Skip to main content

Nginx Open File Cache

Introduction

When serving web content, Nginx constantly needs to check file properties like existence, size, and modification time. Each of these operations requires disk I/O, which can become a performance bottleneck, especially on busy servers. The Open File Cache is a powerful Nginx feature that caches these file metadata operations to significantly reduce disk I/O and improve response times.

In this article, we'll explore how the Open File Cache works, how to configure it properly, and see real-world examples of its benefits.

What is Open File Cache?

The Open File Cache stores metadata about files and directories, including:

  • File existence
  • File size
  • Modification times
  • Directory existence
  • File permissions
  • Other file attributes

By caching this information, Nginx can avoid redundant file system operations, leading to faster processing and response times.

How Open File Cache Works

Here's a simplified workflow of how Open File Cache operates:

Configuring Open File Cache

The Open File Cache configuration consists of several directives that control its behavior. Let's examine the main directives and how to use them.

open_file_cache Directive

This is the primary directive for enabling and configuring the cache:

nginx
open_file_cache max=N [inactive=time];

Parameters:

  • max: Maximum number of entries in the cache
  • inactive: Time after which an unused cache entry is removed (default: 60 seconds)

open_file_cache_valid Directive

This sets how often Nginx should check if cached information is still valid:

nginx
open_file_cache_valid time;

open_file_cache_min_uses Directive

Specifies the minimum number of times a file must be accessed during the inactive period to remain in the cache:

nginx
open_file_cache_min_uses number;

open_file_cache_errors Directive

Determines whether error responses (like "file not found") should be cached:

nginx
open_file_cache_errors on|off;

Basic Configuration Example

Here's a simple example of enabling Open File Cache in your Nginx configuration:

nginx
http {
open_file_cache max=1000 inactive=20s;
open_file_cache_valid 30s;
open_file_cache_min_uses 2;
open_file_cache_errors on;

server {
listen 80;
server_name example.com;
root /var/www/html;

location / {
try_files $uri $uri/ =404;
}
}
}

In this configuration:

  • We cache up to 1000 file entries
  • Entries unused for 20 seconds are removed
  • Cache validity is checked every 30 seconds
  • Files must be accessed at least twice to stay in cache
  • File errors are also cached

Real-World Configuration Examples

Let's look at some practical use cases for Open File Cache.

Static Content Server

For a server primarily serving static content like images, CSS, and JavaScript files:

nginx
http {
# Aggressive caching for static content server
open_file_cache max=10000 inactive=5m;
open_file_cache_valid 2m;
open_file_cache_min_uses 1;
open_file_cache_errors on;

server {
listen 80;
server_name static.example.com;
root /var/www/static;

location ~* \.(jpg|jpeg|png|gif|ico|css|js)$ {
expires 30d;
add_header Cache-Control "public, no-transform";
}
}
}

Dynamic Content Server

For servers with more dynamic content that changes frequently:

nginx
http {
# More conservative caching for dynamic content
open_file_cache max=2000 inactive=1m;
open_file_cache_valid 1m;
open_file_cache_min_uses 2;
open_file_cache_errors off;

server {
listen 80;
server_name app.example.com;
root /var/www/app;

location / {
try_files $uri $uri/ /index.php?$query_string;
}

location ~ \.php$ {
fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
fastcgi_index index.php;
include fastcgi_params;
}
}
}

Performance Impact

Let's examine the performance benefits of enabling the Open File Cache:

Before Open File Cache (Example Stats)

  • Average response time: 120ms
  • Server load during peak: high
  • Disk I/O operations: ~5000/second

After Open File Cache (Example Stats)

  • Average response time: 85ms
  • Server load during peak: moderate
  • Disk I/O operations: ~1200/second

This represents:

  • ~30% improvement in response time
  • ~75% reduction in disk I/O

Fine-Tuning Considerations

Determining Optimal Cache Size

The ideal max parameter depends on your server's file count and memory. Start with:

nginx
open_file_cache max=1000 inactive=20s;

Then monitor and adjust based on:

  • Memory usage
  • Cache hit ratio

You can estimate memory usage with this formula:

Memory used ≈ max_entries × ~160 bytes

Inactive Time Tuning

Set the inactive parameter based on your traffic patterns:

  • For high traffic sites with common files: longer inactive times (5-10 minutes)
  • For sites with diverse file access: shorter inactive times (1-2 minutes)

Debugging Open File Cache

You can enable debug logging to see how the cache is functioning:

nginx
error_log /var/log/nginx/error.log debug;

Look for entries containing "open file cache" to monitor cache hits and misses.

Common Issues and Solutions

High Memory Usage

Problem: The cache is consuming too much memory.

Solution: Reduce the max parameter and increase open_file_cache_min_uses:

nginx
open_file_cache max=500 inactive=20s;
open_file_cache_min_uses 3;

Stale Content Being Served

Problem: Modified files are still serving old versions.

Solution: Reduce the open_file_cache_valid time:

nginx
open_file_cache_valid 10s;

Cache Not Being Utilized

Problem: Low cache hit ratio despite configuration.

Solution: Verify with debug logs and adjust inactive and min_uses parameters:

nginx
open_file_cache max=1000 inactive=1m;
open_file_cache_min_uses 1;

Summary

The Nginx Open File Cache is a powerful optimization technique that can significantly improve your server's performance by reducing disk I/O and speeding up file operations. Key takeaways:

  • Open File Cache stores file metadata to avoid redundant disk operations
  • Configure the cache size, validity period, and minimum uses based on your needs
  • Different types of content (static vs. dynamic) benefit from different caching strategies
  • Proper monitoring and fine-tuning are essential for optimal performance

Additional Resources

Exercises

  1. Benchmark Exercise: Set up a simple Nginx server and compare the performance with and without Open File Cache using tools like ab or wrk.

  2. Configuration Exercise: Create an Nginx configuration for a website with both static assets and dynamic PHP content, implementing appropriate Open File Cache settings for each section.

  3. Monitoring Exercise: Set up Nginx debug logging and analyze the Open File Cache hit/miss ratio for your server.

  4. Optimization Challenge: For a given server setup, find the optimal Open File Cache parameters that balance memory usage with performance improvement.



If you spot any mistakes on this website, please let me know at [email protected]. I’d greatly appreciate your feedback! :)