Around 1995(?) Erol's Internet used a static RAM based ram-drive device to process email for its tens (hundreds?) of thousands of users. Its larger brother was used to handle Usenet. Unfortunately the Usenet feed was growing like crazy and soon that large drive could not handle it.

... In 2010 some slightly nutty young engineers who heard about that story from the grey beards they worked with at a future very well known company on a very large mysql instance used a monster ramdisk as a single master to achieve a crazy boost in performance. The hard data persistence was achieved via replication to the regular spinning rust slaves. While it worked really well for their application no one ever battle tested bad crashes in production...

... that led to a product around 2013(?)-2014(?) from Violin Memory which combined the ramdisk with if I recall correctly spinning disks to dump the data in case of a power loss. The devices were absolutely amazing but did not create a foot hold in the market. I think they sold a few hundred units total. The product was abandoned in favor of flash arrays