Most tape formats detail the maximum local capacity (without compression) and the maximum compressed capacity. These figures are approximate maximum capacities for the tape drive and these maximums are got under great conditions.
Because real-world systems barely meet great conditions, you may not be able to achieve the specified maximums. As an example, the type of information you are attempting to compress has a great effect on capacity. Some kinds of data just don't compress well.
If you are seeing noticeably lower capacity, it may be due to a few of the following reasons:
The tape drive’s data compression is not enabled. Tape drives that compress information use compression by default. Nevertheless there are techniques for tape drive compression to be turned off through the backup application. Take a look at your application to determine if it has got a setting for hardware compression. In most situations, you will be wanting to make sure hardware compression is turned on.
You may be writing info that does not compress well.Maximum capacities for tapes are typically based primarily on a standard 2:1 info compression proportion (or 2.5:1 for Exabyte M2 drives and some Sony AIT drives). Some types of information compress at a higher ratio; others compress at a lower proportion. For example, executable files and graphics files usually don't compress well.
The tape drive might be attempting to compress information that's already compressed.If your backup programme compresses data before sending it to the tape drive, the tape drive can't compress it further. Actually the extra attempt at compression may actually cause the information to expand. Don't use both software and hardware info compression. If the tape drive is about to compress data, switch off the software compression in your backup application.In the same way, compressed files on your hard disk won't compress any further when fed through the tape drive’s hardware compression chip. If you're backing up a high share of already compressed files, such as MP3, AVI, and JPG files, then you will not see any farther compression at the tape drive level. In fact , as the information is compressed twice, it may very well expand. Try turning off hardware compression and software compression in your backup application.
Your system may struggle to stay abreast of the tape drive.If your PC does not send information to the tape drive as quick as the tape drive can write info to the tape, the tape drive stops and waits for the computer. Everytime the tape drive stops, it writes gap tracks (tracks of uncertain data) to aid in repositioning when more info becomes available. If the tape drive has to stop and restart frequently, tape capacity is affected. Check if there are transfer bottlenecks in your system. As an example, if you're backing up info over a 100bT network, a common transfer rate could be much more slowly than you are expecting. In this situation, converting the network to at least 1GbE and for should improve both transfer rates and tape capacity. For the latest servers and LTO5 drives, a full 6Gb/sec should be provided to the tape drive.
Your tape could be ready for retirement.If you use a tape that's well worn, the tape drive might be performing high numbers of rewrites to correct errors. Excessive rewrites reduce the tape’s capacity. Try cleaning the tape drive with the correct cleaning tape for your machine employing a new tape, and ensure you are using good quality info cartridges.
Your tape drive may need to be cleaned.A buildup of debris in the tape drive or on the recording heads can lead to increased mistake rates and rewrites. If you haven't cleaned your tape drive recently, try cleaning it with the right Cleaning Cartridge for your tape drive model.