My 250GB SSD is nearly full. Should I buy one bigger one (500GB) or is it possible to buy another 250GB and group them in RAID 0? Are there any advantages or disadvantages of either approach?

You can use RAID0 if you don't care about losing everything if one of the drives dies or your raid card dies. 

You can use RAID0 if you don't care about losing everything if one of the drives dies or your raid card dies. 

Are two SSD in RAID0 faster than single SSD? If yes, would it be noticeable in any way in P3D? I'm willing to take a risk of failure if RAID0 would bring such advantages. If not, I'll just go with a single bigger SSD and call it a day.

Unfortunately, no. In RAID0 all drives are added together to make one big disk. RAID1 mirrors data across disks and would offer slightly better read speeds but the array is limited to the smallest disk size. Honestly, the best improvement in performance you would see moving from a SATA SSD would be a PCIe/NVMe drive such as the Samsung 950 Pro.

Considering how fast a single SSD is, I don't think any further speed benefits of a RAID0 arrangement is worth the risk of a striped array failure. Access times are already near-instantaneous on a SSD, and the added throughput of the RAID0 will only really affect loading times - which are good as it is. The risk vs. reward of a SSD in RAID0 doesn't seem to add up for me either.

 

I agree with those recommending against it... and if you are looking for peak performance, I would recommend something along the lines of what Kylan was recommending above. A single fast drive on a fast interface.

Stay away from RAID. You are just asking for Trouble. The golden rule for flight sim is OS on one drive and the Simulator on another and not in a folder called Programs or Programs X84 or something like that.

I'm convinced not to go with RAID0 option Thanks all!

No matter what drive FSX or P3D are installed into, their working files are stored on the OS drive. I think those SSDs already approach fastest transfers. On RAID0 (striped) doesn't really further improve it much. To go RAID1 (mirror) although doesn't increase the volume size, it does mean if any one drive blows you simply power up the PC with a new one installed in its place, no more reinstalls. Both would have to fail, or the controller fail, for the data to be lost. Drive failure happened to me recently on my RAID10 4x2Tb barracudas giving 4Tb. RAID10 is a striped pair mirrored with another striped pair. One drive started going wrong and was automatically mapped out of the system by the controller, I replaced it with a new one and the array rebuilt, all while I continued to work on and run FSX and P3D. FSX and P3D are installed into "C:\Program Files (x86)\.." and the problem of getting write access to that read only area is overcome simply by adding the "Modify" permission to the "Users Group" on those folders.