filesystem snapshot question

Howdy list,

I've read some of the documentation written about
the new filesystem snapshot feature in 5.x, and I've
used it a few times on my laptop. Very very slick.

However, I can't seem to find a good explanation of
exactly HOW it works and what performance issues are
involved. Snapshots don't seem to take up much disk
space. (even though `ls -al` will report that the
snapshot is as large as the slice on which it resides
at the time the snapshot was created)

So when a snapshot is active on a filesystem, does
every disk write happen twice? Once to the real
filesystem and once to the snapshot with old data?

I mean, it CAN'T actually be making a copy of the
filesystem. :) My disk doesn't have enough free space.

Is there a way to actually figure out how much data
the snapshot has allocated at any given time?

Does a snapshot physically grow on the disk as
changes are made to the real filesystem?

Just hoping to understand things better.

Thanks!

-- 
Jesse Guardiani, Systems Administrator
WingNET Internet Services,
P.O. Box 2605 // Cleveland, TN 37320-2605
423-559-LINK (v)  423-559-5145 (f)
http://www.wingnet.net