10PB is a crazy amount of data. Far more than any normal business would ever have to deal with. Presuming you aren't crazy, you must have an unusual business plan to legitimately need to handle that much data. That means it's tough for us to say much - any assumptions we might have about it could be invalid depending on your actual business needs. You're just going to have to tell us some more about your business case before we can say anything useful about it.
Machine data takes a lot of space, depending on how long you need/want to hold onto it - any given even might only be a few 100 bytes, but with 1000s or 10s of 1000s of devices sending even syslog turns into a metric buttload of data :)
