Think about snapshots this way. Suppose there were no snap reserve at all. Whenever you delete a file that is also in a snapshot, you get no disk space back because the data blocks occupied by the file cannot be free until all snapshots that contain the file are also gone. Overwriting a file that is in a snapshot works similarly. The new version of the file consumes new disk space while the old version remains frozen in a snapshot. So overwriting the file consumes space while giving none back.
This isn't the sort of behavior that folks are used to, so NetApp invented the snap reserve. This is just a bookkeeping trick. Whenever you delete a file that is still in a snapshot, the space is subtracted from the snap reserve and added to the filesystem free space. So it just looks like you got some disk space back, but you really didn't. A similar thing happens when you overwrite a file.
You are correct that the actual size of a volume is the reported size of the volume plus the size of the snap reserve. The filer will not let you exceed the filesystem size limit. So if the filesystem fills, but the snap reserve still has free space, you can simply decrease the snap reserve to "enlarge" the filesystem.
If you want to guarantee that you will always be able to use all of your space, then set the snap reserve to 0%. If you do this however, then if the volume fills up, ordinary users may not be able to free up any disk space by deleting files. Getting disk space back then requires deleting snapshots, which only a sysadmin can do.
Sorry for the newbie questions, one last question
When I do a df, for instance
/vol/vol0/ 127105232 748236 26356996 1% /vol/vol0/ /vol/vol0/.snapshot 31776304 4548 31771756 0% /vol/vol0/.snapsh ot
Does vol0's capacity include the snapshot capacity? That is to say, vol0 is really 127105232? Or, is vol0 really that plus the reserved snapshot capacity, in this case, 127105232 + 31776304?
Second I do understand from the documentation that if that snapshot exceeds the specified size, it will eat up the volumes capacity. In this case, both vol0 and snapshot would reflect the exceed capacity, right? --
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