[dm-devel] Device mapper for temporarily buffering writes on fast device before pushing to slower device

[Drew Hastings]

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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