Is it possible to gain a speed benefit of an SSD if you create a software RAID with a big SSD + two small HDDs?
At the moment I have a system with only two 320GB HDDs. And I thought about inserting a 640GB SSD in.
Maybe the SSD could be the main disk and it only syncs to the other two disks if there is idle time?
How could I create such a RAID on Debian?
Solution:
Despite what other posters have said, you absolutely can gain a speed increase using an SSD in a RAID – if the RAID software is smart enough. What you want is a technology called "Write Mostly", which basically does write to both disks, with reads coming from the faster disk. Here is one explanation of it.
Also, depending on how critical "up to the second" data is, you may be able to use some kind of process to "buffer" writes on the secondary disk. I achieved this by abusing DRBD. You can read about the experiments I did for a similar option here.