EROFS On-disk Format

EROFS uses a flexible, hierarchical, block-aligned on-disk layout that is built with the following goals:

• DMA- and mmap-friendly, block-aligned data to maximize runtime performance on all kinds of storage devices;

• A simple core on-disk format that is easy to parse and has zero unnecessary metadata redundancy for archive use unlike other generic filesystems, ideal for data auditing and accessing remote untrusted data;

• Advanced on-disk features like compression (compressed inodes and metadata compression) are completely optional and aren’t mixed with the core design: you can use them only when needed.

The entire filesystem tree is built from just three core on-disk structures:

• Superblock — located at a fixed offset of 1024 bytes; the only structure at a fixed position in the filesystem.

• Compact/Extended inodes — per regular file, device, symlink, or directory; addressed in O(1) time via a simple NID-to-offset formula.

• Directory entries — 12-byte records, sorted lexicographically by filename at the beginning of each directory block (each data block of a directory inode).

Optional features extend this foundation without breaking the core design:

• Extended attributes (xattrs) support per-inode metadata. Several mechanisms — including a shared xattr pool, long prefix tables, and a per-inode Bloom filter — keep storage overhead low even when xattrs are used extensively.

• Chunk-based layout splits large files into fixed-size, independently-addressed chunks, enabling cross-file deduplication and multi-device storage.