are you talking about facebook flashcache module where you use an ssd as a cache for your hdd?
Yes, it works great in our testing that we have done, and if disk read access is a bottleneck for you, then it is a worthwhile consideration providing that you have a sensible disk layout (partitions or LVM logical Volumes).
Just follow the instructions to build the kernel module, then each time you update the kernel re-build the module.
follow the instructions for installing/configuring - it all works just fine.
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