Volume Level :
Volume guarantee :
This is also called space guarantees. This is the amount of space reserved by the volume from the aggregate. The possible volume guarantee values are volume, none and file. File is deprecated in ONTAP v8.3.
If the volume guarantee type is volume, then the space for the entire volume is reserved from the aggregate.
Here is a screenshot from the NetApp TR 3483
If the volume guarantee is set to none, then no space is reserved from the aggregate when the volume is created and the space is consumed from the aggregate as and when data grows on the volume.
Here is a screenshot from the NetApp TR 3483. note the volume is created with -s none flag
Snapshot reserve:
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.
Autosize
The volume autosize feature in ONTAP allows for the volumes to be resized automatically when a threshold is reached. The default threshold is 98%. However, this is a configurable paramenter. The command is volume autosize <volname> <vserver name>https://docs.netapp.com/ontap-9/index.jsp?topic=%2Fcom.netapp.doc.dot-cm-cmpr-930%2Fvolume__autosize.html
AutoDelete
The volume setting allows Data ONTAP to delete Snapshot copies if a threshold is met. The threshold is called a trigger.The possible triggers are Volume, snap_reserve and space_reserve.
Volume: When the volume is nearly full.
snap_reserve: When the snap_reserve is full
space_reservation: The overwrite reserve space (fraction reserve) is full.
The order in which snapshot copies are deleted are determined by the delete_order flag. The delete_order can be newest_first or oldest_first. Certain snapshots can be (specified by the value -defer-delete)the last snapshot copied to be deleted.
The snapshots are deleted until the target volume capacity is reached.
One other aspect of autodelete is the flag called Commitment. This option specifies which Snapshot copies and LUN or file clones can be automatically deleted to reclaim back space.
if set to try, snaps that are not locked are deleted.
if set to disrupt, snap copies not locked by data backing functionalities are deleted.
if set to destroy, snap copies locked by data backing functionalities are deleted.
Fractional reserve
To understand fractional reserve, let us understand what happens when fractional reserve is disabled.Say you have a volume that 100GB in size and you create a 50GB volume. Now the active file system has 50GB data. Create a snapshot and the free space remains at 50GB.
Next change 40GB of data and the snapshot will consume 40GB of data and the free space will reduce to 10GB. Now, change 20GB of data and the writes will fail because the volume is out of space.
Now, let us introduce fraction reserve. The fractional reserve is the amount of space reserved for overwrites in a volume. This comes into picture when the volume has one or more LUNs and a snapshot is created. Let us repeat the above example but with 100% fractional reserve.
Say you have a volume that 100GB in size and you create a 40GB volume. Now the active file system has 40GB data and free space is 60GB. Create a snapshot and the free space remains goes to 20GB as 100% of the LUN size (40GB) is used for reserves. Next change 10GB of data and create a snapshot, the snapshot will consume 10GB of data and the free space will reduce to 10GB. Now, change 20GB of data and the writes will work because the writes will directed to the reserve .
