Understanding the Snapshot reserve area of a FlexVol volume or Infinite Volume and what Snapshot spill is can help you correctly size the Snapshot reserve. For FlexVol volumes, it can help you decide whether to enable the Snapshot autodelete capability.
When Snapshot copies use more space than the Snapshot reserve, they spill over and use space in the active file system. The Snapshot reserve area of a volume is the space reserved exclusively for Snapshot copies. It is not available to the user data or metadata area of the volume. The size of the Snapshot reserve is a specified percentage of the current volume size, and does not depend on the number of Snapshot copies or how much space they consume.
If all of the space allotted for the Snapshot reserve is used but the active file system (user data and metadata) is not full, Snapshot copies can use more space than the Snapshot reserve and spill into the active file system. This extra space is called Snapshot spill.
The following illustration shows a FlexVol volume with no Snapshot spill occurring. The two blocks on the left show the volume's used and available space for user data and metadata. The two blocks on the right show the used and unused portions of the Snapshot reserve. When you modify the size of the Snapshot reserve, it is the blocks on the right that change.
The following illustration shows a FlexVol volume with Snapshot spill occurring. The Snapshot reserve area is full and Snapshot copies spilling over into a Spill area that is part of the user data and metadata area's available space. The size of the Snapshot reserve remains the same.
For more information about Snapshot copies, see the Clustered Data ONTAP Data Protection Guide