Bcache (or block level cache) is a Linux kernel block layer cache. It allows one or more fast disk drives such as flash-based solid state drives (SSDs) to act as a cache for one or more slower hard disk drives. It is designed around the performance characteristics of SSDs.

## install
$ sudo yum install bcache-tools | sudo apt-get install bcache-tools

## using, assing HDD is /dev/sda and an SSD is /dev/sdb
# wipe (from util-linux) devices
$ wipefs -a /dev/sda1 ; wipefs -a /dev/sdb1

# format backup/hdd and cache/ssd devices
$ make-bcache -B /dev/sda1 ; make-bcache -C /dev/sdb1

# attach the cache device to our bcache device 'bcache0'
$ echo C_Set_UUID_VALUE > /sys/block/bcache0/bcache/attach

# create and mount fs
$ mkfs.ext4 /dev/bcache0
$ mount /dev/bcache0 /mnt

# optionally use faster writeback (instead of default writethrough)
$ echo writeback > /sys/block/bcache0/bcache/cache_mode
# same but permanently
$ echo /dev/sda1 > /sys/fs/bcache/register

# monitor 
$ bcache-status -s

from bcache and/vs. LVM cache