diff --git a/helpers/helpers.v2.1.d/backup b/helpers/helpers.v2.1.d/backup
index 2a024fa8f..642c66e65 100644
--- a/helpers/helpers.v2.1.d/backup
+++ b/helpers/helpers.v2.1.d/backup
@@ -4,85 +4,48 @@ CAN_BIND=${CAN_BIND:-1}
 
 # Add a file or a directory to the list of paths to backup
 #
-# usage: ynh_backup --target=/path/to/stuff [--is_big] [--not_mandatory]
-# | arg: -s, --target=        - file or directory to bind or symlink or copy. it shouldn't be in the backup dir.
-# | arg: -b, --is_big           - Indicate data are big (mail, video, image ...)
-# | arg: -m, --not_mandatory    - Indicate that if the file is missing, the backup can ignore it.
+# usage: ynh_backup /path/to/stuff
 #
-# This helper can be used both in a system backup hook, and in an app backup script
+# NB : note that this helper does *NOT* perform any copy in itself, it only
+# declares stuff to be backuped via a CSV which is later picked up by the core
 #
-# `ynh_backup` writes `target` and the corresponding path inside the archive (dest_path) into a CSV file, and it
-# creates the parent destination directory
+# NB 2 : there is a specific behavior for $data_dir (or childs of $data_dir) and
+# /var/log/$app which are *NOT* backedup during safety-backup-before-upgrade,
+# OR if the setting "do_not_backup_data" is equals 1 for that app
 #
-# Example in the context of a wordpress app :
-# ```
-# ynh_backup "/etc/nginx/conf.d/$domain.d/$app.conf"
-# # => This line will be added into CSV file
-# # "/etc/nginx/conf.d/$domain.d/$app.conf","apps/wordpress/etc/nginx/conf.d/$domain.d/$app.conf"
+# The rationale is that these directories are usually too heavy to be integrated in every backup
+# (think for example about Nextcloud with quite a lot of data, or an app with a lot of media files...)
 #
-# ynh_backup "/etc/nginx/conf.d/$domain.d/$app.conf" "conf/nginx.conf"
-# # => "/etc/nginx/conf.d/$domain.d/$app.conf","apps/wordpress/conf/nginx.conf"
+# This is coupled to the fact that $data_dir and the log dir won't be (and
+# should NOT) be deleted during remove, unless --purge is used. Hence, if the
+# upgrade fails and the script is removed prior to restoring the backup, the
+# data/logs are not destroyed.
 #
-# ynh_backup "/etc/nginx/conf.d/$domain.d/$app.conf" "conf/"
-# # => "/etc/nginx/conf.d/$domain.d/$app.conf","apps/wordpress/conf/$app.conf"
-#
-# ynh_backup "/etc/nginx/conf.d/$domain.d/$app.conf" "conf"
-# # => "/etc/nginx/conf.d/$domain.d/$app.conf","apps/wordpress/conf"
-#
-# #Deprecated usages (maintained for retro-compatibility)
-# ynh_backup "/etc/nginx/conf.d/$domain.d/$app.conf" "${backup_dir}/conf/nginx.conf"
-# # => "/etc/nginx/conf.d/$domain.d/$app.conf","apps/wordpress/conf/nginx.conf"
-#
-# ynh_backup "/etc/nginx/conf.d/$domain.d/$app.conf" "/conf/"
-# # => "/etc/nginx/conf.d/$domain.d/$app.conf","apps/wordpress/conf/$app.conf"
-#
-# ```
-#
-# How to use `--is_big`:
-#
-# `--is_big` is used to specify that this part of the backup can be quite huge.
-# So, you don't want that your package does backup that part during ynh_backup_before_upgrade.
-# In the same way, an user may doesn't want to backup this big part of the app for
-# each of his backup. And so handle that part differently.
-#
-# As this part of your backup may not be done, your restore script has to handle it.
-# In your restore script, use `--not_mandatory` with `ynh_restore`
-# As well in your remove script, you should not remove those data ! Or an user may end up with
-# a failed upgrade restoring an app without data anymore !
-#
-# To have the benefit of `--is_big` while doing a backup, you can whether set the environement
-# variable `BACKUP_CORE_ONLY` to 1 (`BACKUP_CORE_ONLY=1`) before the backup command. It will affect
-# only that backup command.
-# Or set the config `do_not_backup_data` to 1 into the `settings.yml` of the app. This will affect
-# all backups for this app until the setting is removed.
-#
-# Requires YunoHost version 2.4.0 or higher.
-# Requires YunoHost version 3.5.0 or higher for the argument `--not_mandatory`
 ynh_backup() {
-    # TODO find a way to avoid injection by file strange naming !
 
-    # ============ Argument parsing =============
-    local -A args_array=([t]=target= [b]=is_big [m]=not_mandatory)
-    local target
-    local is_big
-    local not_mandatory
-    ynh_handle_getopts_args "$@"
-    is_big="${is_big:-0}"
-    not_mandatory="${not_mandatory:-0}"
-    # ===========================================
+    local target="$1"
+    local is_data=false
 
-    BACKUP_CORE_ONLY=${BACKUP_CORE_ONLY:-0}
-    test -n "${app:-}" && do_not_backup_data=$(ynh_app_setting_get --key=do_not_backup_data)
+    # If the path starts with /var/log/$app or $data_dir
+    if ([[ -n "${app:-}" ]] && [[ "$target" == "/var/log/$app*" ]]) || ([[ -n "${data_dir:-}" ]] && [[ "$target" == "$data_dir*" ]])
+    then
+        is_data=true
+    fi
+
+    if [[ -n "${app:-}" ]]
+    then
+        local do_not_backup_data=$(ynh_app_setting_get --key=do_not_backup_data)
+    fi
 
     # If backing up core only (used by ynh_backup_before_upgrade),
     # don't backup big data items
-    if [ $is_big -eq 1 ] && ([ ${do_not_backup_data:-0} -eq 1 ] || [ $BACKUP_CORE_ONLY -eq 1 ]); then
+    if [[ "$is_data" == true ]] && ([[ ${do_not_backup_data:-0} -eq 1 ]] || [[ ${BACKUP_CORE_ONLY:-0} -eq 1 ]]); then
         if [ $BACKUP_CORE_ONLY -eq 1 ]; then
             ynh_print_info --message="$target will not be saved, because 'BACKUP_CORE_ONLY' is set."
         else
             ynh_print_info --message="$target will not be saved, because 'do_not_backup_data' is set."
         fi
-        return 0
+        return 1
     fi
 
     # ==============================================================================
@@ -90,12 +53,8 @@ ynh_backup() {
     # ==============================================================================
     # Be sure the source path is not empty
     if [ ! -e "$target" ]; then
-        ynh_print_warn --message="Source path '${src_path}' does not exist"
-        if [ "$not_mandatory" == "0" ]; then
-            return 1
-        else
-            return 0
-        fi
+        ynh_print_warn --message="File or folder '${target}' to be backed up does not exist"
+        return 1
     fi
 
     # Transform the source path as an absolute path
@@ -154,46 +113,45 @@ with open(sys.argv[1], 'r') as backup_file:
     return $?
 }
 
-# Restore a file or a directory
+# Restore a file or a directory from the backup archive
 #
-# usage: ynh_restore --target=/path/to/stuff [--not_mandatory]
-# | arg: -t, --target=     - Path where was located the file or the directory before to be backuped or relative path to $YNH_CWD where it is located in the backup archive
-# | arg: -m, --not_mandatory    - Indicate that if the file is missing, the restore process can ignore it.
-#
-# Use the registered path in backup_list by ynh_backup to restore the file at the right place.
+# usage: ynh_restore /path/to/stuff
 #
 # examples:
-#     ynh_restore -t "/etc/nginx/conf.d/$domain.d/$app.conf"
-#     # You can also use relative paths:
-#     ynh_restore -t "conf/nginx.conf"
+#     ynh_restore "/etc/nginx/conf.d/$domain.d/$app.conf"
 #
-# If `DEST_PATH` already exists and is lighter than 500 Mo, a backup will be made in
-# `/var/cache/yunohost/appconfbackup/`. Otherwise, the existing file is removed.
+# If the file or dir to be restored already exists on the system and is lighter
+# than 500 Mo, it is backed up in `/var/cache/yunohost/appconfbackup/`.
+# Otherwise, the existing file or dir is removed.
 #
 # if `apps/$app/etc/nginx/conf.d/$domain.d/$app.conf` exists, restore it into
 # `/etc/nginx/conf.d/$domain.d/$app.conf`
-# if no, search for a match in the csv (eg: conf/nginx.conf) and restore it into
+# otheriwse, search for a match in the csv (eg: conf/nginx.conf) and restore it into
 # `/etc/nginx/conf.d/$domain.d/$app.conf`
 #
 # Requires YunoHost version 2.6.4 or higher.
-# Requires YunoHost version 3.5.0 or higher for the argument --not_mandatory
 ynh_restore() {
-    # ============ Argument parsing =============
-    local -A args_array=([t]=target= [m]=not_mandatory)
-    local target
-    local not_mandatory
-    ynh_handle_getopts_args "$@"
-    target="/${target#/}"
-    not_mandatory="${not_mandatory:-0}"
-    # ===========================================
+    target="$1"
 
     local archive_path="$YNH_CWD${target}"
+
+    # If the path starts with /var/log/$app or $data_dir
+    local is_data=false
+    if ([[ -n "${app:-}" ]] && [[ "$target" == "/var/log/$app*" ]]) || ([[ -n "${data_dir:-}" ]] && [[ "$target" == "$data_dir*" ]])
+    then
+        is_data=true
+    fi
+
     # If archive_path doesn't exist, search for a corresponding path in CSV
     if [ ! -d "$archive_path" ] && [ ! -f "$archive_path" ] && [ ! -L "$archive_path" ]; then
-        if [ "$not_mandatory" == "0" ]; then
-            archive_path="$YNH_BACKUP_DIR/$(_get_archive_path \"$target\")"
-        else
+        if [[ "$is_data" == true ]]
+        then
+            ynh_print_info --message="Skipping $target which doesn't exists in the archive, probably because restoring from a safety-backup-before-upgrade"
+            # Assume it's not a big deal, we may be restoring a safety-backup-before-upgrade which doesnt contain those
             return 0
+        else
+            # (get_archive_path will raise an exception if no match found)
+            archive_path="$YNH_BACKUP_DIR/$(_get_archive_path \"$target\")"
         fi
     fi
 
@@ -239,7 +197,7 @@ ynh_restore_everything() {
     cat ${YNH_BACKUP_CSV} | tr --delete $'\r' | grep --only-matching --no-filename --perl-regexp "^\".*\",\"$REL_DIR.*\"$" \
         | while read line; do
             local ARCHIVE_PATH=$(echo "$line" | grep --only-matching --no-filename --perl-regexp "^\".*\",\"$REL_DIR\K.*(?=\"$)")
-            ynh_restore --target="$ARCHIVE_PATH"
+            ynh_restore "$ARCHIVE_PATH"
         done
 }