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From: "Stephen Fuld" <SFuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: ancient OS history, ARM is sort of channeling the IBM 360
Date: Wed, 26 Jun 2024 16:25:35 -0000 (UTC)
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Lawrence D'Oliveiro wrote:

> On Tue, 25 Jun 2024 22:28:40 -0700, Stephen Fuld wrote:
> 
> > But if you are using the "standard" Unix file system, doesn't it
> > read the block(S) into its cache, then when the user program does
> > the read, transfer the data from its cache into the user's
> > variables?  That is the extra overhead to which I was referring.
> 
> That tends to happen anyway, even on OSes which insist on
> record-oriented I/O. For example, on DEC’s VMS, the record blocking
> layer is called “RMS” (“Record Management Services”), and that
> usually copies records between the user’s buffers and its own
> internal buffers (“move mode”). It is possible to request “locate
> mode”, where it returns the address of a record directly within its
> internal buffers. But there are many restrictions on this, among
> other things:
> 
> * It only works for reads, not for writes
> * It doesn’t work for records crossing block boundaries
> * It doesn’t work for compressed records
> 
> So this record-copying overhead is not, in itself, a point against
> Unix- style streaming I/O.


The fact that other OSs "made the same mistake"  :-) isn't a point for
treating all I/O as a stream of bytes.  I don't know VAX, but I don't
understand why not for writes.  The no crossing block boundries is a
side effect of fixed block disks.  This couldn't happen in OS/360 with
CKD disks.  I agree about compression, of course, as unless you do the
compression in the I/O hardware stream, you need to "move" the data
anyway.




-- 
 - Stephen Fuld 
(e-mail address disguised to prevent spam)