This is a version of QEMU that includes a block device (disksim) that introduces I/O delays according to a simulator. This allows you to use a ramdisk while obtaining performance consistent with what a real disk would produce.
To use this, you need DiskSim 4.0 source from:
http://www.pdl.cmu.edu/DiskSim/
and the patches from:
http://scobyseo.blogspot.pt/2009/12/how-to-compile-disksim-40-ssdsimms-in.html
This post includes also detailed information on how to compile DiskSim.
After having compiled qemu with --enable-disksim, use the following to configure a simulated disk:
-drive file=disksim:barracuda.parv:stats.txt:image.raw
where barracuda.parv is the configuration file for DiskSim, stats.txt will be the simulation results file, and image.raw is the supporting file, that should be located in a ramdisk.