Difference between revisions of "Btrfs - Tips and tricks"

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[[Category:File systems]]
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#REDIRECT [[Btrfs#Tips and tricks]]
Go back to [[Btrfs]].
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== Snapshots ==
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=== Automatic snapshots on each boot ===
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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.
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==== Requirements ====
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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.
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{{hc|/etc/fstab|
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<nowiki># ...
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UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX  /                btrfs  defaults,subvol=__current/ROOT  0 0
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UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX  /run/btrfs-root  btrfs  defaults,compress=lzo            0 0
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# just for the below example:
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UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX  /var              btrfs  defaults,subvol=__current/var    0 0
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UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX  /opt              btrfs  defaults,subvol=__current/opt    0 0
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UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX  /home            btrfs  defaults,subvol=__current/home  0 0
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# ...
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</nowiki>}}
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==== Snapshot script ====
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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.
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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.
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When called like this ...
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{{bc|# bash /usr/local/bin/snapshot_current_system_state.sh '/run/btrfs-root' '__current/ROOT' '__snapshot/__state_at_last_successful_boot'}}
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the following structure will result:
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{{hc|<nowiki>$ btrfs subvolume list '/'</nowiki>|
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<nowiki>ID 257 gen 1134 top level 5 path __current/ROOT
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ID 258 gen 1137 top level 5 path __current/var
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ID 259 gen 1129 top level 5 path __current/opt
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ID 260 gen 1137 top level 5 path __current/home
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ID 277 gen 1128 top level 5 path __snapshot/__state_at_last_successful_boot/__current/ROOT
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ID 278 gen 1128 top level 5 path __snapshot/__state_at_last_successful_boot/__current/var
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ID 279 gen 1129 top level 5 path __snapshot/__state_at_last_successful_boot/__current/opt
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ID 280 gen 1130 top level 5 path __snapshot/__state_at_last_successful_boot/__current/home
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</nowiki>}}
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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:
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{{bc|<nowiki>
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/run/btrfs-root/__current/ROOT
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/run/btrfs-root/__current/var
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/run/btrfs-root/__current/opt
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/run/btrfs-root/__current/home
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/run/btrfs-root/__current/net
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/run/btrfs-root/__current/bak
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/run/btrfs-root/__snapshot/__state_at_last_successful_boot/__current/ROOT
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/run/btrfs-root/__snapshot/__state_at_last_successful_boot/__current/var
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/run/btrfs-root/__snapshot/__state_at_last_successful_boot/__current/opt
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/run/btrfs-root/__snapshot/__state_at_last_successful_boot/__current/home
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</nowiki>}}
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The script below takes 3 parameters:
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# The path of the btrfs filesystem root. E. g. {{ic|/run/btrfs-root}},
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# The name of the btrfs root subvolume as specified in /etc/[[fstab]]. E. g. {{ic|__current/ROOT}},
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# The path where the newly created snapshots will reside without its 1st parameter portion. E. g. {{ic|__snapshot/__state_at_last_successful_boot}} (if the actual path is ''/run/btrfs-root/__snapshot/__state_at_last_successful_boot'')
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{{Warning|This script will delete all snapshots of the same names as the regular subvolume names in the path its third parameter is pointing to. Be careful that the ''3rd parameter is not pointing at the place where your subvolumes reside in'' --- In this example, ''/run/btrfs-root/__current'' as the 3rd parameter would be incorrect and lead to data loss.}}
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{{hc|/usr/local/bin/snapshot_current_system_state.sh|
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<nowiki>
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#!/bin/sh
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# example call: # bash /usr/local/bin/snapshot_current_system_state.sh '/run/btrfs-root' '__current/ROOT' '__snapshot/__state_at_last_successful_boot'
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if [ $# -ne 3 ]
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then
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  /usr/bin/echo -e "This script requires three parameters:\n1st parameter: The path of the btrfs filesystem root. e. g. /run/btrfs-root\n2nd parameter: The name of the btrfs root volume as specified in /etc/fstab. E. g. __current/ROOT\n3rd parameter: The path where the newly created snapshots will reside without its 1st parameter portion. E. g. __snapshot/__state_at_last_successful_boot\nCAUTION: This script will delete all snapshots of the same name as the regular volume names in the path parameter 3 is pointing to."
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  exit 0
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fi
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btrfs_root="${1}" # example: '/run/btrfs-root'
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path_to_root_volume="${2}" # example: '__current/ROOT'
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path_to_snapshots="${3}" # example: '__snapshot/__state_at_last_successful_boot'
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# take no snapshots when booted into a snapshot
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if [ -e '/SNAPSHOT-TIMESTAMP' ]
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then
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  exit 0
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fi
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# anti recursive snapshots
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for subvolume in $(/usr/bin/btrfs subvolume list '/' | /usr/bin/awk '{print $NF}') # scan
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do
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  path_to_snapshot="${btrfs_root}/${path_to_snapshots}/${subvolume}"
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  if [ -d "${path_to_snapshot}" ]
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  then
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    /usr/bin/btrfs subvolume delete "${path_to_snapshot}"
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  fi 
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done
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subvolumes="$(/usr/bin/btrfs subvolume list '/' | /usr/bin/awk '{print $NF}')" # rescan
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for subvolume in $subvolumes
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do
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  snapshot_directory="${btrfs_root}/${path_to_snapshots}/$(/usr/bin/dirname ${subvolume})"
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  if [ ! -d "${snapshot_directory}" ]
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  then
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    /usr/bin/mkdir -p "${snapshot_directory}"
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  fi 
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  /usr/bin/btrfs subvolume snapshot "${btrfs_root}/${subvolume}" "${btrfs_root}/${path_to_snapshots}/${subvolume}"
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  if [ "${subvolume}" = "${path_to_root_volume}" ]
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  then
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    timestamp="$(/usr/bin/date +%d.%m.%Y-%H:%M:%S)"
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    /usr/bin/echo -e "Arch Linux --- state at last successful boot (nonpersistent) [${timestamp}]\n" > "${btrfs_root}/${path_to_snapshots}/${path_to_root_volume}/etc/issue"
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    /usr/bin/echo "${timestamp}" > "${btrfs_root}/${path_to_snapshots}/${path_to_root_volume}/SNAPSHOT-TIMESTAMP"
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    sed_path_to_snapshots="$(/usr/bin/echo ${path_to_snapshots} | /usr/bin/sed --posix --regexp-extended 's/\//\\\//g')"
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    for subvolumeX in $(echo $subvolumes | /usr/bin/sed --posix --regexp-extended 's/\//\\\//g')
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    do
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      /usr/bin/sed --posix --regexp-extended "s/subvol=${subvolumeX}/subvol=${sed_path_to_snapshots}\/${subvolumeX}/g" --in-place "${btrfs_root}/${path_to_snapshots}/${path_to_root_volume}/etc/fstab"
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    done
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  fi
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done
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/usr/bin/sync
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</nowiki>}}
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====systemd unit file====
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Following [[systemd]] unit file will run the script every time the system manages to successfully boot into ''multi-user.target'':
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* Example unit file {{ic|/etc/systemd/system/snapshot_current_system_state_upon_boot.service}}:
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{{bc|<nowiki>
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[Unit]
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Description=Takes a snapshot of each btrfs subvolume mounted under / after multi-user.target has been reached.
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After=multi-user.target
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[Service]
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Type=oneshot
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ExecStart=/bin/sh /usr/local/bin/snapshot_current_system_state.sh '/run/btrfs-root' '__current/ROOT' '__snapshot/__state_at_last_successful_boot'
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[Install]
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WantedBy=multi-user.target
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</nowiki>}}
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The unit file has to be symlinked by systemd:
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# systemctl enable snapshot_current_system_state_upon_boot.service
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=== Booting into snapshots ===
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In order to boot into a subvolume the ''rootflags=subvol='' option has to be used on the kernel line. The ''subvol='' mount options in {{ic|/etc/fstab}} of the snapshot to boot into also have to be specified correctly.
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{{Note|If using the example [[Btrfs - Tips and tricks#Snapshot script|script above]], the {{ic|/etc/fstab}} of the snapshot of the root subvolume is set automatically.}}
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==== [[GRUB]] ====
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* Example menue entry {{ic|/etc/grub.d/40_custom}}:
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{{bc|<nowiki># ....
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menuentry "Arch Linux --- state at last successfull boot (nonpersistent)" {
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  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
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  initrd /initramfs-linux.img
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}
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# ...
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</nowiki>}}
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{{ic|/etc/grub.d/40_custom}} has to be executable:
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# chmod 700 /etc/grub.d/40_custom
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{{ic|/boot/grub/grub.cfg}} has to be recreated:
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# grub-mkconfig -o /boot/grub/grub.cfg
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Latest revision as of 13:12, 19 July 2016