🚀 go-pugleaf

>
>On  Mon, 25 May 92 12:10:42 ADT  Regis BOSSUT <RPARBS@FRFUPL11.BITNET>
>writes:
>
>> a) Is it somewhere possible to make zero seeks 0ms time consuming, so
>> that I am able to transfer data at the 2400 kB/s rate for a single
>> cylinder ?
>
>I cannot speak for the VM/CMS environment, however, have you looked
>at using Read Multiple count/key/data to read an entire track at time?
>If the command were correctly chained from a read record zero with
>multi-track on you might be able to read an entire cylinder with a
>single channel program.  Of course, in MVS multi-track operations are
>not allowed unless the data set is allocated on a cylinder boundary.

Reading a whole track or successive records is almost equivalent. The
main drawback of reading entire tracks (with MT off) is that you have to
wait for 1 rev between tracks in order to fetch record 0 of next track.

We don't have here the 3380 or 3880 manuals, so I have to rely on your
knowledge. Can someone send me examples of channel programs reading multiple
tracks at a time (with MT on) ?

>
>> c) The CCW 'Set sector' needs one byte which is different from the
>> record number. What is the meaning of this sector number ? Tables for
>> sector numbers corresponding to record number can be found in module
>> DMSDIP.
>
>Set sector allows the device to disconnect from the channel until the
>sector comes under the read/write heads at which point the device
>attempts to reconnect to a channel to transfer data.  If all channels by
>which the device can transfer data happen to busy when the sector comes
>by then the device has to wait an extra revolution to try again (thus
>the term RPS miss.)  On older control units and devices, the device
>had to reconnect on the same path as the original request, however,
>newer configurations allow the device to connect over whichever path
>happens to be not busy when the chance comes.  There are well known
>"rules of thumb" for how busy channels can be on average before you
>begin to suffer from RPS misses.  Of course there is nothing that
>requires that your channel program issue a set sector and you will
>never have an RPS miss, however, your channel utilization will begin
>to look very bad and overall throughput will suffer.  VM may not allow
>you this degree of control and potential for abuse, I don't know.

During my investigations, I incorrectly programmed the 'set sector'
with a value of 0: RTMSF showed a disconnect time of 0 and large CN times, but
VM did not disuade me to do that...

My question might not be explicit enough. How many sectors are there on
a track ? Are they equally spaced ? How can I calculate the sector number
from the record number ?

/Regis BOSSUT

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