"There's no bandwidth cost associated with transferring data TO Tarsnap"
Except for those of us who's outbound bandwidth isn't free and/or unlimited. It makes no difference what Amazon charges Tarsnap for it - I can't upload all of my 128G SSD every hour - I just don't have the bandwidth to do it, and if I _did_ have the bandwidth, it'd probably send me broke pretty quickly (or have my ISP throttle me or cut me off).
It's not a Tarsnap cost, but it can be a Tarsnap user cost, and I suspect cperciva considers that just as much a "real cost" as actual monetary expenses to Tarsnap.
The point of a dedupe-based service like Tarsnap is that while logically you upload the whole disk, physically only the changed blocks are sent to Tarsnap.
Except for those of us who's outbound bandwidth isn't free and/or unlimited. It makes no difference what Amazon charges Tarsnap for it - I can't upload all of my 128G SSD every hour - I just don't have the bandwidth to do it, and if I _did_ have the bandwidth, it'd probably send me broke pretty quickly (or have my ISP throttle me or cut me off).
It's not a Tarsnap cost, but it can be a Tarsnap user cost, and I suspect cperciva considers that just as much a "real cost" as actual monetary expenses to Tarsnap.