[TUHS] File-as-record (was: Happy birthday, Dennis Ritchie!)

Paul Winalski paul.winalski at gmail.com
Tue Sep 12 03:26:48 AEST 2017


On 9/8/17, Greg 'groggy' Lehey <grog at lemis.com> wrote:
> On Saturday,  9 September 2017 at 13:16:30 +1200, Wesley Parish wrote:
>> 'fraid so. The Unix directory structure and the correlating
>> free-form file competed with the file-as- record-structure and
>> directory-as-record-structure in the seventies and eighties. The
>> competition had finished by the nineties, and hardly anybody
>> remembers it now.
>
> Sorry, I don't understand this.  Can you give an example of
> file-as-record and directory-as-record?  Some of it suggests MVS, but
> not quite.

At least in the IBM world, disk drives originally didn't have their
tracks formatted into fixed-length sectors as they are today.  Early
on in the business world, everything was record-oriented.  Except for
the very largest ones, computers didn't have the either the speed or
the memory to do database processing as we now know it.  For most
business shops, there was sequential file processing, random access
(where you had to know the cylinder and head position of the record
you wanted), and indexed sequential access (by the value of a key
field in the record).

Disks in those days (pre-mid 70s) were not formatted in fixed-length
sectors as they are today.  Instead, records could be arbitrary length
(limited by the capacity of a single disk track, of course) and had
three fields:  a count field containing the number of bytes in the
record, a key field for indexed access, and a data field.  This format
was called CKD (count/key/data).  On IBM disks, records were addressed
by a number of the form CCCHHRR, where CCC is the cylinder, HH the
head, and RR the record on the track (for the data cell drive, there
was also a two digit bin number).  The disk controller could also do a
"seek by key" operation.  This would scan the track for a record whose
key field matched the given key.  It was used to implement indexed
sequential access.

For System/360/370, the OSes for the smaller, slower systems, such as
Basic Operating System (BOS) and Disk Operating System (DOS and
DOS/VS) did not have an on-disk file system at all.  You had to
manually allocate the files on the disk and you used directives in job
control language (JCL) to tell the OS where your file was and to
assign it a logical unit number for the program to use (FORTRAN's I/O
reflects this).  The OSes for larger systems such as OS/MVS (and
perhaps its predecessor, OS/MVT?) had a catalog file that could
automate the process of assigning files to locations on disk.  This
was the closest these OSes came to a modern directory-oriented file
system.

In the business world this is not as grim as it might seem.  Most
businesses had a small number of files that didn't change very often.
For example, a college has accounts payable/receivable, student
academic records, alumni/charitable giving, and payroll and personnel.
These get allocated once and, provided the initial disk allocation is
sufficient, don't have to move.

On the programming side, there wasn't either the memory capacity or
processing power to implement a modern disk file system.  One of the
first computers I worked with was a System/360 model 25 running
DOS/360.  The machine had 48K of core memory, 12K of which was for the
OS, leaving 36K for programs.  No virtual memory.  The OS had an 8K
resident kernel and two 2K overlay areas:  the physical transient area
(for controlling hardware; the forerunners of what we now call device
drivers) and the logical transient area (for the parts of program I/O
that had to operate in privileged mode, and for most OS system service
functions).  For I/O, each program contained library routines called
"access methods", which took logical file I/O operations (open, close,
read, write, seek, etc.), built the appropriate I/O channel programs,
and executed the EXCP (execute channel program) OS primitive.  The
access methods operated on logical I/O unit numbers, which as
previously noted were associated with places on-disk via JCL
statements before the program started execution.  The access methods
were SAM (Sequential Access Method; records read/written in on-disk
order), DAM (Direct Access Method; allowed seeking to an arbitrary
record using CCCHHRR address), and ISAM (Indexed Sequential Access
Method; allowed seeking to a record by key, and then reading/writing
sequentially from there).

The access methods and their capabilities mirrored the CKD disk format
of the time.  As disk hardware became denser and faster, and computers
got more memory, it became more efficient to format the disk as a set
of fixed-length sectors, which could be packed more densely and
accessed faster than CKD records.  Record blocking and unblocking was
moved to the OS or application.  Indexed access could be accomplished
a lot faster using B-trees in the file.

UNIX took the final step in this evolution by providing exclusively
byte-stream I/O in the OS, and leaving record processing up to the
application.  Those of us from the record-oriented world found this a
pain in the butt at first (application programmers were forced to roll
their own record processing, something that the access methods did for
you in the OS/360 world).  But by 1990 it was generally recognized
that stream I/O is the more appropriate OS primitive.

-Paul W.


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