freeman@spar.SPAR.SLB.COM (Jay Freeman) (04/09/88)
I have found an apparant peculiarity of the tape backup software that comes with the Apple 40SC Tape Backup system, knowledge of which might save users a little time and a few tape cartridges. (I hope I have this one figured out, perhaps some Apple net-watcher will correct me if I am wrong.) Briefly, I think that the "Backup Volume" command backs up entire tracks at a time, and either (a) (of course!) backs up every track that has undeleted data in any sector (even if most of the track is full of deleted data) or (b) backs up every track from track 0 up to and including the highest-numbered track that has any undeleted data in any sector (even if some tracks in between contain nothing but deleted data). The impact here is that if your undeleted files are scattered around on your hard disc (for example, if you have just deleted several megabytes of temporary files), the "Backup Volume" command will write to tape many more bits than there is valid data on your disk; thereby taking more time and perhaps using more tape cartridges than would otherwise be necessary. The work-around is to do a complete "Backup Files", reformat the hard disk, and then "Restore Files". My evidence for this phenomenon comes from my Mac II, which has an (Apple) 80 MByte internal hard disc and the (Apple) 40SC Tape Backup unit. Recently, with only 30 MBytes on the disc (as attested by "Get Info"), I found that the tape backup software insisted on using two cartridges for a volume backup. The cartridges are supposed to be 38.5 MBytes, so that was puzzling, the moreso because I had just done a complete files backup to one cartridge with no problem. After trying various interim hacks (reboot, rebuild desktop) to no avail, I decided to reformat and restore the files. I thought it would be wise to make an extra file backup before reformatting, and I did so using the same cartridge that was not big enough for a volume backup -- even more puzzling. But after reformatting and restoring files, the volume backup went onto one cartridge with no problem. Furthermore, the amount of time for the volume backup decreased to about 32 minutes (roughly right for 30 MBytes), whereas before the software had interrupted the backup and asked for a new cartridge after 38 minutes, when the backup was only about 95 percent complete. I had indeed just deleted many megabytes from my file system. Based on the speed of the volume backup, I conjecture it is a track-based backup, and the hypothesis given at the start of this item follows. (It's really just conjecture.) (Incidentally, that's not a bad way for a volume backup program to run; this posting is not a bug report, just a description of an undocumented behavior.) Anyhow, it looks as if occasional reformat/restore will speed up volume backups. Perhaps one of the commercial utilities that groups all of a file's blocks close together on disk would do as well. Other details: I was running system 5.0 (ordinary finder -- not multifinder), had recently reformatted the disk (for other reasons) with <forgot name> version 1.5, that came with system 5.0, and was running version 1.1 of the tape backup software. I was running the tape backup software off a floppy disc, with that floppy as the system disc, so both the file and volume backup were indeed backing up the entire hard disc. (File backup won't back up open files, like the active system file and the tape backup program itself.) As for the "haunted hard disc" header, well, obviously, any manifestation of the previous existence of a deleted file is a ghost, and obviously, a hard disk full of ghosts is haunted. Right? Right! :-) -- Jay Freeman <canonical disclaimer, these are my opinions only>