I'm trying to decide whether or not to use software compression in my amanda configuration. At the moment, I'm backing up to a DDS-3 and I've told Amanda that the tape capacity is 15GB to allow for hardware compression. Of course I don't really know what level of compression the drive is able to achieve with my mix of data.
At the moment this is not an issue because the most data ever gets taped in one run is ~10GB but as I'm adding clients and disks this will increase and I'm concerned about maximising my use of the limited capacity of the DDS-3 drive. Obviously using software compression helps with this because Amanda knows exactly how big each compressed dump is and I can also tell her the truth about the tape capacity. However my big concern is restoring. I know that even with current processors gzipping several GB of data takes some time and the same applies in reverse - or does it ? Does it take significantly longer to extract one file from a gzipped tar file on a DDS-3 than it does to extract one file from an uncompressed tar file or can a reasonable CPU gunzip in a pipe as fast as the DDS-3 can deliver data ? And then of course there's the fact that I won't be restoring very often anyway, so the extra backup capacity obtained may be worth the price of slower restores. Anyway, my dilemma should be clear. I'd appreciate feedback from anyone else who's considered these issues. What did you decide, and why ? A second question that arises is the issue of existing tapes. I've read that once a DDS-3 tape has been written in hardware compressed mode, the tape is marked accordingly and will ever after be written with compression, no matter what the drive is told to do. I've also read that this mark can only be removed by using a magnetic tape eraser. Is this correct ? Regards, Niall O Broin
