unix
9 minLinux Disk Full Errors: Blocks, Inodes, Deleted Files, and Safe Cleanup
Diagnose ENOSPC when disk usage looks confusing, including exhausted inodes and deleted files still held open.
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unix
18 minLinux Under Load: Epoll, File Descriptors, OOM, TCP Queues, and VFS
Connect kernel readiness APIs, resource limits, cgroups, socket queues, page cache, and inode behavior to production symptoms.
Q&A
Q&AWhy can a Linux filesystem be full when du shows free space?
Inodes may be exhausted, or deleted files may still be held open by running processes.
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Compare df -h with df -i. If block usage and directory totals disagree, inspect open-but-deleted files with lsof +L1. A process retains allocated blocks until it closes the file; restart or signal the owning service through its safe lifecycle after confirming the file is disposable.
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Q&AHow do you check whether a service is listening on a port?
Inspect local socket state and bind address before changing firewall policy.
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On Linux, `ss -lntup` lists listening sockets; `netstat` may be available via net-tools on older systems. Confirm the process, address, and network namespace. A service bound only to 127.0.0.1 is not externally reachable; also check container port mapping and host policy.
Q&A
Q&AWhy can epoll handle many mostly-idle sockets more efficiently than select?
epoll tracks interest and reports readiness changes instead of requiring repeated full-set scans.
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select requires rebuilding and scanning descriptor sets on each wait and has platform-specific descriptor limits; poll also examines a supplied list. epoll maintains kernel-side interest and ready structures, which suits large mostly-idle sets. It does not remove per-connection memory costs, application scheduling, or edge-triggered drain requirements.