If you are talking about setting up a SSD and A HDD in say a raid 0 configuration - DON't DO IT!
1) Say SSD is 120 gigs and HDD is 1TB - you would end up with a 240 Gig Raid0 drive, Only 120 gigs of the 1 TB drive would be used while the remainder of the HDD (880 Gigs) would be rendered useless.
2) Performance gain would be VERY questionable.
There is with, Intel Z series MB, a setup called SRT where the SSD is used as a Cache for the HDD. Overall performance varies from about 2x to 4x the performance of the HDD. However all reviews of SRT have had as a bottom Line " if SSD large enought for OS + programs USE it as a OS + Program drive NOT as a cache drive.
What some do is set the Controller in Bios to Raid, set up the SSD as a OS + Program drive. Then they add a Pair of HDDs in a raid0 setup. This is acceptable (If you need the HDDs in raid0). With an Intel chipset the Windows Trim cmd will still be passed to the SSD, in a NON intel based MB, it will not be passed.
TOTAL agree with "You should NEVER create a RAID array with a SSD and a HDD."
But it is the size that is limited by the "smallest" drive, 2 x smallest drive. AND any HDD space above the SSD becomes USELESS as you can NOT use it
Performance wise it should be close to an average of the two drives (real life probably less than avg). The HDD will not slow the SSD down, but then the SSD will not speed up the HDD.
For example Raud0 with a 64 K strip size. Reading a 50 K file will be read at the speed of which drive it is on - You just have NO control of which drive it will be on.
For say a 640 K file, Half of the file will be on the SSD and read at the speed of the SSD. The other half of the file will be on the HDD and read in at the speed of the HDD.
Raid-0 has been over hyped as a performance enhancer.
Sequential benchmarks do look wonderful, but the real world does not seem to deliver the indicated performance benefits for most
desktop users. The reason is, that sequential benchmarks are coded for maximum overlapped I/O rates.
It depends on reading a stripe of data simultaneously from each raid-0 member, and that is rarely what we do.
The OS does mostly small random reads and writes, so raid-0 is of little use there.
There are some apps that will benefit. They are characterized by reading large files in a sequential manner.
Raid-0 is actually very simple. Stripes(of user defined size) will be alternately written to the devices. If the devices are of different speeds, that is ok, like one half on a SSD, and the other half on a HDD. When it comes time to read,
the ssd stripes will be read faster than the HDD stripes. Where raid-0 potentially can be faster is when the app reads data sequentially, and has the ability to read ahead, thus overlappintg the i/o to the drives. Few apps actually are coded this way, so the benefit does not show up in actual usage.
A real drawback to putting a SSD in a raid array is the loss of the trim command.
I suggest the best strategy is to use the SSD as an os and app drive, and use the HDD as a storage drive for large files.