On Friday 22 November 2002 07:48, Niall O Broin wrote: >On Friday 22 November 2002 11:01, Paul Bijnens wrote: >> > If there is one flipped bit (read error), everything >> > compressed with gzip after that position is lost. With HW >> > compression you can continue restoring after that position >> > plus some overhead. >> >> This comes up now and then, but I have my doubts about it. It >> assumes that on the tape are only your databits, and there is no >> error detection > >To paraphrase a best selling book - "blessed are those who have > not seen, and yet believe" > >This has happened to me. My own fault in that cabling/termination > on the SCSI chain was not up to scratch but it led to very > occasional bad bits going to the tape which didn't give any > errors when writing but when attempting to read (e.g. to verify) > I got GUN tar errors from the gzipped tar file. Tar can > resynchronise after bit errors but it seems that gzip cannot. > This had me puzzled for a while until I redid the cabling and > termination on the SCSI chain when all became sweetness and > light. > > >Kind regards, > > >Niall O Broin
Which should be reason enough to see if the use of bzip2 could be incorporated into amanda. AIUI, bz2 can re-synch, losing only the actual file that the error effected. Early on, there were a couple of instances where bz2 dropped the ball, but its been stable for at least a year now. Since its compression is even better than gzip's best, the question becomes "can amanda tolerate the increased compression time that using bz2 would result in?" Machines keep getting faster all the time and eventually either the drives get faster or we have all sorts of time leftover to do the compression. I've set an order 'SSSsss' into my amanda.conf to make it start with the big ones. So the first one to be processed was then 1.5 gigs original, dumped w/o compression (it was music, mp3's and oggs) and flushed to tape. By the time that was flushed to tape, the other 30 DLE's, 2/3rds of them compressed by server-best, were all done and waiting in the write tape queue. I don't believe the drive would ever have to pause if I could switch it to using bz2, at least under last nights conditions. Food for thought, since bz2 can recover a dropped bit or several... -- Cheers, Gene AMD K6-III@500mhz 320M Athlon1600XP@1400mhz 512M 99.19% setiathome rank, not too shabby for a WV hillbilly
