I found stochastic multi-queue and writeboost to be insufficient for this purpose. I'm wondering if anything exists that fits this description:
Device mapper creates a "cache" device on a fast device (SSD/NVME, etc)... and writes to the device *always* hit the fast device. Writes are later flushed to the slower device as fast as the slow device can handle them.
The problem with the caching solutions I have found with device mapper is that they require the block to be identified as "hot" first, even when the cache itself is not 100% utilized. Read performance is largely irrelevant.
Is there a device mapper implementation that works in this fashion; where 100% of writes happen to the fast device? I was thinking COW effectively does this, but the problem is it does not automatically/periodically flush writes, you have to do them all at once.
I had seen some conversations about trying to get the multiqueue (mq, not smq) policy to do this with zero values for thresholds, but it did not seem like any were successful in ensuring the first write would hit the fast device.
Thanks!
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