On Thu, Oct 23, 2014 at 10:34:38AM -0400, Gene Heskett wrote: > On Thursday 23 October 2014 01:28:01 Tom Robinson did opine ... > > If you are feeding the tape device compressed files, and the drives > compressor is enabled too, this will quite often cause file expansions on > the tape itself. The drives compressor, because it is intended to handle > the compression on the fly, is generally not sophisticated enough to do > any further compression and will add to the datasize, expanding what > actually goes down the cable to the drives heads.
Tom is using an LTO drive (-5 I think). Most modern tape drives, including all LTO's do not exhibit the bad behavior of the DDS drives with their run-length encoding scheme. IIRC, they have enough cpu smarts and memory to first collect the data in memory, try to compress it to another another memory buffer, and if it is enlarged the block is saved "uncompressed". Note, instead of a flag at the start of the tape indicating compressed or uncompressed, there is a flag for each tape block. jl -- Jon H. LaBadie [email protected] 11226 South Shore Rd. (703) 787-0688 (H) Reston, VA 20190 (609) 477-8330 (C)
