Open file cache in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the open_file_cache directive in nginx?

The open_file_cache directive caches information about open files to reduce file system lookups and improve performance.

intermediate

Which parameters are commonly set with open_file_cache?

Common parameters include max (maximum number of cached files), inactive (time after which cached items are removed if not used), and min_uses (minimum number of uses before caching).

intermediate

What does the open_file_cache_valid directive control?

It sets how often nginx checks if cached file information is still valid, by specifying a time interval.

beginner

How does enabling open_file_cache affect nginx performance?

It reduces the number of file system calls, which lowers latency and CPU usage, making nginx serve files faster.

beginner

What happens if open_file_cache is not used in a high-traffic nginx server?

Nginx will perform more frequent file system lookups, increasing CPU load and slowing down response times.

What does the open_file_cache directive do in nginx?

A Caches information about open files to reduce file system lookups

B Caches HTTP responses for faster delivery

C Limits the number of open connections

D Manages user sessions

The open_file_cache directive caches file info to reduce file system calls.

Which parameter sets the maximum number of cached files in open_file_cache?

A inactive

B max

C min_uses

D valid

The max parameter defines the maximum number of cached files.

What does open_file_cache_valid control?

A Maximum cache size

B Timeout for client connections

C Minimum uses before caching

D How long cached file info is considered valid before rechecking

open_file_cache_valid sets the interval for cache validation.

If open_file_cache is disabled, what is a likely effect?

A More cached files

B Faster file serving

C More file system lookups and higher CPU usage

D Reduced memory usage

Without caching, nginx does more file system calls, increasing CPU load.

Which directive controls how many times a file must be used before caching with open_file_cache?

A min_uses

B inactive

C max

D valid

min_uses sets the minimum number of uses before caching a file.

Explain how the open_file_cache directive improves nginx performance.

Think about how caching avoids repeated work.

Describe the key parameters of open_file_cache and their roles.

Focus on what controls cache size, duration, and validation.

Practice

1. What is the main purpose of the open_file_cache directive in nginx?

easy

A. To limit the number of open connections to clients

B. To store file information in memory to speed up file access

C. To cache HTTP responses from upstream servers

D. To compress files before sending them to clients

Solution

1. Step 1: Understand the role of open_file_cache

   The open_file_cache directive caches file metadata like file descriptors and attributes.

2. Step 2: Identify its effect on performance

   By caching file info, nginx reduces disk access and speeds up serving static files.

3. Final Answer:

   To store file information in memory to speed up file access -> Option B

4. Quick Check:

   open_file_cache purpose = cache file info [OK]

Hint: Remember: open_file_cache speeds file access by caching file info [OK]

Common Mistakes:

• Confusing open_file_cache with HTTP response caching
• Thinking it limits client connections
• Assuming it compresses files

2. Which of the following is the correct syntax to enable open file cache with 1000 max cached files and 60 seconds inactive timeout?

easy

A. open_file_cache max=1000 inactive=60;

B. open_file_cache 1000 60s;

C. open_file_cache max=1000 inactive=60s;

D. open_file_cache max=1000 inactive=60sec;

Solution

1. Step 1: Recall correct open_file_cache syntax

   The syntax requires named parameters like max= and inactive= with time units.

2. Step 2: Check each option for correctness

   open_file_cache max=1000 inactive=60s; uses correct parameter names and time unit s for seconds.

3. Final Answer:

   open_file_cache max=1000 inactive=60s; -> Option C

4. Quick Check:

   Use named params with units for open_file_cache [OK]

Hint: Use named parameters with units like 'inactive=60s' [OK]

Common Mistakes:

• Omitting parameter names
• Missing time units for inactive
• Using incorrect time unit abbreviations

3. Given this nginx config snippet:

open_file_cache max=2 inactive=30s;
open_file_cache_valid 10s;

What happens when nginx tries to serve 3 different files within 10 seconds?

medium

A. All 3 files are cached because open_file_cache_valid is 10s

B. All files are cached but expire after 30 seconds

C. No files are cached because max is less than 3

D. Only 2 files are cached; the third file info is not cached

Solution

1. Step 1: Understand max parameter effect

   The max=2 limits cache to 2 file entries only.

2. Step 2: Analyze caching of 3 files

   When 3 files are requested, only 2 can be cached; the third is not cached.

3. Final Answer:

   Only 2 files are cached; the third file info is not cached -> Option D

4. Quick Check:

   max=2 limits cache size to 2 files [OK]

Hint: max limits cached files count; extras are not cached [OK]

Common Mistakes:

• Assuming all files cache regardless of max
• Confusing open_file_cache_valid with max
• Thinking inactive controls cache size

4. You configured open_file_cache max=100 inactive=60s; but notice no caching effect. What is a likely cause?

medium

A. You forgot to enable open_file_cache_valid directive

B. The max value is too high to cache files

C. The inactive time is too short to cache files

D. open_file_cache only works with dynamic content

Solution

1. Step 1: Check required directives for caching

   Besides open_file_cache, open_file_cache_valid must be set to define cache validity.

2. Step 2: Identify missing directive effect

   Without open_file_cache_valid, cached entries expire immediately, so no caching effect.

3. Final Answer:

   You forgot to enable open_file_cache_valid directive -> Option A

4. Quick Check:

   open_file_cache_valid enables cache duration [OK]

Hint: Always set open_file_cache_valid to activate caching [OK]

Common Mistakes:

• Ignoring open_file_cache_valid directive
• Misunderstanding max and inactive roles
• Thinking open_file_cache works only for dynamic content

5. You want to optimize nginx for a high-traffic static site. Which open_file_cache settings best improve performance while avoiding stale data?

hard

A. open_file_cache max=1000 inactive=60s; open_file_cache_valid 30s;

B. open_file_cache max=10 inactive=300s; open_file_cache_valid 300s;

C. open_file_cache max=5000 inactive=5s; open_file_cache_valid 1s;

D. open_file_cache max=100 inactive=3600s; open_file_cache_valid 3600s;

Solution

1. Step 1: Balance cache size and freshness

   High-traffic static sites benefit from a large cache (max=1000) and moderate inactive timeout (60s) to keep data fresh.

2. Step 2: Set cache validity to avoid stale data

   open_file_cache_valid 30s ensures cached info refreshes often enough to prevent stale data.

3. Final Answer:

   open_file_cache max=1000 inactive=60s; open_file_cache_valid 30s; -> Option A

4. Quick Check:

   Large max + moderate inactive + short valid = best performance [OK]

Hint: Use large max, moderate inactive, short valid for fresh cache [OK]

Common Mistakes:

• Setting max too low for traffic
• Using too long inactive or valid causing stale cache
• Setting valid too short causing overhead