MongoDB File Descriptor Exhaustion: ulimit -n Thresholds, Collection/Index Proliferation & systemd Hardening

Diagnosing OS file descriptor exhaustion under large collection counts and connection scaling; configuring systemd LimitNOFILE and limits.conf.

Reviewed by:Senior Database Architect & SRE (dataiotech Labs)

Tested Versions: MongoDB 6.0 / 7.0 / 8.0

Operational Verification & Reliability Metadata

Last Technical Review: 2 Kasım 2026

Tested VersionsMongoDB 7.0.8, 8.0.1 (Ubuntu 24.04 / RHEL 9)

Required PrivilegesOS root/sudo

Risk Assessment

CriticalFile descriptor starvation prevents WiredTiger write operations, forcing daemon termination

Expected OutputMax open files in /proc/$(pidof mongod)/limits verified at 64000

Rollback Procedure:Revert /etc/security/limits.conf and systemd unit

MongoDB Docs: UNIX ulimit Settings

First 60 Seconds Emergency Protocol:

The error string is a symptom, not the root cause. Do not restart the server blindly. Run the diagnostic queries below to pinpoint the actual failure mode.

▸Symptom

mongod drops inbound socket handshakes and logs:

plaintext

Failed to accept socket: Too many open files
WiredTiger error (24): handle-open: open: Too many open files

▸What You See in Production

Database rejects incoming connections. File operations halt across collections and index files.

▸Root Cause Matrix

Probability	Root Cause	Description
%60	Default `ulimit -n 1024` Exhaustion	WiredTiger maintains dedicated file descriptors per collection and index.
%30	Socket File Descriptor Starvation	Spikes in concurrent sockets consume available process descriptors.
%10	Missing systemd LimitNOFILE	systemd limits override `limits.conf` without explicit overrides.

▸EMERGENCY RUNBOOK (0-5 min)

bash

# 1. Inspect live process limits
cat /proc/$(pidof mongod)/limits | grep "Max open files"

# 2. Elevate descriptors on running process via prlimit
sudo prlimit --pid=$(pidof mongod) --nofile=64000:64000

▸PERMANENT BLUEPRINT

1systemd Hardening: Set LimitNOFILE=64000 in systemd unit overrides.
2limits.conf Tuning: Add mongod soft/hard nofile 64000.

▸Verification

bash

cat /proc/$(pidof mongod)/limits | grep "open files"

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