From Fedora Project Wiki


This page describes a test case for bcache-tools: "/ on bcache using LVM"

Bcache 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. The bcache-tools package contains the utilities for manipulating bcache

Testing covers not only bcache-tools but also the interaction between bcache-tools and other packages: kernel, util-linux, dracut and lvm2.

Starting with a stable system with /home on LVM on bcache, we can go forward and have / on bcache. Consider thought that bcache is EXPERIMENTAL!


This test requires the following test case as preparation: /home on bcache (LVM). Furthermore it requires the volume group BCACHE to have at least 2 GB free space!


If you haven't done so, you should first execute the use case /home on bcache (LVM).

How to test

To get / on bcache:

  1. create a Logical Volume: lvcreate -L 2G -n ROOTFS BCACHE
  2. create a filesystem: mkfs -t ext4 -L ROOTFS /dev/BCACHE/ROOTFS
  3. mount it: mount /dev/BCACHE/ROOTFS /mnt
  4. copy your current root filesystem to /mnt: cp -ax / /mnt
  5. edit /mnt/fstab so your root fs is mounted like: LABEL=ROOTFS / ext4 defaults 1 1

Now we have a duplicate root filesystem on /dev/BCACHE/ROOTFS. To use it we need to build a new initramfs:

  1. rename your current initramfs: mv /boot/initramfs-uname -r.img /boot/initramfs-uname -r.img.sav
  2. build a new initramfs: dracut -N

Now reboot your system.

  1. while booting edit the default grub entry and replace the root=UUID=... parameter in the "linux" line by: root=LABEL=ROOTFS
  2. remove any rd.lvm=0 occurrence
  3. now start the modified grub entry

If all goes well, your system now boots fine and it's running on LVM on bcache! Now we have to do some final housekeeping:

  1. update grub config: grub2-mkconfig -o /boot/grub2/grub.cfg for BIOS systems and grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg for UEFI systems
  2. rename your current initramfs (again): mv /boot/initramfs-uname -r.img /boot/initramfs-uname -r.img.sav
  3. build a new initramfs with only the needed drivers included: dracut

Now your system should be ready!

  1. reboot your system to see if it boots OK
  2. apply updates: yum update. Specifically try to update the kernel, because it depens on the proper interaction between dracut, bcache-tools and util-linux,
  3. wget

Expected Results

  1. All steps complete without errors
  2. wget should have the same speed of all time


The original root partition /dev/sda3 is no longer used, so we can reclaim its disk space by adding it to the volume group:

  1. First wipe all metadata: wipefs -a /dev/sda3
  2. Make /dev/sda3 a bcache backing device: make-bcache -B /dev/sda3
  3. Note the fact that a new /dev/bcache2 device is created
  4. Retrieve the cset.uuid from your /dev/sdb1 caching device: bcache-super-show /dev/sdb1
  5. Attach /dev/sdb1 also to /dev/bcache2: echo <cset.uuid> > /sys/block/bcache2/bcache/attach

There are three bcache devices now: /dev/bcache0, /dev/bcache1 and /dev/bcache2 each using /dev/sdb1 as a caching device. The numbering of the bcache devices happens dynamically during boot, but until you reboot you can safely assume /dev/bcache2 uses /dev/sda3 as a backing device.

  1. use "bcache-status -s" to see details about your bcache devices.
  2. Now add /dev/bcache2 to out volume group:\
    1. create a Physical Volume: pvcreate /dev/bcache2
    2. add it to our Volume Group: vgextend BCACHE /dev/bcache2

Now /dev/sda3 is an cached integral part of the volume group. You can allocate more space to / or /home.