Ok - Take Indexes.
n servers, each with multiple partitions, some closed, some open, each with varying amounts of spare disk space.
For each server I need to work out the number of days capacity remaining for Indexes based on current trended usage. I.e. If I have 100GB left and recent trends show 1GB used per day, then I have 100 days Index capacity.
This would be used in reverse to order further storage for the next 3 months and build storage planning. I have 100 days, I need to ensure sufficient capacity for 6 months, therefore I order for an extra ~80 days - 80GB.
Mailbox Archiving Indexes are probably the easiest as the change rate would be consistent. For journaling we have live and standby servers to rotate workload. In this case I'd need to roll the usage and capacity statistics together to get a capacity by days for journaling for a site as a whole.
Hosts - that's also fairly easy - A host has a maximum mailbox archiving capacity e.g. 5000 mailboxes, or a maximum expected mailbox journaling ~ 12,000 mailboxes.
SQL - That's a bit more difficult - need to take into account white space, maintenance space, disk space, t-log, DB.
Centera - TBD
Cheers,
Si