DeepSpeed FastFileWriter File Descriptor Leak, Phantom ENOSPC

DeepSpeed's FastFileWriter leaks file descriptors during checkpoint saves, eventually causing 'No space left on device' (ENOSPC) errors even when disk space is available. The leaked file descriptors exhaust the per-process fd limit, making the filesystem appear full. Denpex detects the fd leak pattern from ENOSPC errors with available disk space.

DeepSpeed FastFileWriter File Descriptor Leak. Phantom ENOSPC is a Data Integrity failure seen during ML training runs. DeepSpeed's FastFileWriter leaks file descriptors during checkpoint saves, eventually causing 'No space left on device' (ENOSPC) errors even when disk space is available. The leaked file descriptors exhaust the per-process fd limit, making the filesystem appear full. Denpex detects the fd leak pattern from ENOSPC errors with available disk space. Common tags: Deepspeed, Fd Leak, Enospc, File Descriptor.

FastFileWriter opens checkpoint files for writing but doesn't close them after the write completes. Each checkpoint save opens new file descriptors without closing the previous ones. After enough saves, the per-process file descriptor limit (ulimit -n, typically 1024 or 65536) is exceeded. The OS returns ENOSPC because the filesystem can't allocate new inodes or directory entries when fd limits are hit. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.

• Training fails with 'No space left on device' but df shows plenty of free space
• The error occurs after many checkpoint saves, not at the beginning
• lsof shows thousands of open file descriptors for checkpoint files that should have been closed

• DeepSpeed ZeRO-3 with frequent checkpoint saves
• DeepSpeed with NVMe offload and FastFileWriter
• Long-running training jobs with many checkpoint iterations
• DeepSpeed versions before v0.14.3

Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.

• ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
• ✓Verified signal present: OSError: [Errno 28] No space left on device when saving checkpoints
• ✓Verified signal present: df -h shows ample free space on the checkpoint storage
• ✓Verified signal present: lsof -p <pid> | wc -l shows file descriptor count near the ulimit -n limit
• ✓Verified signal present: The error occurs after a specific number of checkpoint saves (consistent across runs)
• ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.

Select the traceback or open the failed terminal, then run Denpex locally to see the initiating rank, collateral failures, exact fix, and verification command without uploading the log.

DeepSpeed changes when parameters, gradients and optimizer state are created, partitioned, gathered and offloaded. The hub separates ZeRO, memory, checkpoint and pipeline failures by lifecycle phase.

• FastFileWriter opens checkpoint files for writing but doesn't close them after the write completes
• Each checkpoint save opens new file descriptors without closing the previous ones
• After enough saves, the per-process file descriptor limit (ulimit -n, typically 1024 or 65536) is exceeded
• The OS returns ENOSPC because the filesystem can't allocate new inodes or directory entries when fd limits are hit