SnapShot Magic

[Alex Zbyslaw]

Bob wrote:

>/usr is a 33GB HW/Raid  partition, how can it possibly hold 6 33GB 
>snapshots???
>
>If I mount each one of these, they all report to df that they are indeed 33BB 
>file systems!
>
>How is this magic achieved? 
>
>Is there a doc somewhere with an explaination?
>  
>
Snapshots only hold the originals of blocks that have changed, but still 
look like a complete file system when you mount them.

 From man mount:

>                         Further details can be found in the file at
>                          /usr/src/sys/ufs/ffs/README.snapshot.
>  
>
which should have a pointer to the original paper.

--Alex