[Premium] The systemd Setting That Silently Breaks PostgreSQL Parallel Queries

The official documentation explains exactly why you're seeing "could not remove shared memory segment" warnings. Here's what it says and how to fix it.

Table of Contents

1. Warning That Appears Out of Nowhere

2. When It Could Escalate to Sev-1

3. What the Manual Says: The Root Cause

4. RemoveIPC Setting

5. Why This Causes Random Failures

   • When It Happens

   • Common Triggers

6. Impact on PostgreSQL

   • Parallel Query Failures

   • Inconsistent Behavior

   • Potential Server Instability

7. How to Diagnose the Problem

   • Step 1: Check if postgres is a System User

   • Step 2: Check RemoveIPC Setting

   • Step 3: Correlate Warnings with Sessions

8. Two Fixes from the Manual

   • Fix 1: Make postgres a System User (Preferred)

   • Fix 2: Disable RemoveIPC

9. Why Package Installations Usually Work

10. When You Hit This Issue

11. Quick Verification After Fix

12. Prevention Checklist

13. The Key Takeaways

14. References

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Warning That Appears Out of Nowhere

You’re reviewing your PostgreSQL logs and find this scattered throughout:

WARNING: could not remove shared memory segment
PostgreSQL.1450751626”: No such file or directory

The database seems to be running. Queries complete. But this warning keeps appearing sometimes during parallel queries, sometimes during routine operations.

You check disk space. Plenty available.

You check memory. Nothing unusual.

You restart PostgreSQL. The warnings come back.

This is a Sev-3/Sev-2 issue that should be fixed proactively before it combines with other factors to cause a real Sev-1.

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When It could Escalate to Sev-1

Scenario 1: Critical Parallel Queries Fail

• If your application depends on parallel queries for time-sensitive operations

• Queries fail or return incomplete results

• Business-critical processes break

Scenario 2: Combined with Other Issues

• RemoveIPC + high connection count + memory pressure

• The unreliable behavior becomes the tipping point

What’s happening?

The PostgreSQL manual has the answer, and it points to an unlikely culprit: systemd.

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What the Manual Says: The Root Cause

Deep in Chapter 19 ”Server Setup and Operation”there’s a section called “systemd RemoveIPC” with this warning:

If systemd is in use, some care must be taken that IPC resources (including shared memory) are not prematurely removed by the operating system.

Translation: systemd can delete PostgreSQL’s shared memory segments while the database is still running.

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RemoveIPC Setting

The manual explains the mechanism:

The setting RemoveIPC in logind.conf controls whether IPC objects are removed when a user fully logs out. System users are exempt. This setting defaults to on in stock systemd, but some operating system distributions default it to off.

Here’s what that means:

RemoveIPC=yes (default)→ When a user logs out, systemd removes all their IPC resources (shared memory, semaphores)

RemoveIPC=no → IPC resources persist after logout

System users → Exempt from RemoveIPC regardless of setting

As per manual:

Alternatively, if the user account was created incorrectly or cannot be changed, it is recommended to set RemoveIPC=no in /etc/systemd/logind.conf or another appropriate configuration file.

Problem

If the postgres user is not a system user, systemd treats it like a regular user. Every time an admin session ends SSH disconnect, maintenance script completion, cron job finish systemd may decide to clean up the postgres user’s IPC resources.

Including shared memory segments that PostgreSQL is actively using.

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Why This Causes Random Failures

The manual describes the exact symptom:

A typical observed effect when this setting is on is that shared memory objects used for parallel query execution are removed at apparently random times, leading to errors and warnings while attempting to open and remove them.

The example warning:

WARNING: could not remove shared memory segment
PostgreSQL.1450751626”: No such file or directory

When It Happens

A user logging out might happen as part of a maintenance job or manually when an administrator logs in as the postgres user or something similar, so it is hard to prevent in general.

Common triggers:

→ SSH session as postgres ends: User logged out

→ su - postgres session ends: User session terminated

→ Cron job running as postgres completes: Background session ended

→ Maintenance script finishes: User session closed

→ Ansible/automation task completes: Remote session terminated

The randomness is maddening. It’s not predictable because it depends on when systemd decides the user has “fully logged out.”

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The Impact on PostgreSQL

1. Parallel Query Failures

Parallel queries create shared memory segments for worker communication. When systemd removes mid-query:

WARNING: could not remove shared memory segment 
PostgreSQL.1450751626”: No such file or directory

The query may still complete, but resources aren’t properly cleaned up.

2. Inconsistent Behavior

The manual notes:

Different types of IPC objects (shared memory vs. semaphores, System V vs. POSIX) are treated slightly differently by systemd, so one might observe that some IPC resources are not removed in the same way as others.

This means you might see:

- Parallel queries fail intermittently

- Some operations work, others don’t

- Errors that appear and disappear without pattern

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3. Potential Server Instability

The manual’s warning is stark:

At least one of these two things has to be ensured, or the PostgreSQL server will be very unreliable.

Very unreliable. That’s the official documentation telling you this isn’t a minor issue.