cop so that no two tape mounts are made to the same drive at the same time by different tasks.
What virtualization buys
Virtualizing tape devices allows host applications to access a logical tape drive while a virtual appliance accesses actual physical tape drives. Because the biggest bottleneck in disk-to-tape operations is writing data to tape, virtualization at this level allows logical mounts to be presented to the host whether physical tape drives are available or not. The virtual solutions can gather tape data residing in the disk cache and later write the data to tape, as mainframe tape management systems have done for years.
The biggest drawback to the virtualization products is that they're limited to either mainframe (for example, Bus-Tech, Diligent Technologies Corp., IBM's VTS or StorageTek's VSM) or open systems (for example, Advanced Digital Information Corp.'s Pathlight VX, Data Domain Inc.'s DD200, EMC's Clariion Disk Library or Quantum Corp.'s DX100). These systems aren't used for sharing but for enabling separate backup domains by OS or application platform. They're point solutions that improve backup windows and speed recovery time, but they don't provide simultaneous sharing.
Another drawback is that duplicate virtual appliances are needed at the disaster recovery site to recover data, and these devices are typically pricey. They may also require purchasing backup software upgrades. For example, Veritas charges by the number of tape drives (real or virtual) needed, so a 40 drive virtual environment at $3,000 per drive would cost $120,000. Veritas also adds $2,000 for a SAN-shared drive. If the 40 virtual drives were shared, the total cost would be $200,000.
Limits to tape sharing
The key to tape sharing is the ability to establish static mount points, so that only one system can read/write to one tape at a time. The process of mounting/dismounting tape drives is time consuming. With disk arrays, several hosts can simultaneously access a set of disks and although each obtains the data it requested, the context switching is transparent to the host. This is impossible with current tape technologies, because when host systems need access to the same set of tapes or tape drives, each must wait until the previous request has completed. In this scenario, aggregate data rate transfers drop dramatically, often to fewer than 100 kilobytes per second (KBps).
Another operational risk for arrays and ATLs posed by multiplatform support is the need to quiesce the system to update microcode. When this occurs, some or all of the attached hosts are impacted. Every attempt to upgrade shared ATLs or tape drives requires coordination between mainframe, Unix and Windows groups. Array vendors have overcome this limitation by minimizing or eliminating these disruptive microcode upgrades, so that elaborate coordination is no longer necessary.
Read more of this tip in Storage magazine.
For more information:
About the authors: Mike Drapeau and Gary Brown are storage consultants.
This was first published in April 2005