[Info-vax] INIT/SHADOW

[Michael Moroney]

helbig at astro.multiCLOTHESvax.de (Phillip Helbig---undress to reply) writes:

>I can understand that, although the erase takes time, when the shadow 
>set (containing no data, of course) is formed, then it is quicker since 
>no time is needed for the merge.  But how does the shadow set KNOW that 
>no merge is necessary?

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.

>Suppose I have two identical disks (say, BACKUP/IMAGE from a third disk) 
>and mount them into a new shadow set.  Presumably, in this case a merge 
>would take place, even though it is not necessary.

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.

>Would it make any difference if the third (source) disk was previously 
>part of a shadow set or not?

No, the backup will create the destination as a nonshadowed disk.

-Mike (one time shadowing devo)