Paul Koning wrote:
The reason for the interleaving on DECtape is the
start/stop time. To run non-interleaved at high speed you have to leave the tape running
(no "stop" commands) and you have to issue the next command quickly. RT-11
could do that; DOS could not.
When I attempted to evaluate how well a backup to tape
would work, I always included the verify portion to make
sure that the tape had been written correctly in addition to
making sure that the tape could also be read.
For the PDP-11/83 under RT-11, there were essentially
two choices: TK50 and TK70. Aside from the advantage
that the TK70 had over the TK50 with respect to the
capacity - (if I remember correctly) about 256 MB vs 32 MB,
the other problem was that it was found to be impossible to
steam the TK50 during a BUP verify operation. So in
addition to the TK70 being much faster in the first place
relative to the TK50, the verify operation was just not
feasible. Somehow, the PDP-11/83 could read both
the tape and the disk drive, then compare the two current
buffers while the alternate buffers were being read, using,
I presume, DMA requests to the two controllers to transfer
the data to the next pair of buffers.
Of course, RT-11 had special EMT requests to initiate the
read requests and then go away and do something else.
Obviously, I could have copied the BUP backup to a scratch
disk and verified the scratch disk against the original disk and
saved a great deal of time if the TK50 had been the only option.