On Sat, Jun 15, 2002 at 11:35:38AM -0400, Gene Heskett wrote: > Time to compress, or uncompress isn't normally that big a deal > unless the tape server is a 66mhz P1 or some such slowpoke.
Even with a fast machine, compressing 8GB is going to take some time. Of course that doesn't much matter because it'll be done on the client at night anyway, but I'm more concerned by possible slowdown on restores which of course will be dependent on the speed of unzipping. > Doubtfull unless the server is a 1.5+ ghz. OTOH, extraction time is > normally lless than compression time by a large measure. I decided that a little empirical testing was in order. On a sample client gunzipping a file goes at about 3MB/sec. elapsed time. Data goes to my tape drive (from the Amanda report) at a max of about 1.5M / sec so presumably it'll come back off the tape at about the same rate so gunzip should be comfortably able to keep up with it. In fact a little further consideration makes me think that compression may actually speed up restores because I'll have to stream less data off the tape which is of course the bottleneck in a restore operation. > Using a mag tape eraser just wrecks the tape as it destroys the > hidden from you factory tape header. The tape cannot be mounted or > recognized by the drive, ever. Cancel eraser purchase so :-) > To clarify, the tapes OWN header that contains much of this > compressed or not data will remain in the compressed state, but the > flags that determine the status of the data on the tape will be > reset and the rest of the tape will not be written in the > compressed mode. I rescued about 20 dds2 tapes by doing this in a > Seagate 4586np drive, aka a CTL-96. > > The compression led will come on while the label is being read, but > will then go back off. I'm dubious about the logic here. The lore, as I understood it, was that there was a magic header written to the tape which said that it was written compressed and ever after this header's word was law, and even when the tape was rewritten completely it stayed in compressed mode once so written, regardless of the commands sent to the tape. However, what you have done here is simply set the tape to uncompressed and then write AFTER the amanda label. When it gets down to it, there's no difference between that and simply using the tape for a backup. > And I'm not going to claim it works with any drive, these @^%# dats > seem to write their own laws... Well indeed - YMMV and all that. Regards, Niall
