Kernel Log: Coming in 3.10 (Part 2)
Linux 3.10 will include the "block-layer cache" bcache, which can be used to configure one disk as a cache for another disk; a fast SSD, for example, could be used as a cache for a slower hard drive with more capacity. This kind of SSD cache can speed up access to frequently read data and take on write requests until a quieter moment when they can be written to the slower disk.
Bcache is the work of Kent Overstreet of Google, which has been using the tool to improve productivity for some time now; after dm-cache, which was integrated into Linux 3.9 , it is the second cache framework of this kind to be added to the Linux kernel. As device mapper maintainer Alasdair Kergon pointed out at LinuxTag a month ago, the two solutions work in somewhat different ways, which means that one or the other could be the right choice depending on the situation.
Bcache is designed to be better for situations with several small write operations that can then be transferred to the hard drive in a more orderly fashion. A few developers have tried to benchmark the caching solutions recently (1, 2 and others), often including SSD caching software EnhanceIO, which has not yet been integrated into the Linux kernel.