Please add support for bcache

[Massimo-B]

Please add support to create a initramfs with bcache support to be able to boot from bcache devices.

[globalcitizen]

Based on https://evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt it seems that:

To make bcache devices known to the kernel, echo them to /sys/fs/bcache/register:

  echo /dev/sdb > /sys/fs/bcache/register
  echo /dev/sdc > /sys/fs/bcache/register

To register your bcache devices automatically, you could add something like
this to an init script:

  echo /dev/sd* > /sys/fs/bcache/register_quiet

It'll look for bcache superblocks and ignore everything that doesn't have one.

Registering the backing device makes the bcache show up in /dev; you can now
format it and use it as normal. But the first time using a new bcache device,
it'll be running in passthrough mode until you attach it to a cache. See the
section on attaching.

The devices show up at /dev/bcacheN, and can be controlled via sysfs from
/sys/block/bcacheN/bcache:

  mkfs.ext4 /dev/bcache0
  mount /dev/bcache0 /mnt

Cache devices are managed as sets; multiple caches per set isn't supported yet
but will allow for mirroring of metadata and dirty data in the future. Your new
cache set shows up as /sys/fs/bcache/<UUID>

General takeaway:

• Not yet configuration stable (addition of multiple caches may cause a later need for code changes)
• Support should be very easy to add, because auto-scanning as a default behaviour should be safe and there is a small number of common device prefixes that would tend to be used with SSD drives
• I don't have a test system so can't validate any code

Proposal for defaults/initrd.scripts:

if [ -e /sys/fs/bcache ]
then
   # friendly message about bcache being detected goes here
   echo /dev/sd* /dev/nvm* > /sys/fs/bcache/register_quiet
fi

[gitowok]

I've just implemented and tested this based on @globalcitizen's code contribution above. I'm using btrfs on bcache as root filesystem. It needed a slight tweak to work - you cant echo all the devices at once into register_quiet

I edited /usr/share/genkernel/defaults/linuxrc . This needs to be after modules are loaded and devtmpfs is mounted, but critically - BEFORE btrfs device scan. I inserted this code right before the "start iSCSI" section, around about line 417:

# bcache device registration
if [ -e /sys/fs/bcache ]
then
   # friendly message about bcache being detected goes here
   for potential_bcache_device in /dev/sd* /dev/nvm* 
   do
        echo $potential_bcache_device > /sys/fs/bcache/register_quiet
   done
fi

It should be noted that the kernel was built with bcache was compiled as built-in rather than as module. Building as a module would require editting some other files to make sure bcache module is inserted before the above code is executed.

[Flowdalic]

Note that this issue essentially duplicates https://bugs.gentoo.org/605094