SSD redundancy via HDD

Solution 1:

The use-case here is flawed. The one use-case where this makes sense is where your I/O activity is majority-read by a very large degree (like 9:1). In that case, since there are so few writes the HD can keep up with the changes coming from the SSD. If the ratio is any less then that, then the SSD risks outpacing the HD's entire write ability.

What you're looking at isn't RAID, it's replication. Something like drbd is what you're looking for. The system will keep a delta-cache of writes to commit to the HD, and periodically flush those writes to the HD. But as I said, since an SSH can handle w-a-y more writes than an HD can, this only works if you're doing very little writing to the SSD.

Solution 2:

If every 5 minutes is enough and there are not too many writes rsync is your fried here.