Record-access libraries (with query on NFS)

[John Chambers]

> If you access the raw disk device do you disable that read-ahead and
> write-behind aspect of the UNIX filesystem abstraction?

Oh, wow!  A question with a simple answer: Yes.  According to several
manuals, the main difference between /dev/dsk* and /dev/rdsk* is that
there is no buffering for the latter.  Reads always delay for physical
I/O, and writes always go immediately to disk (though with DMA, the
write may not be complete when write() returns).  There's also a
warning that the raw disks should be only accessed in multiples of
a sector.  In fact, most programs use multiples of BUFSIZ, which 
is invariably a multiple of a sector.

The exact wording in one of the manuals describes the "'raw' interface
which provides for direct transmission between the disk and the user's
read or write buffer.  A single read or write call results in exactly
one I/O operation and therefore raw I/O is considerably more efficient
when many words are transmitted."   Note the specific claim that the
transfer is direct between the disk and the buffer in user space,
without going through a kernel buffer.

Of course, this is all at the whim of the driver, so some vendors 
could have screwed it up...

-- 
John Chambers <{adelie,ima,maynard,mit-eddie}!minya!{jc,root}> (617/484-6393)

[Any errors in the above are due to failures in the logic of the keyboard,
not in the fingers that did the typing.]