On Sat, Jun 15, 2002 at 12:01:26PM +0100, Niall O Broin wrote: > > 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.
Don't tell it the capacity is 24GB :), More like 11.8 (post compression). > 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. Why should they be slower? Ungzipping is much faster than gzipping. I ran a small test, a 200MB cdrom image. gzipping, 99 real seconds, 53 cpu seconds (~2MB/sec) unzipping, 23 real seconds 9 cpu seconds (~8MB/sec) I'm also using dds-3. My tapetype definition shows a transfer speed of 1MB/sec. Looks like gunzipping should be able to keep up with the tape drive. > > 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 ? I don't think it is a dds-3 issue. More like a specific format and/or hardware vendor and/or model issue. I may be wrong, (please correct me if so), but I don't recall any instances of this problem with dds-3. I have an HP 6 6 tape changer. I know when running the tapetype program early on I used the same tapes with HW and no compression. My results were clear that my drive did not exhibit this problem (i.e. capacity with HW compression was less that without irrespective of previous use of the tape). jon -- Jon H. LaBadie [EMAIL PROTECTED] JG Computing 4455 Province Line Road (609) 252-0159 Princeton, NJ 08540-4322 (609) 683-7220 (fax)
