One of the more interesting replies to my recent discussion of NetApp's dangerous VTL--which NetApp apparently feels is likely to cause significant loss of backup data--concerned the concept of "tape smart sizing". To quote NetApp's CTO: "I can also confirm that Tape Smart Sizing works exactly as advertised."
Since I dismissed it as little more than a David Blaine type trick, it might be useful to ask: what exactly is advertised? Again, just quoting NetApp:
"[tape smart sizing] sample[s] the data for compressibility as it’s being written to the VTL and then dynamically adjust the size of that virtual tape to match the estimated compressed capacity of the physical tape it will be written to. The LTO-3 virtual tape is now automatically adjusted depending on what type of data is being written to it and how it will compress in the Real-Tape World. Welcome to not wasting money or space on extra physical tapes when using a VTL."
Incidentally, NetApp also claims that:
Every other VTL out there requires you to to set a fixed size on the virtual tape when configuring the virtual library.
The consequence of this is, according to NetApp, that if you write 200 GB data that is 2:1 compressible to any other VTL, you will use 200 GB of disk space; write it to tape, and you use 100 GB of tape (because tape is compressed). So, it seems that you would require 2 virtual LTO-1 cartridges. Take those 2 virtual cartridges, and they will each half fill a physical cartridge. Thus, wasted tape.
So we have two different bloggers claiming that tape smart sizing is useful, and unique in terms of its ability to save space of physical media. Basically, that "every other VTL" will require that you waste physical tape space as described above.
Let me state this unequivocally, and for the record: this is not true. EMC's virtual tape products have no such issue.