Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!ucbcad!pasteur!ucbvax!utorphys.BITNET!SYSRUTH
From: SYSRUTH@utorphys.BITNET
Newsgroups: comp.os.vms
Subject: buffered vs. direct I/O's
Message-ID: <8801181707.AA16029@gpu.utcs.toronto.edu>
Date: 18 Jan 88 17:06:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 28


In terms of CPU and other resource usage, I would like to know which of these
is more expensive and precisely why. We are considering changing our charging
algorithm to reflect the fact that things other than CPU are our main costs
these days, and I/O operations are among the things we might charge for. I
would assume that buffered I/O is generally to/from things like terminals
and tape drives (if using RMS and not direct QIO's), while direct would be
for devices such as array processors, image processors (and display screens),
etc. Where exactly do the disks come in? I guess most system devices must be
buffered somewhat, but to be honest, this is an area where my understanding
has always been a bit fuzzy. So is it more expensive to do number of single
direct I/O's to a device, or to fill something up and flush it out a bunch
at a time?
     
If I had to make a guess, I'd say there probably isn't that much difference,
since direct I/O's may be smaller but more of them are needed, while buffered
I/O has to collect somewhere before the operation that writes/reads it.
     
Can someone explain this to me clearly and cleanly?
     
Many thanks to all who respond - this may be a question that puzzles others
as well.
     
Ruth Milner
Systems Manager
University of Toronto Physics
     
SYSRUTH@UTORPHYS (BITNET)