Virtual Tape

This section covers both z/OS (IBM Mainframe) tape systems, and Open Systems virtual tape.

The problems with tape

As tape capacity gets bigger, its gets harder to fill a tape. z/OS tapes typically hold 700GB native, while LTO and DLT on Open Systems hold up to 1.5 TB. Its quite hard to fill one of these tapes, unless you have a specialist application such as DFHSM or TSM which are designed to pack lots of small files onto a big tape. Duplexing tape can be difficult, unless your application provides duplexing facilities. FDRABR and DFHSM are applications which do. Most applications will not create two copies at write time, you have to copy the tape later. Duplexing is important for two reasons.

1. Tape has a higher error rate than disk, so its a good idea to have two copies, especially if the tapes hold lots of data.
2. If you duplex data between two sites for disaster recovery, then you need to duplex tape data, as well as disk data.

There are always points in the day when you do not have enough physical drives, and other times when you are hardly using any drives. It would be nice to be able to smooth out the usage of real tape drives.

Virtual Tape

Virtual tape solves these problems, as explained in this page set. Some virtual tape systems do not use real tapes at all, so there are two fundamental types of virtual tape

1. Tape Virtualisation; where the applications write data to virtual volumes on virtual drives and this data is later consolidated onto real volumes on real drives.
2. Tape Elimination; where the applications write data to virtual volumes on virtual drives and this data is stored permanently on disk. This type of virtualisation is often combined with data de-duplication.

As Open Systems tapes tend to be for backup and recovery only, the claim is that backup to disk is faster than to tape, and recovery certainly is. When combined with data de-duplication, disk can actually be cheaper than tape.
The basic principles of virtual tape are illustrated in the first page.
The second page discusses the implementations used by three main suppliers of virtual tape
The next page discusses how to mirror data between sites using virtual tape.
The final page discusses Open Systems virtual tape

Use the links above, or left hand frame to navigate between the pages.

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