RAID 5 inside a mid 2010 Mac Pro ?

Yes you can. A good recommendation is use an internal SSD for booting, which you connect via the lower DVD connection, then use the internal trays for a 4 drive RAID 5 volume. Power consumption on this configuration will be fine. Performance won't be quite as good as with newer CPU's, but more than adequate.

Thanks a lot for the answer !

The MacPro is the 12 core version with 30Gb of RAM, and the SSD is already attached to the DVD connection on the motherboard so it should be fine with a 4 disks RAID5 volume managed with SoftRaid !

I've all setted up with four 4To disks in RAID5 with SoftRaid ans everything is fine.

I did some speed test with Blackmagicdesign Disk Speed Test and I have : 305MB/s in write mode and 400.1 in read. I was not expecting such good results !

Now that the tests are done and they are conclusive, all I have to do is buy a SoftRaid license !

thanks for the feedback!

For those interested, I've installed 2 disks in my Mac Pro's optical drive bay allowing for 6 disk internal SATA arrays. I benchmarked a few configurations and here are the results:

6 disk RAID 5 (what I currently use):
R: 480MB/s
W: 260MB/s

5 disk RAID 5:
R: 450MB/s
W: 273MB/s

4 disk RAID 5:
R: 390MB/s
W: 290MB/s

6 disk RAID 0:
R: 574MB/s
W: 507MB/s

6 disk RAID 4:
R: 553MB/s
W: 270MB/s

All tested using Blackmagic Disk Speed Test on the below machine.
4 disks are: https://eshop.macsales.com/item/Toshiba/MG03ACA400/
2 disks are: https://eshop.macsales.com/item/Toshiba/HDWE150XZSTA/

Mac Pro 5,1
2x 6-core 2.93GHz
48GB RAM
960GB PCIe SSD Boot disk
macOS 10.13.4

FYI: if you have a slot available, putting the two drives on a PCI card would probably double your RAID 0 performance. The DVD slots max out at about 150MB/s. that is why the 574MB/s max on the RAID 0, all drives are essentially bottle-necked by the two disks in the upper PCI bus.

My guess is, on a PCI card, you may get > 900MB/s on a RAID 0