Which alternative is most performant (raid, ssd, 15k rpm disks)

Bottom Line - You get what you pay for. Without going into a rediculous amount of detail, because I'll let the internet do it for me.

There is a huge difference between an Enterprise SSD like the Optimus or Sandisk Lightning or the Intel 3700 and the Samsung 840 or OCZ Vector. There's a pretty darn good review Tweak Town SSD Review.

That review highlights the differences chief among them SUSTAINED performance. There are also a few other points that I think make all the difference:

Reliability - Enterprise SSD are made to run in a datacenter environment which typically means a higher sustained operating and level. They are built to run all out for extended periods of time at higher temps in more demanding environments.So please consider a couple things when making this decision:

edit added

- The use of your drives: The drives that are housing something very volatile will get beat up a lot more, stuff like caching drives or database tempdb. By the nature if those types of use those will get over written a lot and that may be worth the extra investment for extended life.

- How prepared you are to deal with replacement and such when the time comes. Depending on your architecture and it's built in reliability (fail-over clusters and the like) maybe you can live with it being down for a while as you order replacement parts. Maybe you can afford to have some the shelf just in case. If you choose to put all your eggs in one basket for what ever reason then I usually counsel people to make that basket as bullet proof as possible. In the end you have to do what is right for the business and your situation.

Data Protection - Enterprise SSDs have protections against power loss and the like built in. They will make sure all writes get committed before they go offline.

The Dell controller should support mixed drives - it actually says it supports mixed drives.

If you have support on your server and you purchase your drives from them then your drives will be supported in your contract. This is a business database on a business class server in a Datacenter, so you have 24x7 support right. Oh and one other thing you should always have hot spares in the box ready to replace a failed device the second it fails. After that you just wait for the courier to show up with your replacement drive and pop it in. When asked how much extra that costs you just make a comparison to the amount of downtime you'll experience if you were to actually lose the server for however long it took. That usually justifies the expense.

I know that a lot of the time in IT the cheap quick solution takes a backseat to the proper solution. I would encourage you to talk to your management and find the money to do it right the first time, because when it comes to storage, the sacrificially lamb can oft times be the business itself.

You cannot mix drive types in the same array group on PERC/LSI/HP hardware RAID controllers.

This also applies to mixing SATA, SAS and SSD.

PERC manual:

"Mixing SAS and SATA drives within a virtual disk is not supported. Also, mixing hard drives and SSDs within a virtual disk is not supported."