Is there a way to guarantee redundancy to an SSD using an HDD?
Raid 1 is the best choice to guarantee redundancy in HDDs, but SSDs are too expensive to guarantee redundancy via RAID. If I was to couple an SSD with an HDD, could I guarantee redundancy using the HDD as a failover device, and lazily mirroring the data on the HDD? (e.g.: every 5 minutes the data should be synchronized, rather than in real time like with Raid 1).
Answer
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.
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