From:	MERC::"uunet!CRVAX.SRI.COM!RELAY-INFO-VAX" 26-JUN-1992 12:43:37.46
To:	"info-vax@crvax.sri.com"
CC:	
Subj:	Single Directory Disks

Some of us old timers will perhaps remember single directory
Files-11 devices. These were invented (I believe) for the old
TU56 DECtapes...the ones that could fall off the reel, get
rewound covered with dust, be replaced, and still read all
data.... On disks, under RSX, setting DV_SDI in the UCB
characteristics caused a file access to ignore the directory
part of the access, and use filename only (using [000,000])
for the directory part). This was actually useful with things like
memory disks, RT emulation, and so on.
   It seems that in VMS, there's almost no residue of this
behavior left. If you set the dev$m_sdi bit in ucb$l_devchar,
things work basically normally. There are two exceptions:
  1. Directory listings stop after one directory (due to the
	details of how RM0SCAN.MAR is coded), and
  2. RMS does not try to read directories with read-logical
	and cache them, but instead gets the ACP or XQP to read
	the directory. This slows things down a little, causes
	MONITOR FCP to show more XQP activity, and allows
	anything that wants to affect the XQP operation to in
	principle affect directory operations also.

VMS Backup operation seems to be unaffected by the bit. There is
code that could cause problems in other parts of RMS, but it seems
to be bypassed presently because the code tests to see if it's
on a disk first, then skips around the single directory test
if so. The assumption appears to be that the only single directory
device is magtape.

   I think this is a good change - nobody much used single directory
disks in RSX anyhow - but one that is in the "interesting arcana"
category that should be shared...
Glenn
Everhart@raxco.com