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DLT/SDLT/LTO Tapes Drive Data Compression Q & A
| Q. What is data compression and how does compression work? A. Data compression permits increased storage capacities by using a mathematical algorithm that reduces redundant strings of data. There are many different compression schemes that can be performed by software or hardware. For tape drives, the compression algorithm is typically implemented in hardware and will remove redundancy from data by encoding patterns of input characters more efficiently. The more repeat patterns in the data, the more the data can be compressed. Thus the amount of data compression obtained will vary depending upon the characteristics of the data. Q. Can I assume that I will get the advertised 2:1 compressed capacity? A. No; you may get more or less. A more definitive statement that could be used for advertising capacity, which “assumes 2:1 data compression,” would be “Assuming Your Data Compresses 2:1.” Q. Why wouldn’t my tape drive get the capacity indicated at the advertised compression ratio? A. Your data is unique – not like anyone else’s. Therefore when your data is compressed the ratio will also be unique. The amount of compressed data stored on your tape will be different; the capacity is unpredictable. Furthermore, data with random bit strings, encrypted or pre-compressed data are unlikely to show any capacity improvement from the tape drive compression. Capacity can even decrease slightly when highly random data is compressed Q. Do all tape drives feature hardware based data compression? A No; check your drive’s manual for feature information relating to the exact model-number of your drive or contact the drive manufacturer. Q. Some tape drive vendors claim > 2:1 data compression. Why? A. The effects of data compression vary, because some types of data are more “compressible” than others. Different mathematical compression lgorithms are better at compressing certain data types than others. Some vendors may conclude that their compression scheme is more powerful; however, it may not be more powerful for your mix of data. When comparing technologies, it is best to compare native-to-native capacity figures. Compression claims may or may not be relevant to your application. Q. My backup software features data compression and my tape drive also has a compression feature. Should I use software compression and/or the tape drive’s hardware based compression? A. When compression is available in the tape drive hardware/firmware, the compression feature of any software package should be turned off. This reduces the processing burden on your computer. Also, hardware based compression is typically much more effective. Do not double compress; compressed files could even be bigger after compression. This can even cause the tapes to hold less-than their native capacity. Some tape drives prevent lowering capacity due to files that grow larger from compression by writing the uncompressed file when this happens. The drive tags each block (compressed/not compressed) for appropriate handling during playback – e.g., LTO Ultrium drives have this feature. Q. Does data compression affect the tape drive’s data transfer rate? A. Sustained data transfer rates will increase at the same ratio as data capacity when employing hardware based data compression (up to the speed of the compression hardware). The same may not be true for compression implemented by software running on your computer. Software-based compression typically degrades performance, since the software or separate utility has to compress the data before saving it to tape. Retrieval time is also longer, since the software must decompress. Today’s hardware-based compression chips are typically many times faster than the tape drives they work with. Therefore compression does not introduce any overhead into the writing or reading processes. Since compression reduces the number of bits written to or read from the tape, significant performance gains are realized. Q. Does the tape media need to store more data bytes with compression? A. No; the tape may appear to hold twice as much data, but the capacity (density) of bits/bytes written on the tape media will not be increased. Q. Can data compressed on my drive be read on other [same type] drives? A. Yes, if you used the drive’s hardware based compression process and the other drive has the compression feature. All drives within a family of drives will employ the same data compression hardware. If another compression capable drive can read uncompressed data from your device, it will be able to decompress your compressed data too. Of course if you are using a software program to compress your data, the same software program must be available to the other drive. Q. What is decompression? A. An inverse procedure of the compression software or the compression hardware that returns compressed data to its original size and condition. Q. What is loss-free compression? A. Loss-free compression schemes allow full recovery of the original data. Only loss-free (lossless) algorithms are used for computer data, insuring that decompressed data output is exactly the same as the uncompressed data input. Q. What is lossy compression? A. Lossy compression is not applicable to tape drives used by computers. Lossy compression allows some of the original data to be lost. Lossy compression can be used where the data is meant to be interpreted by humans (not computers). Employing lossy compression schemes can provide a much better compression ratio. Lossy compression can be used for data such as graphic images, video, pictures, photographic images and music. Q. How do I compute my Compression Ratio? A. Compression ratio is the ratio of a file’s original size divided by its compressed size. A compression ratio of 2:1 means the compressed file is ˝ as large as the original file. | ||
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