out_of_memory could not create semaphores: No space left on device — 53200

Symptoms

The server could not create the semaphores it needs at startup because the kernel ran out of semaphore space. PostgreSQL raises SQLSTATE 53200 (out_of_memory).

• Semaphore allocation failed at startup.
• Common with kernel SEMMNS/SEMMNI limits too low.
• Note: this is not a disk-space issue despite the wording.

What the server log shows

FATAL:  could not create semaphores: No space left on device
DETAIL:  Failed system call was semget(5432001, 17, 03600).

Why PostgreSQL raises this — what the manual says

Section 18.4.1 Shared Memory and Semaphores:

“The maximum number of semaphores in the system is set by SEMMNS, which consequently must be at least as high as num_os_semaphores plus one extra for each set of 16 required semaphores (see the formula in Table 18.1).”

PostgreSQL allocates semaphore sets sized by max_connections and related settings. When the kernel’s semaphore limits are exhausted, semget() fails (misleadingly “No space left on device”) and the server reports 53200.

Common causes

• Kernel semaphore limits (SEMMNS/SEMMNI) too low.
• Other processes consuming the available semaphore sets.
• max_connections set very high.

How to fix it

1. Raise kernel semaphore limits per the OS tuning guide.
2. Lower max_connections if appropriate.
3. Free leaked IPC resources from crashed processes (ipcs/ipcrm).

Related & next steps

• SQLSTATE 53200 — Out Of Memory (full reference)
• Message: could not create shared memory segment
• Glossary: Semaphore

Reference: PostgreSQL 18 Section 19.4 “Managing Kernel Resources”.