Virtual Tape Technology
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. Physical tapes can typically hold up to 5TB native. 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.
- 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.
- 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 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
- 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.
- 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.
Does Virtual Tape have a future? Some say that the integration between backup products and de-duplication makes it more cost effective now to store backup data permanently on cheaper SAS or SATA disk, especially on disk subsystems where hard drives can spin down when they are not being read, as this saves on power consumption. The argument is, why use a VTS to make a hard drive emulate tape, when the data can simply be stored on disk using more efficient block sizes? Most backup products now support disk to disk, or D2D backups with de-duplication. This means that a VTS that just uses disk storage is a waste of time. There are no immediate signs that disk based VTS devices are disappearing, but it is one to watch.
Another emerging strategy is to recognise that backups and archives are two different solutions to different problems. Backups tend to be short retention, say 30 days, while archives are long retention, measured in years. Disks are not really suitable for long term retention, tapes are arguably a better solution. So a workable strategy could be to used D2D for backups and tape based VTS for archives. A clear advantage of VTS over native tape here is that the physical tapes can be recycled regularity, so proving that they can be read, and faulty tapes can be replaced if they are duplexed.
One final innovation is to offload all this archived file management and let someone else look after it. CLOUDsafe was released in late 2012 to store mainframe tape data in the Cloud.
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.