Ask HN: How do you manage scalable storage for your SaaS?

Hi HN!

I run https://usebx.com and we are adding a feature that allows users to attach documents to invoices and projects.

I know a lot of people use Amazon S3 for storing things such as this, but I'm interested to know if anyone here manages this sort of storage themselves at scale.

Key things I'm interested in are:

How do you ensure high availability?

How do you add storage when you're low on space?

What software you use to manage your environment?

How do you secure content belonging to particular users?

What infra do you use to serve the content to users?