Re: DLT tape drive, Ontape and Informix 7.24

Paul Watson <P.Watson@LIV001.dss.gov.uk> offerred: +dpimpo@my-dejanews.com wrote: +> Informix tech support told us: Only requirement for tape drive to work with +> ontape is that it is an "auto-rewind" device... in other words, the ontape +> utility does not issue the rewind command. + +I don't believe this is the case. If you use a no rewind device on Sun and do +two backups one after the other and mt to forward to start of the second backup +then ontape -r rewinds the tape before the restore starts. However, I've never +tried this on anyother OS's. It is primarily an issue with multiple-volume backups, and with restores. When ontape switches to writing the second volume of an archive set, it first reads the header from the tape to be sure that the tape was actually switched (i.e. it won't write the second volume over the first volume), then "rewinds" the tape. It does the "rewind" by closing the device, which, on a normal (i.e. not no-rewind) driver will cause the tape to rewind. If you use a norewind driver in this case, ontape will read a bit of the tape, corresponding to the header it would write. It would then close the device, reopen it and start writing the new header. With the norewind driver, that header would be one header's worth into the tape, rather than at the start. If you tried to restore from that tape, the restore would fail. I don't recall offhand if the norewind is an issue with the first tape in a volume or not. It's been a while since I dove into the gritty details. +> IBM told us: no hardware is going to automatically rewind. Generally true, but very misleading. It isn't the hardware that is at issue, but rather the device driver used. The "standard" unix device driver for a tape will issue a rewind on close of the device. A norewind driver will not. Typically, a norewind driver will end in an "n", whilc the regular driver will not. (e.g. on an HP box, /dev/rmt/0h is the "rewind" driver, /dev/rmt/0hn is the "norewind" driver for the same tape drive.) Dave -- Dave Kosenko davek@summitdata.com Director of Training Services (732) 469-4070 Summit Data Group (an Informix Authorized Education Center)