I don't think you can start processing a compressed file from the middle, unless for some simple compression algorithm. Your log file is most likely compressed with compress or gzip that use the LZ77/LZSS sliding window compression algorithm or its variants. The compression algorithm works on building dictionary of words, or phrase book, of repetitive patterns in a look ahead window, and re-encode the matching phrases with a token in the dictionary. And the encoding window moves forward. So I don't it is possible to decode a LZ compressed file backwards. Unfortunately you will have to uncompress the entire file first.