What data compression technique is used and can it be used all of the time — for example, with files that are already compressed?
All DLT tape drives use LZ1, a variant of the Lempel-Ziv compression algorithm. LZ1 was selected over IBM s IDRC because testing indicated that LZ1 provided higher compression and better efficiency. Though the DLT compression is specified as a 2 to 1 ratio, compression can vary depending on the input data. Binary files may compress at 1.5 to 1 while images from a Paint application may compress at 4.5 to 1. There may be no compression with data that has already been compressed with a Lempel-Ziv or more powerful method. Such compressed data, or very random data (including encrypted data), will likely expand by about 5%. IDRC compressed data usually gets compressed slightly more by LZ1. The only disadvantage to using compression all the time is that some files may expand slightly, as noted above. If in doubt, and if there is a significant amount of data, the Log Sense Read/Write Compression Ratio page can get used to monitor the compression rates that are actually achieved.
