[Info-vax] compressed SDLT capacity

[Bob Koehler]

In article <36d33800-965f-4e70-a1c9-c2a309939772 at u18g2000pro.googlegroups.com>, B Hobbs <bdhobbs18 at acm.org> writes:
> 
> I assume that when I do a INITIALIZE/ERASE, VMS sends a SCSI erase
> command to the drive and VMS does other stuff until the drive signals
> that the erase command is complete -- the drive does the work and the
> SCSI bus is quiet.

   INITIALIZE/ERASE is generic across a lot of things that don't
   implement an erase command.  There's a very good chance that it's
   writing a pattern to the tape.  If you use different options, I 
   think you can convince it to write more than one pattern.
 
> How does a DIRECTORY of the SDLT work?  Does the tape drive send all
> the data to VMS and VMS sorts thru the data to assemble something like
> a directory listing?

   DIRECTORY of a labeled tape reads data from the file headers.  On
   a labeled tape there are three physical files for each logical file.
   The first physical file contains the begining of file header.  The 
   second physical file contains the file data.  The third physical file 
   is the end of file header.

   In order to get directory information from a tape, VMS reads the
   first physical file, does a skip over the second file, and reads the
   third physical file, for each logical file that shows up once in a
   DIRECTORY listing.

   A lot of modern tape drives are optimized for data flow, not
   positioning.  Skipping files is a positioning operation.  There have
   been drives supported by VMS that did not implement a skip operation
   and VMS would actually read all the data in order to implement a
   skip.