What would happen if I did a raid on a SSD and an HDD

Depends on a number of things, I would think. Striping two drives into one does increase the odds of losing data in a failure, so a backup is a must.

It also depends on what data is written where and in what order, although it's probably fast because the OS and early-install stuff is striped across the SSD drive. Eventually larger data files will just hit the slower drive and the benefit will drop off a bit.

If you're just using it as a general-purpose home computer and aren't looking to increase reliability, then yeah, you'd get a hybrid performance machine that will not be as good as pure SSD in performance but better than two 7200 RPM disks.

Have I done it before? No. I wouldn't want to decrease reliability of my system and increase odds of data loss for the sake of faster drive performance. Soon enough using the system will turn into a baseline of performance and I'll forget that it's "faster", and by the time I notice newer drives and bus technologies will still leave me drumming my fingers wishing my word processor launched a fraction of a second quicker.

As far as I know, TRIM won't be passed to SSDs in hardware RAID. It's not so much that the lack of TRIM will affect the lifetime of an SSD (although this may be a factor), but that the drive's performance will suffer relatively quickly - which somewhat contradicts the purpose of the SSD.

If you really need RAID 1, then I'd favour SandForce-based SSDs, which tend to cope better without TRIM.

Edit: It's difficult to quantify how much the MTBF would reduce without TRIM, without being an SSD manfacturer. But we do know that the lack of TRIM can increase write amplification, thereby shortening the life of the drive due to extra program-erase cycles. SandForce drives have really low write amplification rates anyway - typically less than 1.0, due to compression - which explains why they cope better than other SSDs without TRIM.

It's also important to clarify that there are two types of SSD failure:

1. Wear of the flash memory due to excessive program-erase cycles over time
2. Failure of the SSD controller hardware or software

It is most likely the second of these which is described here, and which accounts for the relatively high failure rates here (albeit, presumably for consumer drives). In these cases, because failure occurs relatively early, it's unlikely that TRIM would make any difference to the reliability of the drive.