Why not both? Alerting to find issues quickly, a bit of extra storage so you have more options available in case of an outage, and maybe some redundancy for good measure.
A system this critical is on a SAN, if you’re properly alerting adding a bit more storage space is a 5 minute task.
It should also have a DR solution, yes.
A system this critical is on a hypervisor with tight storage “because deduplication” (I’m not making this up).
This is literally what I do for a living. Yes deduplication and thin provisioning.
This is still a failure of monitoring or slow response to it.
You keep your extra capacity handy on the storage array, not with some junk files on the filesystem.
You also need to know how over provisioned you are and when you’re likely to run out of capacity… you know this from monitoring.
Then when management fails to react promptly to your warnings. Shit like this happens.
