next up previous contents
Next: READBACK TESTS Up: Tape Management Previous: TAPE LENGTHS and PASS   Contents


The amount of data that can fit on a tape depends on the length of the tape, the tape speed, and on the number of passes. The Mark III and VLBA systems make longitudinal recordings using many heads, all mounted on a head stack. The width of each head is 38 microns and the ``head pitch'', or spacing between heads, is 698.5 microns. This difference allows many passes to be recorded with each head simply by shifting the whole head stack over by something more than 38 microns between passes (48 microns is used on the VLBA). The Mark III systems typically use 12 passes while the VLBA and Mark IV use 14.

Many recording modes do not use all of the heads at once. Mark III systems use 28 heads while VLBA and Mark IV systems use 32 (they all actually use the same 36 head headstack so the head pitch etc is the same). Mark III Mode A uses all heads in a pass. Modes B and C use half the heads while Mode E uses a quarter. There are many VLBA modes, but they typically use a quarter, half, or all heads, although lesser options are available. If not all the heads are used in a pass, more passes can be made at the same head position, increasing the total time over which data can be recorded on tape. The setup file parameter TPMODE gives the number of passes that can be recorded at each head position. SCHED will figure it out if it is not given and will report it in the summary.

If multiple setups are used in a project, SCHED will figure out which uses the minimum number passes per head position (most tracks per pass) and will use that number of passes per head position for all setups. This wastes tape because some tracks never get recorded, but the alternative is a bookkeeping mess. Therefore, it is strongly recommended that all setups used in a project use the same number of heads per pass (have the same TPMODE).

It is perfectly possible to use setups that record at different speeds in the same schedule. As long as they all use the same number of heads, the tape will be used efficiently. However, a change of recording speed causes the VLBA correlator to need a new job. This is not a fundamental problem, but there is overhead for each job which causes work for the correlator staff. Current guidelines on how often changes of this sort can be made can be found in the guidelines for scheduling document found by going to the VLBA home page from the NRAO home page on the WWW. As of late 1996, changes more often than every 2 hours are discouraged. Note that using different tape speeds in different subarrays at the same time will preclude simultaneous correlation and will present the current job script making program with bookkeeping problems it is not equipped to handle. Don't do it!

The total time that can be recorded on a tape is the time per pass times the number of passes per head position times the number of head positions. The following table gives the total time per tape, and also the total bit rate, for various combinations of these parameters.

  Pass/head   Heads     Track bit   Passes  Tape time  Total bit
   position  recording  rate (Mbps)                   rate (Mbps)
       1        1           2         14     20:32:00      64
       1        1           4         14     10:16:00     128
       1        1           8         14      5:08:00     256
       1        2           8         12      4:24:00     512
       1        2          16         12      2:12:00    1024
       2        1           2         28     41:04:00      32
       2        1           4         28     20:32:00      64
       2        1           8         28     10:16:00     128
       4        1           2         56     82:08:00      16
       4        1           4         56     41:04:00      32
       4        1           8         56     20:32:00      64
   All tapes are assumed to be 17600 ft long.
   All recordings are assumed to be done at high density.
   Modes with 2 heads are MarkIV only.
   VLBA uses 2 drives at 256 Mbps each for 512 Mbps.

For mostly historical interest, below is the old version of the table which includes the tape times for the low density and/or thick tapes. The 512 and 1024 Mbps modes were not available at the time.

     TOTAL RECORDING TIMES per TAPE (14 Head positions, 32 Heads) *
  Tape length   Format/den
  Bit rate per track
     (feet)                  (2 Mbps)   (4 Mbps)   (8 Mbps)
  1 Pass/head pos.  Bit rate: 64 Mbps   128 Mbps   256 Mbps
     17600      All/High     20:32:00   10:16:00    5:08:00
     17600      VLBA/Low     12:19:12    6:09:36    3:04:48
     17600  Mark III/IV/Low  12:09:52    6:04:56    3:02:28
      8800      VLBA/Low      6:09:36    3:04:48    1:32:24
      8800  Mark III/IV/Low   6:04:56    3:02:28    1:31:14

  2 Pass/head pos.  Bit rate: 32 Mbps    64 Mbps   128 Mbps
     17600      All/High     41:04:00   20:32:00   10:16:00
     17600      VLBA/Low     24:38:24   12:19:12    6:09:36
     17600  Mark III/IV/Low  24:19:44   12:09:52    6:04:56
      8800      VLBA/Low     12:19:12    6:09:36    3:04:48
      8800  Mark III/IV/Low  12:09:52    6:04:56    3:02:28

  4 Pass/head pos.  Bit rate: 16 Mbps    32 Mbps    64 Mbps
     17600      All/High     82:08:00   41:04:00   20:32:00
     17600      VLBA/Low     49:16:48   24:38:24   12:19:12
     17600  Mark III/IV/Low  48:39:28   24:19:44   12:09:52
      8800      VLBA/Low     24:38:24   12:19:12    6:09:36
      8800  Mark III/IV/Low  24:19:44   12:09:52    6:04:56

 *  Systems with Mark III hardware use 28 heads and 12 head positions
    so the recording times are lower.

If the tape allocated is insufficient to record for the whole allocated time at the desired bit rate, there are several options. The most common is to use a duty factor of less than one -- to stop the tape for some of the time. Otherwise, it is necessary to cut down the bit rate or to request more tapes (good luck). Some periods of stopped tapes are desirable for readback tests.

next up previous contents
Next: READBACK TESTS Up: Tape Management Previous: TAPE LENGTHS and PASS   Contents
Craig Walker 2014-06-17