rsync hard link preservation across heterogeneous destination filesystems

Confirmed Behavior

When --hard-links is active and the destination lacks hard link support, rsync silently creates independent copies for each file that was hard-linked on the source. No warning or error is emitted; the transfer completes with exit code 0.

Why This Happens

rsync attempts to recreate hard link relationships by matching files with identical content and metadata. When link() fails on the destination (e.g., FAT32, exFAT, read-only NTFS, or permission-restricted directories), rsync falls back to copying each file independently. This is by design—rsync does not treat link creation failure as a fatal condition.

Filesystem Limitations

• FAT32/exFAT: Cannot support hard links at all; the link() system call will always fail.
• NTFS: Supports hard links, but failures can occur if the destination is mounted read-only, lacks permissions, or resides on certain network filesystems.
• Other: Some network or virtual filesystems (e.g., older SMB, some FUSE mounts) may also reject link().

Practical Implications

The result is silent disk space inflation and broken application logic that depends on shared inodes for deduplication or change detection.

Mitigation Steps

1. Verify destination capability before transfer:

   touch testfile && ln testfile testlink && rm testfile testlink

   If this succeeds, hard links are supported.
2. Use verbose output to observe behavior:

   rsync -avh --hard-links --progress src/ dest/

   Check whether files that should be linked appear as separate copies.
3. Post-transfer verification:

   find dest/ -links +1 -type f

   This lists files that retain hard link counts; absence indicates duplication occurred.
4. Consider alternative destinations: If preserving hard links is critical, use a filesystem that supports them (ext4, XFS, NTFS with proper permissions, ZFS, APFS).

Known Limitations

rsync has no flag to abort when hard link creation fails. The --hard-links option only enables link recreation; it does not enforce it. If link preservation is a hard requirement, pre-check the destination or use a filesystem-aware tool that can report unsupported operations.

One Missing Diagnostic Detail

If you can share the destination filesystem type and mount options (e.g., mount | grep /path/to/dest), I can confirm whether hard links are technically possible there.