Most tape formats detail the maximum native capacity (without compression) and the maximum compressed capacity. These figures are approximate maximum capacities for the tape drive and these maximums are obtained under ideal conditions.
Because real-world systems rarely meet ideal conditions, you may not be able to achieve the specified maximums. For example, the type of data you are trying to compress has a great effect on capacity. Some types of data just do not compress well.
Why can’t I get the maximum capacity from my tape drive?
If you are seeing significantly lower capacity, it may be due to one or more of the following reasons:
- The tape drive’s data compression is not enabled. Tape drives that compress data use compression by default. However, there are ways for tape drive compression to be turned off through the backup application. Check your application to see if it has a setting for hardware compression. In most cases, you will want to make sure hardware compression is turned on.
- You may be writing data that does not compress well. Maximum capacities for tapes are usually based on an average 2:1 data compression ratio (or 2.5:1 for Exabyte M2 drives and some Sony AIT drives). Some types of data compress at a higher ratio; others compress at a lower ratio. For example, executable files and graphics files typically do not compress well.
- The tape drive may be trying to compress data that is already compressed. If your backup program compresses data before sending it to the tape drive, the tape drive cannot compress it further. In fact, the additional attempt at compression may actually cause the data to expand. Don’t use both software and hardware data compression. If the tape drive is set to compress data, turn off the software compression in your backup application.In the same way, compressed files on your hard disk will not compress any further when fed through the tape drive’s hardware compression chip. If you are backing up a high percentage of already compressed files, such as MP3, AVI, and JPG files, then you will not see any further compression at the tape drive level. In fact, as the data is compressed twice, it may actually expand. Try turning off hardware compression and software compression in your backup application.
- Your system may not be able to keep up with the tape drive. If your computer doesn’t send data to the tape drive as fast as the tape drive can write data to the tape, the tape drive stops and waits for the computer. Each time the tape drive stops, it writes gap tracks (tracks of undefined data) to aid in repositioning when more data 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. For example, if you are backing up data over a 100bT network, a typical transfer rate might be far slower than you expect. In this case, 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 made available to the tape drive.
- Your tape may be ready for retirement. If you are using a tape that is well worn, the tape drive may 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 device, using a new tape, and make sure you are using good quality data cartridges.
- Your tape drive may need cleaning. A buildup of debris in the tape drive or on the recording heads can lead to increased error rates and rewrites. If you haven’t cleaned your tape drive recently, try cleaning it with the appropriate Cleaning Cartridge for your tape drive model.