It's rare to find a web server with slow disk I/O performance, but Amazon's EC2 micro-instances are one such example. Their EBS disk subsystem access is rated "low", and this can have a detrimental effect on HTTP throughput. [...] This leaves sessions, which can be written to a redundant and fault tolerant storage system. [...] In order to speed up the disk access, a RAM-based disk can be mounted over the session directory. This has the disadvantage of being volatile - the data is lost in case of a server reboot, or the mount point being unmounted. However if tolerable, storing sessions in RAM insulates the application from poor filesystem performance.

He mentions the two types of kernels that can be used, ramfs and tmpfs, and the specifics of using a tmpfs filesystem to implement the technique (complete with command line calls to make it happen).