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:
open_file_cache max=N [inactive=time];
Parameters:
max
: Maximum number of entries in the cacheinactive
: 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:
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:
open_file_cache_min_uses number;
open_file_cache_errors
Directive
Determines whether error responses (like "file not found") should be cached:
open_file_cache_errors on|off;
Basic Configuration Example
Here's a simple example of enabling Open File Cache in your Nginx configuration:
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 $uri/ =404;
}
}
}
In this configuration:
- We cache up to 1000 file entries
- Entries unused for 20 seconds are removed
- Cache validity is checked every 30 seconds
- Files must be accessed at least twice to stay in cache
- File errors are also cached
Real-World Configuration Examples
Let's look at some practical use cases for Open File Cache.
Static Content Server
For a server primarily serving static content like images, CSS, and JavaScript files:
http {
# Aggressive caching for static content server
open_file_cache max=10000 inactive=5m;
open_file_cache_valid 2m;
open_file_cache_min_uses 1;
open_file_cache_errors on;
server {
listen 80;
server_name static.example.com;
root /var/www/static;
location ~* \.(jpg|jpeg|png|gif|ico|css|js)$ {
expires 30d;
add_header Cache-Control "public, no-transform";
}
}
}
Dynamic Content Server
For servers with more dynamic content that changes frequently:
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:
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:
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
:
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:
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:
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
-
Benchmark Exercise: Set up a simple Nginx server and compare the performance with and without Open File Cache using tools like
ab
orwrk
. -
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.
-
Monitoring Exercise: Set up Nginx debug logging and analyze the Open File Cache hit/miss ratio for your server.
-
Optimization Challenge: For a given server setup, find the optimal Open File Cache parameters that balance memory usage with performance improvement.
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