VMS vs. UNIX file system

[Jeff Siegal]

I'm not so much concerned with the issue of stream vs. record-oriented
file access.  One can always (perhaps not easily or quite so
efficiently, but if you need to get the job done...) be used to
simulate the other.

I think a more fundamental advantage of the VMS I/O system is that
QIO's can be queued to execute asynchronously (similarly for RMS
operations).  An AST (software trap) is delivered to your process when
the I/O completes.  The AST tells you which I/O operation completed,
and the error status.

On Unix, you can get asynchronous I/O by going through the buffer
cache, but this doesn't provide a clean way for the error status to be
returned (for example, NFS writes that go over quota can appear to
complete, but return the error later, when performing another
operation), and doesn't allow you full access to the device (for
example, the buffered tape device only provides 1024 byte records.

I believe certain VMS I/O operations are performed directly from/to
process memory, rather than being copied from/to the kernel, but this
is "only" a performance issue (and an arguable one at that).

Jeff Siegal