For database servers with many writes, I am wondering why can't these ramdisk/ramdrive be a solution to avoid wearing out SSDs and to avoid losing data if too much is kept in normal ram. Can these have any benefit at all for db servers ?
The trouble with a RAM disk is that if it fails (or if just turned off), then everything in it just disappears, like the morning mist.
You say that your data "
will be collected again", but into what?
Database != File
In order to store data, the database process needs to have a Data File in which to put "stuff" and that Data File has to be formatted in a way that the database can make use of it.
If the RAM disk has been wiped clean, then the file that used to be there is gone ("Solid, Gone!") There is nowhere for the database to store the incoming data - in all likelihood, the database won't even start up, because it has a great big "hole" where one of it's Data Files used to be!