Difference between revisions of "Btrfs - Tips and tricks"

Revision as of 13:44, 14 August 2013

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Snapshots

Automatic snapshots on each boot

It is possible to automatically save the then current state of a btrfs subvolume during system boot. Two files are needed to accomplish this, a script which snapshots btrfs subvolumes and a systemd unit file to run the script during the boot process.

Requirements

The root filesystem must have been installed into its own dedicated btrfs subvolume and the btrfs-root where all subvolumes reside has to be accessible.

/etc/fstab

# ...
UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX   /                 btrfs   defaults,subvol=__current/ROOT   0 0
UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX   /run/btrfs-root   btrfs   defaults,compress=lzo            0 0

# just for the below example:
UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX   /var              btrfs   defaults,subvol=__current/var    0 0
UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX   /opt              btrfs   defaults,subvol=__current/opt    0 0
UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX   /home             btrfs   defaults,subvol=__current/home   0 0
# ...

Snapshot script

An example script to snapshot the btrfs subvolumes mounted under / is listed further below. The script will automatically detect the names of all subvolumes mounted under / and create snapshots of the same names under a directory specified by the caller.

What the script does, is to delete all snapshots from the state the system was in at last boot, make new snapshots, and alter the /etc/fstab file of the snapshot of the root subvolume to allow it to be booted without manual configuration.

When called like this ...

# bash /usr/local/bin/snapshot_current_system_state.sh '/run/btrfs-root' '__current/ROOT' '__snapshot/__state_at_last_successful_boot'

the following structure will result:

$ btrfs subvolume list '/'

ID 257 gen 1134 top level 5 path __current/ROOT
ID 258 gen 1137 top level 5 path __current/var
ID 259 gen 1129 top level 5 path __current/opt
ID 260 gen 1137 top level 5 path __current/home
ID 277 gen 1128 top level 5 path __snapshot/__state_at_last_successful_boot/__current/ROOT
ID 278 gen 1128 top level 5 path __snapshot/__state_at_last_successful_boot/__current/var
ID 279 gen 1129 top level 5 path __snapshot/__state_at_last_successful_boot/__current/opt
ID 280 gen 1130 top level 5 path __snapshot/__state_at_last_successful_boot/__current/home

Note that var, opt and home are subvolumes created by the caller in this example and mounted under '/'. The script detected and snapshotted them automatically. The resulting directory structure is:

/run/btrfs-root/__current/ROOT
/run/btrfs-root/__current/var
/run/btrfs-root/__current/opt
/run/btrfs-root/__current/home
/run/btrfs-root/__current/net
/run/btrfs-root/__current/bak
/run/btrfs-root/__snapshot/__state_at_last_successful_boot/__current/ROOT
/run/btrfs-root/__snapshot/__state_at_last_successful_boot/__current/var
/run/btrfs-root/__snapshot/__state_at_last_successful_boot/__current/opt
/run/btrfs-root/__snapshot/__state_at_last_successful_boot/__current/home

The script below takes 3 parameters:

1. The path of the btrfs filesystem root. E. g. /run/btrfs-root,
2. The name of the btrfs root subvolume as specified in /etc/fstab. E. g. __current/ROOT,
3. The path where the newly created snapshots will reside without its 1st parameter portion. E. g. __snapshot/__state_at_last_successful_boot (if the actual path is /run/btrfs-root/__snapshot/__state_at_last_successful_boot)

/usr/local/bin/snapshot_current_system_state.sh

#!/bin/sh

# example call: # bash /usr/local/bin/snapshot_current_system_state.sh '/run/btrfs-root' '__current/ROOT' '__snapshot/__state_at_last_successful_boot' 

if [ $# -ne 3 ]
then
  /usr/bin/echo -e "This script requires three parameters:\n1st parameter: The path of the btrfs filesystem root. e. g. /run/btrfs-root\n2nd parameter: The name of the btrfs root volume as specified in /etc/fstab. E. g. __current/ROOT\n3rd parameter: The path where the newly created snapshots will reside without its 1st parameter portion. E. g. __snapshot/__state_at_last_successful_boot\nCAUTION: This script will delete all snapshots of the same name as the regular volume names in the path parameter 3 is pointing to."
  exit 0
fi

btrfs_root="${1}" # example: '/run/btrfs-root'
path_to_root_volume="${2}" # example: '__current/ROOT'
path_to_snapshots="${3}" # example: '__snapshot/__state_at_last_successful_boot'

# take no snapshots when booted into a snapshot
if [ -e '/SNAPSHOT-TIMESTAMP' ]
then
  exit 0
fi

# anti recursive snapshots
for subvolume in $(/usr/bin/btrfs subvolume list '/' | /usr/bin/awk '{print $NF}') # scan
do
  path_to_snapshot="${btrfs_root}/${path_to_snapshots}/${subvolume}"

  if [ -d "${path_to_snapshot}" ]
  then
    /usr/bin/btrfs subvolume delete "${path_to_snapshot}"
  fi  
done

subvolumes="$(/usr/bin/btrfs subvolume list '/' | /usr/bin/awk '{print $NF}')" # rescan
for subvolume in $subvolumes
do
  snapshot_directory="${btrfs_root}/${path_to_snapshots}/$(/usr/bin/dirname ${subvolume})"

  if [ ! -d "${snapshot_directory}" ]
  then
    /usr/bin/mkdir -p "${snapshot_directory}" 
  fi  

  /usr/bin/btrfs subvolume snapshot "${btrfs_root}/${subvolume}" "${btrfs_root}/${path_to_snapshots}/${subvolume}" 

  if [ "${subvolume}" = "${path_to_root_volume}" ]
  then
    timestamp="$(/usr/bin/date +%d.%m.%Y-%H:%M:%S)"

    /usr/bin/echo -e "Arch Linux --- state at last successful boot (nonpersistent) [${timestamp}]\n" > "${btrfs_root}/${path_to_snapshots}/${path_to_root_volume}/etc/issue"

    /usr/bin/echo "${timestamp}" > "${btrfs_root}/${path_to_snapshots}/${path_to_root_volume}/SNAPSHOT-TIMESTAMP"

    sed_path_to_snapshots="$(/usr/bin/echo ${path_to_snapshots} | /usr/bin/sed --posix --regexp-extended 's/\//\\\//g')"

    for subvolumeX in $(echo $subvolumes | /usr/bin/sed --posix --regexp-extended 's/\//\\\//g')
    do
      /usr/bin/sed --posix --regexp-extended "s/subvol=${subvolumeX}/subvol=${sed_path_to_snapshots}\/${subvolumeX}/g" --in-place "${btrfs_root}/${path_to_snapshots}/${path_to_root_volume}/etc/fstab"
    done
  fi
done

/usr/bin/sync

systemd unit file

Following systemd unit file will run the script every time the system manages to successfully boot into multi-user.target:

• Example unit file /etc/systemd/system/snapshot_current_system_state_upon_boot.service:

[Unit]
Description=Takes a snapshot of each btrfs subvolume mounted under / after multi-user.target has been reached.
After=multi-user.target

[Service]
Type=oneshot
ExecStart=/bin/sh /usr/local/bin/snapshot_current_system_state.sh '/run/btrfs-root' '__current/ROOT' '__snapshot/__state_at_last_successful_boot'

[Install]
WantedBy=multi-user.target

The unit file has to be symlinked by systemd:

# systemctl enable snapshot_current_system_state_upon_boot.service

Booting into snapshots

In order to boot into a subvolume the rootflags=subvol= option has to be used on the kernel line. The subvol= mount options in /etc/fstab of the snapshot to boot into also have to be specified correctly.

GRUB

• Example menue entry /etc/grub.d/40_custom:

# ....
menuentry "Arch Linux --- state at last successfull boot (nonpersistent)" {
  linux /vmlinuz-linux root=/dev/disk/by-uuid/<UUID of btrfs-root> rootflags=subvol=__snapshot/__state_at_last_successful_boot/__current/ROOT init=/usr/lib/systemd/systemd ro quiet
  initrd /initramfs-linux.img
}
# ...

/etc/grub.d/40_custom has to be executable:

# chmod 700 /etc/grub.d/40_custom

/boot/grub/grub.cfg has to be recreated:

# grub-mkconfig -o /boot/grub/grub.cfg