[Info-vax] INIT/SHADOW

[Phillip Helbig---undress to reply]

In article <ijq0sd$a9d$1 at pcls6.std.com>, moroney at world.std.spaamtrap.com
(Michael Moroney) writes: 

> Shadowing uses metadata on the disk, the SCB (which is the first block
> of BITMAP.SYS) to determine shadowset membership, the time of the last
> transition and so forth, as well as some info in the home block (whether
> the volume was properly dismounted) to determine whether two disks were
> part of the same shadowset and whether a merge is necessary if they
> were.  (I'll ignore bitmaps/minicopy etc).
> 
> In theory a program could make two non-shadow disks into a shadowset
> needing no merge by sticking the correct magic bits into the SCB.  In a
> way INIT/SHADOW has such code (it initializes the disks identically and
> then makes them appear to be part of a shadowset) but I know of no
> other such code.

> If you take two identical, nonshadowed disks and make a shadowset from
> them, the code will mark one as the source and copy onto the second.
> It doesn't know they're identical.

Sounds logical.  However, HELP says that INIT/SHADOW/ERASE will erase
the volumes sequentially, and suggests for big disks to do just
INIT/ERASE and then later do INIT/SHADOW/NOERASE.  It doesn't say so
explicitly, but the impression is that INIT/SHADOW/NOERASE won't cause a
MERGE (the whole point of doing /INIT/ERASE in parallel is to save
time).  So, in this case, I do INIT/SHADOW on two disks which were not 
previously part of a shadow set, but which have been erased.  There must 
be some information which says "this disk has been erased and since then 
nothing has happened", but probably not part of the SCB, since the disks 
aren't yet part of a shadow set.  Where is this information stored?