Next3

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Next3 is a Linux file system based on ext3 that adds snapshot support while staying compatible with ext3’s on-disk format. It was introduced in May 2010 by CTERA Networks and is available as open-source software under the GPL.

What snapshots do
A snapshot is a read-only copy of the file system frozen at a specific moment. Next3 uses copy-on-write, so when data is changed, new blocks are written while the old blocks remain accessible to the snapshot. Snapshots are quick to create and space-efficient, because unchanged data is shared between the live system and its snapshots. They don’t require pre-allocated storage—the space for snapshots is allocated as needed.

How it works with ext3
Next3 uses the same on-disk structures as ext3, so it can be mounted as read-only by existing ext3 tools. Its directory layout, file allocation bitmap, and metadata tables are compatible with ext3. Snapshots appear as files within the volume’s free space. Because it keeps ext3 compatibility, Next3 does not include newer features like extents.

Performance and overhead
Without snapshots, Next3 performs like ext3. With snapshots, there is a small overhead: a bit more work for metadata writes (copy-on-write) and a small (~1%) overhead for data writes.

Key details
- Partition IDs: 0x83 (MBR) and a GPT GUID
- Directory structure: hashed B-tree with dir_index
- Limits: 2 TB to 16 TB for volume size; 2 TB per file; 254-character filename limit
- Timestamps: record mtime, ctime, and atime from 1901-12-14 to 2038-01-18, with 1-second resolution
- Attributes: Unix permissions, ACLs, and security attributes
- Features: no built-in compression or encryption (encryption is at the block device level), no data deduplication

Software and future
Next3 is implemented for Linux and is open source. As of 2011, a project called Next4 aimed to port Next3’s snapshot capabilities to Ext4, with contributions from the Pune Institute of Computer Technology (PICT) and the Chinese Academy of Sciences.