Debugging Persistent "Too Many Redirects" Errors with Large XML Sitemap Protocol Files in Our Generator

I understand how frustrating it can be to debug persistent "too many redirects" or 500 errors, especially when your core generation process is solid and it only manifests with large-scale XML sitemap protocol implementations. We've certainly encountered similar challenges with high-volume technical SEO deployments.

The fact that smaller sitemaps work perfectly points towards resource exhaustion or specific configuration limits that only become apparent under significant load, likely related to NGINX's handling of numerous static file requests or PHP-FPM's interaction during the serving phase, even if it's just a gateway.

• NGINX File Handling & Caching: For serving many static files, ensure your NGINX open_file_cache directive is appropriately configured in your http or server block. Increase max and set a reasonable inactive timeout. This helps NGINX efficiently manage file descriptors for frequently accessed files. Also, verify worker_rlimit_nofile in your NGINX main configuration to ensure workers can open enough file descriptors.
• PHP-FPM & NGINX Interaction: Even if you're serving static files, if PHP-FPM is involved in the initial request routing or error handling, it can impact the outcome. Revisit request_terminate_timeout in your PHP-FPM pool configuration. If NGINX is reverse proxying to PHP-FPM for the sitemap (even if it's just to check existence or log), ensure fastcgi_read_timeout and fastcgi_buffers in NGINX are generous. The "too many redirects" can sometimes be a default NGINX behavior when a proxied backend (like PHP-FPM) times out or returns an unexpected response, leading NGINX to try and redirect to an error page.
• System-Level File Descriptors: Beyond NGINX's worker_rlimit_nofile, check the global system limits for open file descriptors (ulimit -n for the NGINX user, and sysctl fs.file-max). Serving hundreds of thousands or millions of URLs across many sitemap files could hit these limits even if NGINX is configured correctly.
• Reduce NGINX Processing Overhead: If files are truly static, ensure NGINX is serving them directly without unnecessary PHP processing. A common pattern for redirect loops is a missing file causing NGINX to pass to PHP, which then tries to serve it, fails, and redirects, creating a loop. Double-check your try_files directives.
• Consider External Storage/CDN: For extremely large sitemaps, especially if public access is frequent, offloading these files to an object storage solution like Amazon S3, Google Cloud Storage, or Backblaze B2, and serving them via a CDN (e.g., Cloudflare, CloudFront), can significantly alleviate server load and potential NGINX/PHP-FPM bottlenecks. This also provides better global distribution and reduces latency for users.

Have you had a chance to monitor system-level metrics like file descriptor usage, CPU load, and I/O wait during the periods these errors occur, specifically when attempting to download the large sitemap indexes?

MD Alamgir Hossain Nahid, man, how many of these did we *think* we checked but maybe not deep enough... especially the `open_file_cache` and `ulimit` stuff. good call on the NGINX timeouts when it passes to PHP-FPM too, ngl that's often a hidden snag.