[Info-vax] strange backup behavior

[B Hobbs]

On Aug 7, 12:27 pm, glen herrmannsfeldt <g... at ugcs.caltech.edu> wrote:
> B Hobbs <bdhobb... at acm.org> wrote:
>
> < I've noticed that the more data on our backup SDLT tape, the longer it
> < takes to add additional files to the same tape.  When a file of about
> < 35 blocks (BACKUP-STUFF.COM;67) is backed up at the beginning of the
> < tape, that backup takes less than a minute.  Backing up the same file
> < after about 45 gigabytes of other backup files takes over 30 minutes.
>
> As I understand it, LTO knows where files (tape marks) are on the
> tape using a small EEPROM in the cartridge.  I don't know that SDLT
> does that, which would mean reading the tape until the end, then
> start writing.  I don't know so many details about SDLT, though.
>
> -- glen

SDLT does something similar, but uses the tape before the beginning of
tape (BOT) hole to store info instead of using EEPROMs and short range
radio.

I believe that the SDLT drive reads the info before BOT into memory
when a tape is loaded into the drive, uses that info to speed up
finding files, updates the info in the drive's memory as files are
changed, and then rewrites the info from memory to the before BOT area
as the tape is being unloaded from the drive.

I think the SDLT drive would not update the info on tape between
backup steps unless something was explicitly telling the drive to
update that area.

The Quantum web site appears to have some pretty good info on SDLT
http://www.quantum.com/ServiceandSupport/SoftwareandDocumentationDownloads/SDLT320/Index.aspx#Documentation