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Why Are Disk Usage and Snapshot Size Inconsistent in the File System?

Created on 2025-06-13 20:09:30
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Disk usage and snapshot size can appear inconsistent in a file system due to several reasons:

  1. Snapshot Efficiency: Snapshots only store changes (deltas) made after the snapshot is taken, not a full copy of the data. If no changes occur, the snapshot size remains small, while the disk usage may grow as new data is written.

  2. Data Deduplication & Compression: Some systems optimize snapshots by deduplicating identical blocks or compressing data, reducing the snapshot size even if the disk usage appears larger.

  3. Deleted Files: When files are deleted from the file system, the space may not be immediately reclaimed, leading to a discrepancy between disk usage (showing used space) and snapshot size (which may still reference old data).

  4. File System Overhead: Metadata, journaling, and other file system structures consume space but may not be fully reflected in snapshot size calculations.

Example:

  • A 100GB disk has 60GB of data. A snapshot is taken. Later, 10GB of new data is added, and 5GB of files are deleted. The disk usage might show 65GB (60GB + 10GB - 5GB), but the snapshot size could be much smaller (e.g., 10GB, only storing the new changes).

For efficient storage management, Tencent Cloud offers CBS (Cloud Block Storage) with snapshot capabilities, ensuring optimized storage usage and quick recovery. Additionally, CFS (Cloud File Storage) provides scalable file systems with snapshot support for consistent backups.