Full definition
ext4 structure: replicated superblock, block groups with inode and block bitmaps, inode tables (1 per file with metadata), and data in extents (contiguous ranges) instead of ext3's indirect block map. Journaling in ordered (default), writeback or journal mode protects against power-loss corruption. In Linux recovery, the key is the superblock and inode table. When a file is deleted in ext4, the inode is zeroed but extents continue pointing to data until rewritten. Tools like extundelete, ext4magic and debugfs can recover recently-deleted files before allocation reuses the blocks.
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