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hakanekat
Asked: 2020-05-19 08:03:31 +0800 CST

Bcache seems to need update-initramfs

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I'm setting up bcache on ubuntu 20.04.

Booting from install CD, I created a bcache dirve by:

#modprobe bcache
#make-bcache -B /dev/mapper-vgubuntu-root
#make-bcache -C /dev/sdb

Just after that #fdisk -l showed /dev/bcache0. Then #mkfs.ext4 /dev/bcache0

and put all the files back from the backup.tar to the root of /dev/bcache.

Editted /etc/fstab like:

UUID=------- / ext4 erros=remount-ro 0 1

Then I updated grub by:

#mount -t ext4 /dev/bcache /mnt
#munt -B /dev /mnt/dev
#mount -B /proc /mnt/proc
#mount -B /sys /mnt/sys
#grub-install --recheck /dev/sda
#update-grub

Then I tried to restart the system from the new root /dev/bcache0. However, it fell into grub-rescue>,

saying "unknown file system UUID=-----".

I restarted the system from the install CD and checked whether or not the system recognized /dev/bcahe0.

#fdisk -l showed that the system didin't recognize it after boot.

Then

#modprobe bcache
#echo /dev/dm-0 > /sys/fs/bcache/register
#echo /dev/sdb > /sys/fs/bcache/register

With these commands fdisk -l showed /dev/bcache0.

/dev/mapper/vgubuntu-root is recognized as /dev/dm-0.

From the situation mentioned above, I think I have to update the initrd.img to execute the above scripts before the file system is mounted.

So I added bcache in /etc/initramfs-tools/modules

and wrote the following script to a file in /etc/initramfs-tools/scripts/init-premount/:

#! /bin/sh
echo /dev/dm-0 > /sys/fs/bcache/register
echo /dev/sdb > /sys/fs/bcache/register

I named the file 10_bcache.

Finally I update-initramfs and then restarted the system, however, the system still doesn't recognize /dev/bcache0 and falls into grub rescue mode.

How should I update-initramfs to recognize the bcache device before the system mounts the file system.

initramfs bcache

1 Answers

  1. I've solved this by myself. The cause of the problem is that /boot was put in the bcache drive since I installed 20.04 with default settings. Unless initrd.img is loaded, the bcache drive is not recognized and grub cannot find /boot in it.

    Now I put /boot outside the bcache drive and grub can find it and boot the system.

    The followings are the way I set up bcache.

    First, I manually partitioned the hdd with fdisk.

    Device       Start       End   Sectors   Size Type
/dev/sda1     2048      4095      2048     1M BIOS boot
/dev/sda2     4096   1054719   1050624   513M EFI System
/dev/sda3  1054720   3151871   2097152     1G Linux filesystem
/dev/sda4  3151872 976773134 973621263 464.3G Linux filesystem

    Second, logical volumes were created by:

    # pvcreate /dev/sda4
# vgcreate vgubuntu /dev/sda4
# lvcreate -L 12Gi -n swap1 vgubutu
# lvcreate -l 100%FREE -n root vgubuntu

    fdisk -l shows the result:

    Disk /dev/mapper/vgubuntu-swap1: 12 GiB, 12884901888 bytes, 25165824 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/mapper/vgubuntu-root: 452.26 GiB, 485608128512 bytes, 948453376 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

    Then a bcache drive was created by:

    # apt install bcache-tools
# modprobe bcache
# make-bcache -B /dev/mapper/vgubuntu-root -C /dev/sdc

    /dev/sdc is the SSD.

    fdisk -l shows:

    Disk /dev/bcache0: 452.26 GiB, 485608120320 bytes, 948453360 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x91af541f

    In order to install the system in /dev/bcache0 a partition was made in it by fdisk /dev/bcache0. fdisk -l shows:

    Device         Boot Start       End   Sectors   Size Id Type
/dev/bcache0p1       2048 948453359 948451312 452.3G 83 Linux

    After the preparation mentioned above, I installed the system using the advanced disk setting menu in the installer, where I set the mount points as the following.

    /dev/sda1                       biosgrub
/dev/sda2                       EFI system
/dev/sda3                  ext2 /boot
/dev/mapper/vgubuntu-swap1 swap
/dev/bcache0p1             ext4 /

    After the installation, I rebuilt initramfs in order for the system to recognize the bcache drive before mounting root file system. I editted a file and added a script in /etc/initramfs-tools/.

    /etc/initramfs-tools/modules

    # List of modules that you want to include in your initramfs.
# They will be loaded at boot time in the order below.
#
# Syntax:  module_name [args ...]
#
# You must run update-initramfs(8) to effect this change.
#
# Examples:
#
# raid1
# sd_mod
#
bcache

    /etc/initramfs-tools/scripts/init-premount/10_bcache

    #! /bin/sh
#
# Backing Device
echo /dev/mapper/vgubuntu-root > /sys/fs/bcache/register
# Cashing Device
echo /dev/sdc > sys/fs/bcache/register

    Under the above settings # update-initramfs was done.

    Without the latter script the system falls into BusyBox and you have to execute the above scripts manually because bcache drive doesn't appear until the scripts are executed. I learned this operation from https://wiki.archlinux.org/index.php/Bcache.

    And /etc/fstab is:

    # /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
#
# boot
/dev/sda3                               /boot   ext2    defaults        0   0
#
# swap
UUID=840f9099-58a2-45e5-958b-61becd00eabd none  swap    sw              0   0
#
# bcache root /dev/bcache0p1
UUID=bfff7e4c-c94d-4008-8f6e-0a82f41ccddf /     ext4    errors=remount-ro 0  1
#

# external 500GB SDD
UUID=2757914a-06df-4fbb-9364-2d14bb222766 /mnt/workstation ext4 defaults 0  0

    This is all the things I've done for bcache to work in my system. These are much more complicated than using lvm or mdadm. No.1 reason is that the bcache drive is used for the root file system, which has to be recognized before initramfs mounts file systems. In addition, initramfs has to be placed in an ordinary separate drive outside the bcache drive.

    So, if you use bcache drives other than root file system, such as /home,it will be much easier.

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