What would happen when one head can't write out data fast enough (for some reason) and the tape has to slow down and back up. Now, instead of one head, you have multiple heads that have to resync and get going again.
I dunno, if multiple heads were worth it, I think it would be common.
> What would happen when one head can't write out data fast enough (for some reason) and the tape has to slow down and back up.
The tape does not rewind in this scenario.
It is fairly common for the system to not be able to keep up with the full write speed of the tape. Just as a reminder, LTO7 has a write speed of 300 Mb/s, and filling a 300 Mb/s pipe for five and a half hours is no small task.
There are two ways the system deals with it. First, the drive will slow down the tape to match the data rate. Second, the drive can mark a section of tape as failed and then rewrite that data to the next part of the tape. Instead of rewinding and writing over a section of tape, you’re using up more tape this way.
Tapes, like hard drives and SSDs, come with a certain amount of extra capacity to allow for this as well as other write errors.
> I dunno, if multiple heads were worth it, I think it would be common.
I’m sure that it’s “not worth it”, yes, absolutely. But I don’t see major technical hurdles here.
One technical hurdle would be figuring out how to, say, fill an 800 Mb/s pipe for six hours straight. That’s hard. It gets harder when you realize that a single tape library might have as many as 64 or 80 tape drives. 64 drives, multiplied by 400 Mb/s, is 26 Gb/s.
If you have a 64 drive tape robot, you probably have 65 computers: one controlling the load/unload mechanism, servicing 64 servers each dealing with one drive. (Or perhaps 16 servers that each handle 4, or 8x8.) Each of them has its own high speed drive for cache and independent NIC.
That's part of the solution, yes, but it's difficult to get the whole system set up this way and achieve something near maximum throughput. My understanding is that most people who use tape don't get anywhere near maximum throughput over the long term. Naive use of drives is still quite common... things like backing up with tar, or dumping a database straight to tape, and it's easy to imagine buffer starvation scenarios.
Returning to the idea of increasing throughput on the drives by adding heads, I'm just doubtful that it would be the best way to improve overall system performance. For alternatives, you can either get engineers to optimize usage of existing drives or buy more drives.
I dunno, if multiple heads were worth it, I think it would be common.