diff --git a/CHANGELOG.md b/CHANGELOG.md
index b454a3c..84ccb29 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,9 @@ stopped after 13 hours with "Blob not in index"; the backup itself was fine.
   backups record no ID counters, and they stayed at 1. Every counter of a restored table now goes
   after its data. Found in a disaster recovery simulation of 138 GB from MariaDB to PostgreSQL,
   where all 15,772 files and every count were identical.
+- A full restore now runs in maintenance mode, switched off again at the end (also after an
+  error): a background job, a page visit or a user writing into the instance meanwhile could
+  collide with restored rows ("duplicate key") or be lost.
 - Adding a location whose repository config could not be read, while backup data is there,
   no longer creates a new repository over it: it stops with an explanation. A snapshot that
   cannot be taken over when a location is added is now logged instead of left out silently.
diff --git a/docs/DISASTER-RECOVERY.md b/docs/DISASTER-RECOVERY.md
index d1f48a1..acef877 100644
--- a/docs/DISASTER-RECOVERY.md
+++ b/docs/DISASTER-RECOVERY.md
@@ -94,7 +94,8 @@ tables that do not exist here, install the apps they belong to and run it again,
 out with `--skip-missing-tables`.
 
 **8. Let the restore run.** It restores the database, then the data directory, and takes hours for
-a large backup. If it stops halfway, fix the cause and run the same command again with `--force`.
+a large backup. Nextcloud is in maintenance mode meanwhile, so nobody and no background job
+writes into it; the restore switches it off again at the end, also after an error. If it stops halfway, fix the cause and run the same command again with `--force`.
 If occ then says that Nextcloud or one of the apps requires an upgrade, the database is already
 the old server's: run `occ upgrade` first, then the same command with `--force`.
 
diff --git a/lib/Command/RestoreFull.php b/lib/Command/RestoreFull.php
index 9774e7a..e8867c1 100644
--- a/lib/Command/RestoreFull.php
+++ b/lib/Command/RestoreFull.php
@@ -88,6 +88,7 @@ HELP);
 					$this->installApp($app, $output);
 				}
 			}
+			$output->writeln('Nextcloud is in maintenance mode while the restore runs (switched off again at the end, also after an error).');
 			$result = $this->restoreFull->restoreFull($target, $snapshot, $skipMissingTables);
 		} catch (\Throwable $e) {
 			$output->writeln('<error>' . $e->getMessage() . '</error>');
diff --git a/lib/Service/RestoreFullService.php b/lib/Service/RestoreFullService.php
index 9a0e279..4b036dd 100644
--- a/lib/Service/RestoreFullService.php
+++ b/lib/Service/RestoreFullService.php
@@ -30,6 +30,8 @@ final class RestoreFullService {
 	private const CONNECTION_KEYS = [
 		'dbtype', 'dbhost', 'dbport', 'dbname', 'dbuser', 'dbpassword', 'dbpersistent',
 		'dbtableprefix', 'dbdriveroptions', 'datadirectory',
+		// set by restoreFull() itself for the length of the restore
+		'maintenance',
 	];
 
 	public function __construct(
@@ -91,6 +93,25 @@ final class RestoreFullService {
 		if (!$check['ok']) {
 			throw new \RuntimeException($this->describeBlockers($check));
 		}
+		// Maintenance mode for the whole restore: background jobs (cron.php stops in maintenance
+		// mode), page visits (AJAX cron) and users must not write into tables or the data
+		// directory while they are being replaced; a row written in between would collide with a
+		// restored one or be silently lost. Switched off again at the end, also after an error:
+		// occ only offers app commands outside maintenance mode, and the guide's next step is
+		// to run the same restore again with --force.
+		$wasInMaintenance = $this->config->getSystemValueBool('maintenance', false);
+		$this->config->setSystemValue('maintenance', true);
+		try {
+			return $this->restoreInMaintenance($target, $snapshotId, $skipMissingTables, $check);
+		} finally {
+			if (!$wasInMaintenance) {
+				$this->config->setSystemValue('maintenance', false);
+			}
+		}
+	}
+
+	/** @return array{dbReport:array, dataFiles:int, configKeys:int, dbConsistent:bool, missingApps:list<string>} */
+	private function restoreInMaintenance(Target $target, string $snapshotId, bool $skipMissingTables, array $check): array {
 		return $this->withSharedLock($target, function (callable $refresh) use ($target, $snapshotId, $skipMissingTables, $check) {
 			$beat = LeaseService::throttled($refresh);
 			$repo = $this->targets->repository($target);
diff --git a/tests/integration/full-restore-roundtrip.sh b/tests/integration/full-restore-roundtrip.sh
index 13bcb28..f15e311 100755
--- a/tests/integration/full-restore-roundtrip.sh
+++ b/tests/integration/full-restore-roundtrip.sh
@@ -172,7 +172,9 @@ OUT=$(run_on ngb-frt-restore backup:restore:full offsite "$SNAP" "${RESTORE_OPTS
 	docker exec ngb-frt-restore sh -c 'tail -c 6000 /var/www/html/data/nextcloud.log' || true
 	fail "restore:full"
 }
-ok "backup:restore:full completed"
+printf '%s\n' "$OUT" | grep -q "maintenance mode while the restore runs" || fail "the restore did not say it runs in maintenance mode"
+run_on ngb-frt-restore status 2>&1 | grep -q "maintenance: false" || fail "maintenance mode is still on after the restore"
+ok "backup:restore:full completed, in maintenance mode and out of it again"
 
 # Everything below must work in a FRESH occ process (a new PHP process per docker exec), since
 # earlier bugs here only showed up after config.php's secret was rewritten mid-restore.