diff --git a/docker-compose.yml b/docker-compose.yml
index 02a2b3257..eaacd11d4 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -554,6 +554,10 @@ services:
       - shared_preload_libraries=pg_stat_statements
       - -c
       - pg_stat_statements.track=top
+    # Docker gives a container 64 MB of /dev/shm by default. Postgres keeps
+    # the shared memory of parallel queries and parallel maintenance there,
+    # and a larger plan fails with "could not resize shared memory segment".
+    shm_size: 256mb
     # cap_drop: [ALL] is deliberately NOT applied here (unlike `app`): the
     # official Postgres entrypoint starts as root, chowns the data
     # directory, and drops privileges to the `postgres` user itself, which
diff --git a/docs/ops/measurement-maintenance.md b/docs/ops/measurement-maintenance.md
index aee952f08..2576b3fcd 100644
--- a/docs/ops/measurement-maintenance.md
+++ b/docs/ops/measurement-maintenance.md
@@ -147,6 +147,16 @@ SELECT pg_size_pretty(pg_total_relation_size('measurements')) AS total,
        pg_size_pretty(pg_indexes_size('measurements')) AS indexes;
 ```
 
+## "could not resize shared memory segment"
+
+Up to v1.42.0 the run could stop at once with `could not resize shared memory
+segment ... No space left on device`. Postgres ran the vacuum in parallel and
+kept its working memory in `/dev/shm`, which Docker limits to 64 MB per
+container unless `shm_size` is set. From v1.42.1 the maintenance connection
+runs single-process and needs no shared segment, and the bundled
+`docker-compose.yml` gives the database `shm_size: 256mb`. On v1.42.0, add
+`shm_size: 256mb` to the `db` service and restart the database container.
+
 ## Not part of this
 
 `VACUUM FULL` would shrink the table file as well, but it locks the table for
diff --git a/src/lib/jobs/__tests__/measurement-maintenance-session.test.ts b/src/lib/jobs/__tests__/measurement-maintenance-session.test.ts
index e2b6ffe9c..93ba20002 100644
--- a/src/lib/jobs/__tests__/measurement-maintenance-session.test.ts
+++ b/src/lib/jobs/__tests__/measurement-maintenance-session.test.ts
@@ -24,7 +24,13 @@ describe("maintenance session settings behind a pooler", () => {
       "SET statement_timeout = '0'",
       "SET idle_in_transaction_session_timeout = '0'",
       "SET maintenance_work_mem = '128MB'",
+      "SET max_parallel_maintenance_workers = '0'",
       "SET lock_timeout = '300000'",
     ]);
   });
+  it("keeps maintenance in one process so it needs no /dev/shm segment", () => {
+    expect(MAINTENANCE_SESSION_OPTIONS).toContain(
+      "-c max_parallel_maintenance_workers=0",
+    );
+  });
 });
diff --git a/src/lib/jobs/measurement-maintenance.ts b/src/lib/jobs/measurement-maintenance.ts
index fa5305baf..72a7c6a03 100644
--- a/src/lib/jobs/measurement-maintenance.ts
+++ b/src/lib/jobs/measurement-maintenance.ts
@@ -65,9 +65,19 @@ export interface MeasurementMaintenancePayload {
   reindex: boolean;
 }
 
-/** Session settings of the maintenance connection. */
+/**
+ * Session settings of the maintenance connection.
+ *
+ * `max_parallel_maintenance_workers=0` keeps VACUUM and the index rebuild in
+ * one process. A parallel run shares its `maintenance_work_mem` through
+ * dynamic shared memory in `/dev/shm`, which Docker caps at 64 MB unless the
+ * container sets `shm_size`; a 128 MB budget then fails at once with
+ * "could not resize shared memory segment ... No space left on device".
+ * One process needs no shared segment, so the run works on any host,
+ * whatever its container settings.
+ */
 export const MAINTENANCE_SESSION_OPTIONS =
-  "-c statement_timeout=0 -c idle_in_transaction_session_timeout=0 -c maintenance_work_mem=128MB -c lock_timeout=300000";
+  "-c statement_timeout=0 -c idle_in_transaction_session_timeout=0 -c maintenance_work_mem=128MB -c max_parallel_maintenance_workers=0 -c lock_timeout=300000";
 
 /**
  * The same settings as `SET` statements, for a host behind a connection