But how would you then label the tapes so that the data would be consistent?
Wouldn't this mean that in order for the data and the parity to be consistent that you would only be able to write the data in (n-1) "chunks" - i.e. if you had 3 LTO-8 tape drives, the total raw, uncompressed capacity would be 36 TB, with only 24 TB that's "usable" since 12 TB would be used to store the parity data.
That also means that those three tapes will always have to work in conjunction with each other, so when you want to read data from this group of tapes, you will have to load all three tapes into the three tape drives simultaneously in order for this to work, correct?
And if you have 5 drives, then you would have to load all five tapes in at the same time for this to work, right? (etc. etc. as you increase the number of tape drives)
Conversely, why do that when you can just use par2 instead? You write the parity data to each individual tape so that with a single drive, the tape will contain all of the information self-contained within the tape, and if you are worried, you can also create/write parity of the parity data onto the tape as well.
Isn't that what double parity does?
Single parity is parity on your original data/file.
Double parity would be parity on top of your original data/file AND the first set of parity data that was generated/calculated by the RAID HBA or OS/CPU if you're using software RAID?
And if you are worried about the tape being a single point of failure (e.g. the tape is burned up in a fire), you can have x number offsite, colocated copies for disaster recovery.
This way, you don't have to wait for n number of tape drives to synchronize (and loading n number of tapes simultaneously) to be able to move data around. Instead of waiting to eject and load 5 tapes, while one tape is being ejected, another could already be loaded and it can start reading the next tape already. Whereas with RAIT, you'd have to synchronize the tape drives so that the group of tapes will work as a single unit together.
(And the load/unload mechanism isn't exactly the fastest, even with LTO-8. It can still take upto a minute with my current drive (not a tape library) to bring a tape up online so that it can be read/written to.)