How do I install and use flashcache/bcache to cache HDD to SSD?

There is a conversion tool for enabling bcache on an existing device (disclosure: I wrote it). It's also the way to go for fresh installs, since current distro installers don't know how to create bcache devices (in which case: start by installing to the HDD).

See converting your root filesystem to bcache in the blocks README for instructions.

bcache requires Linux 3.10 or newer, which is in saucy, and can be manually installed from http://kernel.ubuntu.com/~kernel-ppa/mainline/?C=M;O=D (look for the newest tagged release, currently v3.11.5-saucy, and install the debs that match your architecture).

bcache-tools and blocks can be installed from my storage ppa.

Well, we got a bcache answer but no flashcache answer. I chose flashcache because I already had an existing installation, so bcache was out of the question. To me it seemed easier to set up as well. I chose the DKMS method so I wouldn't be stuck rebuilding the module/tools every time I got a kernel upgrade.

These steps are outlined in README-DKMS on Github.

Building the Module

1. The first thing you do is install the building prerequisites:

   sudo apt-get install dkms build-essential linux-headers-$(uname -r) git
   

2. Then snag the source:

   git clone git://github.com/facebook/flashcache; cd flashcache
   

3. Build and install the modules:

   make -f Makefile.dkms
   

Creating a flashcache device

(In this example I use writeback caching -- the default -- but flashcache can do writearound and writethrough as well)

Caching a non-root (not mounted at /) device

sudo flashcache_create -p back fcache /dev/sdbX /dev/sdaX Where fcache is the created device (/dev/mapper/fcache), /dev/sdaX is the SSD partion (or whole disk) and /dev/sdbX is the HDD partition.

Caching your root device

There are a few more steps if you're caching your root device. You will need an Ubuntu LiveCD/USB for this section.

1. cd /path/to/flashcache/source

2. make -f Makefile.dkms boot_conf

3. Edit /boot/grub/grub.cnf and /etc/fstab to boot from /dev/mapper/fcache (I don't believe this step is really necessary, but it says to). Be sure to write down the UUID of your original root device.

4. Reboot with the LiveCD/LiveUSB.

5. Mount your root device: mount /dev/sda4 /mnt (/dev/sda4 is my Linux root partition)

6. cd /mnt/path/to/flashcache/source

7. sudo apt-get install dkms build-essential linux-headers-$(uname -r)

8. make; sudo make install No need to do the whole DKMS setup in the live environment

9. Unmount the root device sudo umount /mnt

10. sudo flashcache_create -p back fcache /dev/sdbX /dev/disk/by-uuid/[UUID] Where fcache is the created device (it should be the same as you put in /etc/fstab and /boot/grub/grub.cnf, in my case it was fcache), /dev/sdbX is the SSD partition (or whole disk) and [UUID] is the UUID of your root partition.

11. Reboot!

Caveats when using the root device

One minor annoyance when flashcache is used for the root device is that grub-probe fails to detect the root device and can mess up your boot menu so that you have two Ubuntu entires. But, it shouldn't matter which you use in all reality since the make -f Makefile.dkms boot_conf step above installs some scripts in your initrd that will detect and use the flashcache device.

Prior warning: bcache can not be used for your existing Ubuntu installation. If you need SSD caching for your operating system you should wait for a future or special version of the Ubuntu installer which could set up your SSD cache. See this answer by Oli here: Does bcache require a fresh installation?

For everyone else who wants a bcache drive for non-OS data:

The first thing to do is upgrade your kernel if your using 13.04 or older using the kernel mainline PPA. Instructions for using this can be found here:

https://wiki.ubuntu.com/Kernel/MainlineBuilds

Make sure you select the 3.9 or higher kernel when booting or it won't work and your drive will disappear.

Install bcache-tools, at the moment you will have to install from the git repository:

git clone http://evilpiepirate.org/git/bcache-tools.git

If you manage to package bcache-tools for debian, let us know and this question will be updated for you. (bcache-tools's build dependencies are uuid-dev and libblkid-dev. They must be installed, to build bcache-tools.)

Once you have all the tools in place, you're ready to try and follow the guide here:

http://atlas.evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt

This guide shows how to format the drives (SSD AND hard drive) and then attach them together to make a new device block using bcache. Post a comment if you find the instructions confusing and I'll update this answer with simplifications.