[linux-lvm] what use is virtualsize -- or how is it to be used?

["Linda A. Walsh" <lvm@tlinx.org>]

I just saw it in the man page, and first thing I tried was creating a 
logical volume
by just giving a --virtualsize, arg -- thinking it would 'auto-extend 
it' as needed.

Nep.

Next was looking on the net and seeing an example that gave it some 
small size (-L 1G),
with a virtual size of 2G.   So what is that supposed to do?

I just now thought to copy a bunch of files ( that differed from 
yesterday to today) to a tmp
partition) but had it fail miserably when it REALLY failed upon reaching 
the physical
limit -- not the virtual limit!)....


I was trying to use rsync to copy from a snap from yesturday of /home 
that was mounted on
/ohome to 'home.diff' (the partition that I used 'virtual size' with, 
... I gave it a real
size of 1G, and a virtual size of 1T.

Using rsync, I had --compare-dest=/ohome (where I mount yesturday's 
snap) and copied
from /home to /home.diff (the virtual partition)....   Well the diffs 
were > 1G, so had
hoped /home.diff would expand to it's virtual size...  Anyway, got I/O 
errors, after I
ran out of space.  Ok, so *that*'s not what virtual size is for...either! 

Anyway, I unmounted the now 'corrupt' /home.diff, and tried to remove it...
Got all sorts of i/o errors using lvremove Home.diff. (-f didn't work 
either).

Got it to remove with lvremove -f /dev/PV/Home.diff
That gave I/O errors too, but successfully completed...

So what's virtual size good for, anyway?