What Is a recommended setting for using the NetApp ONTAP LUN fractional reserve?

The recommended setting is to set the space guarantee for the volume to volume. For volumes that contain LUNs, this is a foundational best practice. The space-guarantee=volume setting ensures that the entire size of the volume is pre-allocated from its containing aggregate. This prevents the volume from running out of space due to other volumes in the aggregate consuming all the available space, which could cause disruptive write failures to the LUNs. This creates a stable and reliable storage container, which is critical before managing space-saving features like fractional reserve within it.

A. LUN space reservation is disabled.

This enables LUN thin provisioning. While often used with a low fractional reserve, it is a space-saving choice with management trade-offs, not a universal recommendation for all LUN environments.

B. Space allocation is set to enable.

This LUN-level setting controls the pre-allocation of space blocks for performance. It is not a primary recommendation related to the use of the volume-level fractional reserve feature.

D. Volume Snapshot copy automatic deletion is disabled.

Disabling Snapshot autodelete is generally not recommended, as it removes an important safety mechanism that protects against the volume filling up, which is especially risky with thin-provisioned LUNs.

1. NetApp Technical Report 4068 (TR-4068)

"LUN and SAN Configuration Guide for ONTAP 9":

Section 3.1

"Volume Provisioning": "For all SAN environments

the volume space guarantee should be set to volume. This is the default for LUN creation. This setting preallocates space in the aggregate for the entire size of the volume

preventing a situation in which a LUN can go offline because the aggregate runs out of space."

2. NetApp ONTAP 9 Documentation

"Logical storage management":

Section: "Create a FlexVol volume": In the description of the -space-guarantee parameter for the volume create command

the documentation states: "For volumes containing LUNs

the recommended setting is volume."

3. NetApp ONTAP 9 Documentation

"SAN administration":

Section: "LUN provisioning and space management": This section details the interaction between volume guarantees

LUN reservations

and fractional reserve

reinforcing that a volume-level guarantee provides the most stable environment for LUNs. It states

"A thick-provisioned volume (space guarantee set to volume) provides the most assurance that space will be available."