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No, that's not possible, as a snapshot is not new disk space, but existing areas of that LV which are not modified. Instead all new data is put onto space with the VG, and all the while the size of changes between the snapshot and the current data is less than the size allocated to the snapshot, it will be valid. You can take a small snapshot and then dd it to a seperate location if that's what you want to achieve.
snapshot space needs to be reserved within each VG, as above, you can't snap to somewhere else, as there is no data actually written anywhere. the point of a snapshot is to freeze a copy in time, so you would, assuming your tape is fast enough, write directly from the snap to the tape. you do not need intermediary disk space if you can write to tape before the snap invalidates itself, which for a database, depends entirely on how it's used during that time. I think you'd do well to do some formal study of LVM and what technologies actually back up the concept of an LVM snapshot, and it should fall into place much better.