Speed vs Resiliance
Firstly seeing hot HR data mentioned made me laugh. Storage gets down to cost no matter. How much would it cost for this data to be a little slower, how much would it cost if this data wasn't there at all and how much if this data wasn't around for 5 minutes, half an hour, an hour, 3 hours, half a day.... Then you can start to begin about planning your needs based on your needs to get the requirments for your business. Does the HR system need to be in the same storage array - well if it has the capacity but a isolated smaller system in the same datacenter may be better and isolate that data compeletely from other data that has no need to be in the same area. Call it data health and safty. Yes I know it should be an issue but it does add to your exposure with regards to system usage etc as such adding to potentual issues beyond what you realy need. Isolation is also good security practice as well as ensuring intergrity. It also allows you to accomodate resiliance and needs based on the data and usage as apposed to one shoe fits all as every department will say there data is business critical and of the utmost importance but try getting everybody to look out the windows of your entire company and describe the same thing, they all have there own viewpoint. you have to look at the numbers of actual buiness cost based upon external income impact and not that some HR director things 1 minute of there time is worth a million to the company as without him they would have no staff, arragance takes many forms remember but this is were accountants can have a positive use in actualy having to do some useful work and get you those real world figures department heads are mindfuly blind too in some respect. They manage departments not the whole company so you need to scale them with accountant numbers to see what there data is realy worth.
You can then plan if anything needs to be data warehoused and backup frequency/policy and failover access to the data etc.
What people need and what they want are always wrong and if there right then you have found somebody you can respect, so good luck.
Like rationalising a database, rationalising your data needs goes along way in seeing what you realy need from a business perspective. Just becasue you can, dosn't always mean you should. Also having the fastest discs in the land means nothing if you have a poor network that supports that data. So that real expensive storage array with automated backup may be fine, but if the network upgrade is due next year then you realy did get things in the wrong order. This is why isolation helps in many ways more than you think. Ever had marketing get a new HD camera and upload 1000x more data than they do in a year in the space of a day impacting main data storage access as it filled up the network and casued a router to overheat/fail. Well best avoided; Only allow people to shoot themselfs in the foot as much as possible as they are terrible shots and will end up shooting somebody else's foot instead and without even knowing they have.
Treat your data like Gold, your users like idiots and plan around that as much as possible bearing in mind there is alot of fools gold out there ;).