tedj@hpcilzb.UUCP (11/26/87)
What is a "streaming tape drive"? What other kinds of tape drives are there? -Ted
roy@phri.UUCP (11/29/87)
In article <870052@hpcilzb.HP.COM> tedj@hpcilzb.HP.COM (Ted Johnson) writes: > What is a "streaming tape drive"? > What other kinds of tape drives are there? Tape drives come in two basic flavors; streaming and start-stop. Data is written on a tape in blocks, with gaps between them. In an start-stop drive, the CPU prepares a block of data to be written (or a buffer for data to be read) and nudges the tape controller. The controller starts the tape, and when it is up to speed, starts writing (or reading) the data one byte at a time. When it's done with the block, it stops the tape and tells the CPU it's ready for the next command. The inter-block gaps give the tape time (and room) to stop and then get up to speed again for the next block. The problem is, the tape spends a lot of time starting and stopping. To fix is to keep the tape moving (i.e. streaming) between blocks. You start the same way; the CPU prepares the data buffer and tells the tape controller to go at it. The controller starts the tape moving, does the read/write, and tells the CPU it's ready but keeps the tape moving, depending on the CPU to have the next buffer ready by the time the gap is past the head. If the CPU can keep up, you save a lot of time (not to mention wear and tear on the drive and the tape itself). If the CPU can't keep up, the controller has to stop the tape and wait for the next command; unfortunately, by the time it realizes this, the tape has gone too far and the controller has to reverse it to get back into the gap. I don't have the specs in front of me, but I think the maximum gap for 1600 bpi tape is 3/4"; if your tape moves at 25ips (fairly slow) you zip through a gap in 30 ms. You mileage will vary for other densities and formats. A reposition takes much longer than a normal start-stop cycle and is the bugaboo of all streaming tape drives; if you can keep it streaming, you win vs. start-stop, but if you can't, you loose big. Most (all?) streamers have a start-stop mode to fall back to. My TU-80 (horrible beast) has three modes: 100 or 25 ips streaming, and 25 ips start-stop. With an RA-81 (another horrible beast) on a Vax-11/750, and the standard utilities, there is no way you can keep the tape streaming and unfortunately, the decision to switch modes is made in the controller; it keeps trying to stream, blows a gap, repositions, falls back to start-stop, and then decides to take another try at streaming. With Don Speck's multi-process triple-buffered dump, we can get it to stream at 25 ips pretty reliably, but the stupid controller keeps trying to attemp 100 ips, and fails. I hope that answers your question. -- Roy Smith, {allegra,cmcl2,philabs}!phri!roy System Administrator, Public Health Research Institute 455 First Avenue, New York, NY 10016