Network tape restore is faster than disk to disk copy

As an asdide, note that 100MB/min is far below the minimum speed for tape streaming with LTO 3, so you're probably losing a fair amount of capacity with the tape stopping and starting (ie you're probably getting better than 1.5:1 compression, but this is lost in gaps in the data on the tape). This will probably be rather worse with LTO 4, as I think the minimum speed has gone up.

Disk - Disk - Tape will help with the minimum speed problem, and will give you some capacity for free.

If you're not doing it, strongly consider some kind of scheduled defrag of the disks on the servers you're backing up. 1000 MB/min is not a great level of throughput for gig ethernet on reasonably modern hardware. I'd expect that on even 2 year old machines you should be able to get 1800MB/min (that's only reading from the server disk at 30MB/sec), so there's scope for improvement.

Edit: For LTO 3, you really want a 256KB block size for best performance.

WRT shoe shining, There's no time for the tape to rewind if the buffer runs empty briefly, so it'll leave a gap on the tape.

Here are some numbers that you are probably looking for, as quoted by Jeff Dean, a Google Fellow:

Numbers Everyone Should Know

L1 cache reference                             0.5 ns
Branch mispredict                              5 ns
L2 cache reference                             7 ns
Mutex lock/unlock                            100 ns (25)
Main memory reference                        100 ns
Compress 1K bytes with Zippy              10,000 ns (3,000)
Send 2K bytes over 1 Gbps network         20,000 ns
Read 1 MB sequentially from memory       250,000 ns
Round trip within same datacenter        500,000 ns
Disk seek                             10,000,000 ns
Read 1 MB sequentially from network   10,000,000 ns
Read 1 MB sequentially from disk      30,000,000 ns (20,000,000)
Send packet CA->Netherlands->CA      150,000,000 ns

It's from his presentation titled Designs, Lessons and Advice from Building Large Distributed Systems and you can get it here:

• Dr Jeff Dean Keynote PDF or on slideshare.net

The talk was given at Large-Scale Distributed Systems and Middleware (LADIS) 2009.

Other Info

• Google Pro Tip: Use Back-Of-The-Envelope-Calculations To Choose The Best Design
• Stanford 295 Talk Software Engineering Advice from Building Large-Scale Distributed Systems

It's said that gcc -O4 emails your code to Jeff Dean for a rewrite.