At 12:32 AM 7/11/2001 -0700, Roger B.A. Klorese wrote:
>It started with Volume Manager 1.2, released for public consumption in
>ealty 1992.  The term nay also have been used with Tolerant's products,
>which would take us back to the mid 80s.

Obviously you have quite a history with the Veritas product line, that 
definitely exceeds mine. ;)


> > (d) That's what a lot of vendors have actually done with other things.
> >      Call it whatever you want, as long as you don't pick a name that the
> > rest of the
> >      industry is using to mean something else.
>
>Renaming things is nearly always a bad idea.
>
> > I am in favor of anything that reduces confusion, and I think that one
> > company using the same term to refer to two different technologies is
> > confusing.
>
>But it refers to one *concept*: a stable point-in-time image.  Whether or
>not it is copy-on-write is implementation triviality.

I suppose this is where we'll have to agree to disagree.  To say that 
they're both the same thing implies that they have the same level of 
functionality.  If they did have the same level of functionality, then I 
might agree with you, but they don't.  They both offer a stable, 
point-in-time image, but they don't bother offer the same level of 
recoverability.  The biggest example of this is that snapshots (copy on 
write) can't protect you against a large disk failure on the primary, where 
copies can.  Many people overlook this fact, and get burnt when the have a 
major drive failure on the primary mirror.  "But we had snapshots!"  Sorry, 
Charlie.  Snapshots are only good as long as the disk(s) from which they 
are taken are good.

Therefore, I believe that the proper thing to call copy on write is a 
"snapshot," since it's not really a copy -- it's just a "picture" of the 
disk that helps you remember what it looks like.  But if the original is 
trashed, all your snapshot will help you find is a mess.  It's no better 
than a snapshot of a dead person.  It's a nice memory. ;)  (OK, so the 
anology breaks down, but don't they all?)

A "third mirror," or "business continuity volume" is an entirely separate 
copy that can be used to pick up where you left on when your entire array 
goes kaput (sp?).  I actually like the term BCV, as I think it best 
describes what a "third mirror" is, without using the term "third 
mirror."  However, I think I would infuriate the rest of the industry if I 
tried to use it as the generic term for a third mirror.




---
W. Curtis Preston
Principal Consultant for Storage Designs,   your storage experts
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