Originally Posted by rowan

NCQ is the drive deciding the best way to order requests coming from the controller, generally to minimise head travel over the platters. It improves performance under load (ie more than one request outstanding), but in a mirrored setup it's better if the controller decides which drive to send the request to in the first place... drive #1 and drive #2 can't communicate and figure out which drive probably has its head closest to the requested sector, that's up to the controller...

SSD are improving but they're still very expensive. Some of them have trouble with writing because flash can only delete a relatively large block (such as 128k), so even writing out a single sector of 512 bytes will require reading in the 128k, deleting the block, modifying the 512 bytes, then writing out the 128k again. Newer hybrid versions get around this with an onboard RAM cache that bunches up write requests to get them onto the flash more efficiently.

I've never heard of a pure RAM drive in significant capacities, although I'm sure it's been done... the cost would be huge, even compared to SSD... for example 64GB worth of DDR2 Kingston RAM sticks would cost $USD1100+...