I'm seeking insights into just how data is accessed within a 3590 tape,
and more specifically, how ADSM accesses it.
    We currently have an AIX system with a client and server on that same
host, with fast/wide SCSI connections to 3590 tape drives in a 3494 tape
robot.  All data (some 10 GB) is currently contained on one 3590 tape.
We find that in retrieving an HSM-managed file (small) which has been migrated
to tape that it takes a nominal 3 minutes to get the file back to disk, that
time including the 32 seconds that it takes for the tape mount to complete.
    More recently we had an experience where we needed to retrieve a bunch of
files, averaging 1MB in size, to copy to a non-ADSM 8mm tape.  All of this
data is on a single 3590 tape, as stored by ADSM, and in total is some 1.5GB.
We started that job yesterday and, some 22 hours later, it is less than half
done.  The 3590 tape has remained mounted all the time.
    It's obvious that tape, be it 3590 or any other kind, makes a poor random
access medium.  We just didn't expect it to be this bad - almost
debilitatingly bad.  These experiences beg some questions which someone may
have answers to...

1. Just how does data access occur on a 3590 tape?
   The usual 3590 manuals go no further than quoting specs (128 data tracks on
   a tape, accessed 16 at a time, making for 8 pathways; plus servo tracks;
   etc.).  Does anyone know of a written description, preferably available
   online, which delves into the mechanics of going after data on a 3590?
   (Can a host program, for example, take a shortcut to one of the pathways
   rather than plow through the whole tape to get to a given file.  Is there
   any kind of direct addressability on 3590s?  Etc.)
2. Just how does ADSM manage data on 3590 tapes?
   I can find no information on this.  Does ADSM store database information as
   to where on a tape a given file lives, or is it limited to knowing that
   it's somewhere on a given tape?
3. Is there a way to get from ADSM a list of the files as they are stored on
   the tape?
   If we could get this we could go after the files in the order in which they
   appear on the tape and make retrieval sequential instead of random, thus
   dramatically reducing retrieval time (and drive monopolization and tape
   head wear, etc.).

    Any information in these areas would be appreciated.
             Richard Sims, Boston University OIT