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

• Official Nginx documentation on open_file_cache
• The Nginx Cookbook for more optimization techniques

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.