...I understood restore performance suffered with a VTL - the way it has
been described to me is that, should a restore need to come from a volume
that has been destaged from disk to tape in the VTL, then a restore of a
single file from the volume  would first have to wait for the vtl to
rebuild the tape on disk? Or have I got the wrong end of the stick?
 
Um.  Both.

Most VTL's are disk-only devices that emulate tape, and do not have the staging 
issue you describe.
 
Many VTL's will make restores FASTER  because the tape mount time goes from 
potentially minutes to a second or less.  (You also don't have to worry about 
collocating data in a VTL, so your migration times are generally faster as 
well.)
 
Now that goes with a caveat - you have to PIN YOUR VENDOR TO THE WALL and get 
documentation about throughput rates.  ALL VTL's work about the same way, but 
they all have different hardware inside the box, so you can get drastically 
different results.  You can easily create a case where restoring 1 VERY LARGE 
file will take longer on a slow VTL than with fast tape (Say a TS1120, which 
run get more than 100MB/sec.) 
 
It depends on 
       WHICH VTL you are talking about,
       the speed of the disk in it, 
       the size of the cache in it
      the speed of your SAN connection and/or HBAs
       compared to which tape drive, and 
      whether you are talking about restoring lots of little files or a few 
huge ones.
 
 
A VTS (don't they make this confusing?) is an IBM-only mixture of disk/tape 
that emulates tape.  It has to pull data off tape and stage it back to disk 
before you can restore.  Normally the VTS is used in a mainframe environment.
 
IBM also makes VTLs, the TS7510 and TS7520, for use in open environments.  They 
are all disk.