What's new in Linux 3.9
Linus Torvalds has released Linux kernel 3.9 – ten weeks after Linux 3.8. He and his fellow kernel hackers have therefore kept to their usual rhythm, as most of the recent kernel versions have taken around seventy days to develop.
The kernel's Device mapper now includes a cache target called "dm-cache" (1, 2, 3). This option enables a drive to be set up as a cache for another storage device, for example, an SSD as a cache for a hard drive. This feature is able to speed up data writes, as it allows the faster SSD to first cache data and then, in a quiet moment, transfer it to the slower hard drive. The cache target is also able to store frequently read data from the hard drive on the SSD in order to speed up access to it.