Hi Jim,

A couple of comments on your comments. All our systems use Veritas, so 
I've had to use it for a while now.

> 
> It's also not a Veritas feature: it's one provided by the Logical
> Volume Manager. It just happens that most sites that are stupid or
> unlucky enough to use LVM use it with VxFS filesystems...

True that the filesystem itself does not give these capabilities, but 
Veritas do supply their own volume manager (VxVM) with all the GUI tools 
etc.


........

> 
> I think this is a side-effect of the VxFS implementation. The snapshot
> filesystem is probably little more than the old location(s) for the
> old block(s) of the file(s) which have been deleted or updated. IIRC,
> the filesystem on the NetApp file servers does the same trick.
> 

The snapshot filesystem is a real honest to goodness filesystem (albeit a 
little weird in implementation). If you were to back it up immediately 
after creation, all references would be fulfilled by data on the original 
filesystem. If anything gets changed on the original, the old data is 
moved over to its equivalent position in the "snapshot" filesystem, so 
that you will always see a filesystem frozen in time. All the docs say 
that this could lead to needing 101 percent of the original fs size, due 
to extra "snapshot" structures....hmmm!



> You should also appreciate that VxFS is *huge*. When I briefly looked
> at it ~5 years ago, it was ~100,000 lines of code. The Berkeley FFS is
> less than a tenth that size. Personally, I don't think VxFS is worth
> the cost - either in dollars or kernel overheads - for the alleged
> benefits it brings. After all, how often does your enterprise-wide
> server crash these days? And of those failures, how many are caused by
> disk problems? What good is a "crash resistant" filesystem when the
> disk drives it lives on have died?
> 

The size and cost are good points. The crash resistance is increased 
considerably with the aid of the volume manager since you can start 
putting your plexes all over the place for mirroring/striping etc.

Martin Percival

Tandem Computers European Unix Support 

PS Please don't assume from the above that I like Veritas, I work in 
   support after all :-)