ONTAP 9.4, AFF300, autogrow/shrink enabled across the board, autodelete enabled on iSCSI LUN serving volumes only.
To reserve, or to not reserve?
Customer historically did 20% snap reserve on all volumes. I can see a snap reserve if there is an SLA which says a minimum number of snapshots must be retained. I enforce quotas on all my shares, so running out of space in the volume is not an issue.
It doesn't seem like setting the snap reserve is doing anything for me. Quotas, volume efficiency, autogrow/shrink, and autodelete should ensure I have adequate space for snapshots. At worst, they captivate storage.
if auto grow enabled i agree that it's not providing much benefit. but i do want to highlight:
1. if auto delete enabled (which you said sometimes do) and you are using snapmirror/vault .you need to also see what the policy is. first delete or first grow?. if it's to first delete. you may loose baseline snapshots for Snapmirror/Vault (i don't think the reserve would have prevent it. but don't know for sure)
2. i'm using the snap reserve full alerts from OCUM as indication for large amount of file delete or changes (to detect ransomware attacks)