SCSI Subsystem Proactive Maintenance

PROBLEM STATEMENT

The OpenVMS SCSI subsystem has become difficult to maintain, understand,
or extend and needs to be made simpler in all these categories.

Basic flaws which now exist have the following causes:

1. No comprehensive written high level design (architecture) nor component
designs exist save the code sources themselves. Some oral design tradition
exists, but essentially all key individuals who originated this tradition
have left, so that even the oral design tradition is weak and partial.  Much
of the subsystem was implemented with no available overall picture of the
subsystem at the top level. Thus no comprehensive design exists for VMS SCSI
now at all.

2. The code base has had a long history of changes in spite of this,
which have not been implemented consistently. Not surprisingly, component
interfaces are not clean or consistent, and information passed by side
effects of data manipulations is common.

3. The difficulty of understanding the environment of changes has made them
slow to implement and often fragile when implemented. Some areas remain
maintainable, but the difficulty of maintenance is growing, and the learning
curve for the code base is steep. This has led to schedule pressure which
has at times made it necessary to perform partial, not complete, fixes for
problems. 

As a result, the SCSI group has difficulty making fixes or enhancements to
the code base. People new to the SCSI group must undergo a long process of
code reading to get up to speed, which limits what resources can be applied
to designs or to reviews. Partly due to these problems, customers see many
desired features slowly or not at all. Most notably, support for wide SCSI,
extra SCSI LUNs, added features on disks and tapes, new device types, and
commodity SCSI devices is able to be added only slowly. Also, it has been
difficult to provide information needed for third parties to write new class
drivers for VMS SCSI or to be as quick as we should be to handle the
customer problem report backlog. The problems are thus visible both
internally and to customers.