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Could that be mediated with many read heads at different locations?

by yters 6y ago

dragontamer 6y ago
Tapes only have one head. Otherwise, they couldn't wind or rewind. It'd probably be mediated with a thicker tape with a thicker head: the thicker the tape, the more "data per inch" passes through the head, which allows bandwidth to increase. That would change the LTO-tape dimensions however, so that probably isn't an option. Fortunately, by shrinking down the size of data (more data per square mm), the bandwidth necessarily increases. So maybe this technology will have a faster read/write speed than today's tech.
- klodolph 6y ago
  I don’t see a reason why you couldn’t have multiple heads. As it stands, in order to write a complete LTO tape, you make multiple passes across the tape. A second write head would allow you to make fewer passes across the tape to fill it. The data on an LTO tape is written in a serpentine manner, or boustrophedon if you prefer, with overlapping series of tracks separated by the servo tracks used to position the head.
  - JustSomeNobody 6y ago
    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.
    - klodolph 6y ago
      > 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.
      - dsr_ 6y ago
        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.
        klodolph 6y ago
        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.

Could that be mediated with many read heads at different locations?

by yters 6y ago

Could that be mediated with many read heads at different locations?

Tapes only have one head. Otherwise, they couldn't wind or rewind. It'd probably be mediated with a thicker tape with a thicker head: the thicker the tape, the more "data per inch" passes through the head, which allows bandwidth to increase. That would change the LTO-tape dimensions however, so that probably isn't an option. Fortunately, by shrinking down the size of data (more data per square mm), the bandwidth necessarily increases. So maybe this technology will have a faster read/write speed than today's tech.

klodolph 6y ago
I don’t see a reason why you couldn’t have multiple heads. As it stands, in order to write a complete LTO tape, you make multiple passes across the tape. A second write head would allow you to make fewer passes across the tape to fill it. The data on an LTO tape is written in a serpentine manner, or boustrophedon if you prefer, with overlapping series of tracks separated by the servo tracks used to position the head.
- JustSomeNobody 6y ago
  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.
  - klodolph 6y ago
    > 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.
    - dsr_ 6y ago
      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.
      - klodolph 6y ago
        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.