The VFS hot-data tracking patches are now up to their third revision. While the Linux 3.7 merge window is currently open, the hot-data tracking patches are not quite ready yet with still a TODO list of eight items open after the v3 patches were published. It looks like hot-data tracking will burn the Linux 3.8 kernel at the earliest.
Linux kernel hot-data tracking is for tracking data "temperature" information in the VFS layer. The Linux kernel VFS layer stores some statistics like the number of reads/writes, last read/write times, read/write frequency, etc. Those various data points are then turned into a single data "temperature" to determine what data on the disk is "hot" -- i.e. the most used data on the disk. The hot data can then be moved to a solid-state drive (SSD) automatically or dealt with in other manners. The long-term development goal is for allowing Btrfs to intelligently use SSDs in a heterogeneous volume. Btrfs has long desired hot-data relocation and now it's finally coming to reality.
While the initial patches implement hot-data tracking only for the Btrfs file-system, with a bulk of the hot-data work being done in the VFS layer, other file-systems could tap into this feature too. With all the patches applied, the Btrfs file-system needs to be mounted with a hot_track mount option for the feature to be actually flipped on. Hot-data statistics are also exposed over debugfs.
The patches for those interested can be found on the kernel mailing list.