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This has existed in various forms through the years: https://en.wikipedia.org/wiki/RAM_drive (section: Dedicated hardware RAM drives)

It's non-existence (or at least extreme difficulty to locate)for current gen tech suggests that it doesn't improve performance enough to be worth the tradeoffs, but I'm not an expert in that at all.

This has been done for decades. For various reasons it hasn't become mainstream. Cost of DRAM is one of those reasons.

They exist mostly for servers. The capacity is much smaller since DRAM is much more expensive.

http://www.ddrdrive.com/

A company called Violin Memory used to make one for servers:

http://www.storagesearch.com/ssd-23.html

You can create persistent ramdisk with tmpfs.

Add some batteries and a cronjob with rsync and you have the product you are looking for.

there even was NVDIMM