...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.