User:Fg

Good afternoon!

I am a beginner student who likes Gentoo.

BTRFS snapshots allow systems to return to a previous state. One scheme is creating a top level directory of subvolumes that contains snapshots and directories to exclude from snapshots. To access directories, the command mount -o defaults,subvol=@rootlevelsubvolume /dev/sdxn /dir may be used. A partition containing @debian, @debian-var-cache, @gentoo, @gentoo-241103, @gentoo-var-cache, @gentoo-var-tmp, @mnt, @var-lib-flatpak, and a @home is one example. BTRFS allows heavy use of subvolumes rather than other file system alternatives requiring partition editing. Exacting the free space on the system would be tedious otherwise. A shell script may be used to create a snapshot before updates and save large amounts of recovery time.

FILE equip.bash

#!/bin/bash
OPTIONSMOUNT="defaults,noatime,compress-force=zstd:1"
PARTITIONBTRFS="/dev/sda3"
MNTDIR="/mnt/btrfs"
FSSUBVOLUME="@gentoo"


# Prepare mount
if [[ ! -d ${MNTDIR} ]]; then
    mkdir ${MNTDIR}
fi
if find ${MNTDIR} -mindepth 1 -maxdepth 1 | read; then
    echo "$MNTDIR must be unmounted first. Aborting automated snapshot."
    exit 1
fi


# Protect root subvolume with fallback
DATEFALLBACK="121212"
DATERFC=$(date --rfc-3339=date | cut -c3- | sed -e 's/-//g')
if [[ DATERFC < DATEFALLBACK ]]; then
    DATERFC = DATEFALLBACK
fi
DISTRODATEDIR="${MNTDIR}/${FSSUBVOLUME}-${DATERFC}"


# Snapshot today
mount -o ${OPTIONSMOUNT} ${PARTITIONBTRFS} ${MNTDIR}
if [[ -d ${DISTRODATEDIR} ]]; then
    btrfs property set -ts ${DISTRODATEDIR} ro false
    btrfs subvolume delete ${DISTRODATEDIR}
fi
btrfs subvolume snapshot -r ${MNTDIR}/${FSSUBVOLUME} ${DISTRODATEDIR}
umount ${MNTDIR}
exit 0

After execution, it outputs:

root #./equip.bash

Create readonly snapshot of '/mnt/btrfs/@gentoo' in '/mnt/btrfs/@gentoo-241103'

root #./equip.bash

Delete subvolume 302 (no-commit): '/mnt/btrfs/@gentoo-241103'
Create readonly snapshot of '/mnt/btrfs/@gentoo' in '/mnt/btrfs/@gentoo-241103'

A LiveCD may be used to revert to a snapshot:

FILE egress.bash

#!/bin/bash
OPTIONSMOUNT="defaults,noatime,compress-force=zstd:1"
PARTITIONBTRFS="/dev/sda3"
MNTDIR="/mnt/btrfs"
DISTROSUBVOLUME="@gentoo"
NAMINGSCHEME="${DISTROSUBVOLUME}[-][0-9]{6}"

if [[ ! -d ${MNTDIR} ]]; then
    mkdir ${MNTDIR}
fi
if find ${MNTDIR} -mindepth 1 -maxdepth 1 | read; then
    echo "$MNTDIR must be unmounted first."
    exit 1
fi
mount -o ${OPTIONSMOUNT} ${PARTITIONBTRFS} ${MNTDIR}
MOSTRECENT=""
MOSTRECENT=$(ls -l ${MNTDIR} | awk '{print $NF}' | egrep $NAMINGSCHEME | sort | tail -n 1)
if [ -z "${MOSTRECENT}" ]; then
    echo "$MNTDIR snapshot not found."
    echo "No files were changed."
    echo "Exiting."
    umount ${MNTDIR}
    exit 1
fi
btrfs subvolume delete ${MNTDIR}/${DISTROSUBVOLUME}
btrfs subvolume snapshot ${MNTDIR}/${MOSTRECENT} ${MNTDIR}/${DISTROSUBVOLUME}
umount ${MNTDIR}
exit 0

This outputs:

root #chmod +x egress.bash && ./egress.bash && chmod -x egress.bash

Delete subvolume 301 (no-commit): '/mnt/btrfs/@gentoo'
Create a snapshot of '/mnt/btrfs/@gentoo-241114' in '/mnt/btrfs/@gentoo'

This is a GRUB2 entry for the system:

FILE /etc/grub.d/40_custom

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail -n +3 $0' line above.
#
menuentry '@gentoo 6.1.114' --class gentoo --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-12345678-0123-5678-0123-567890123456' {
        # command substitution and date formatting are broken                              
        date                                                                               
        echo -e "\t\t\t\tLoading environment ..."                                          
        loader_subvol_dist="@gentoo"                                                       
        export  loader_subvol_dist                                                         
        loader_kvers=6.1.114-gentoo-dist-hardened                                          
        export  loader_kvers                                                                                                                                                           
        loader_kernel="vmlinuz-$loader_kvers"                                                                                                                                          
        export  loader_kernel                                                              
        loader_disk="initramfs-$loader_kvers.img"
        export  loader_disk                  
        loader_uuid_grubpart=12345678-0123-5678-0123-567890123456
        export  loader_uuid_grubpart
        loader_rootuuid=12345678-0123-5678-0123-567890123456                                                                                                                         
        export  loader_rootuuid                                                            
        date                                                                                                                                                                           
        echo -e "\t\t\t\tLoading modules ..."                                              
        load_video                                                                                                                                                                     
        if [ "x$grub_platform" = xefi ]; then                                                                                                                                          
                set gfxpayload=keep                                                        
        fi                                                                                 
        insmod gzio                                                                                                                                                                    
        insmod part_gpt                                                                                                                                                                
        insmod ext2                                                                                                                                                                    
        date                                                                               
        echo -e "\t\t\t\tLoading GRUB2 ..."                                                
        set root='hd0,gpt2'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-baremetal=ahci0,gpt2  $loader_uuid_grubpart
        else
          search --no-floppy --fs-uuid --set=root  $loader_uuid_grubpart
        fi
        date
        echo -e "\t\t\t\tLoading $loader_kernel\n\t\t\t\t\t\t\t\t\t\tand subvolume $loader_subvol_dist..."
        linux   /$loader_kernel root=UUID=$loader_rootuuid ro rootflags=subvol=$loader_subvol_dist
        date
        echo -e "\t\t\t\tLoading $loader_disk ..."
        initrd  /$loader_disk
}

Take care.