noflushd

"noflushd is a simple daemon that monitors disk activity, spins down disks whose idle time exceeds a certain timeout, and stops writes to them in order to keep them spun down as long as possible. It's mainly used on laptops to save power, but can also be useful to desktop users with noisy hard disks. noflushd requires a kernel thread named kupdate which is present in Linux kernel version 2.2.11 and later. For earlier kernels, bdflush version 1.6 provides equal functionality." (Description on Freshmeat)