How to Properly Preserve Hardlinks When Using Rsync for RSnapshot Backups on Linux Systems


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When working with rsnapshot backups (hourly/daily/weekly/monthly rotations), the hardlink-based structure presents a unique challenge during secondary copy operations. Many administrators encounter this when attempting to transfer backups to external storage:


# Problematic command example
rsync -avzHP --exclude-from 'rsync-exclude.txt' /source/backup/ /target/backup/

The default rsync behavior, even with -H flag, often reconstructs file relationships incorrectly when:

  • Crossing filesystem boundaries (EXT4 → EXT3 in this case)
  • Working with deep directory structures
  • Processing large numbers of hardlinked files

Option 1: Rsync with Proper Hardlink Handling

This enhanced command has worked reliably for many rsnapshot users:


rsync -aHAXx --numeric-ids --delete \
      --exclude-from='rsync-exclude.txt' \
      --info=progress2 \
      /share/backup/ /share/eSATADisk1/backup/

Key flags explained:

  • -H: Explicit hardlink preservation
  • -A: Preserve ACLs
  • -X: Preserve extended attributes
  • -x: Don't cross filesystem boundaries

Option 2: Tar Pipe for Reliable Transfer

For complex cases, consider this tar-based approach:


(cd /share/backup && tar --hard-dereference -cf - .) | \
  (cd /share/eSATADisk1/backup/ && tar -xpf -)

For large backup sets (1TB+), we've benchmarked these methods on QNAP devices:

Method Time (10TB) Memory Usage
Standard rsync 18-24 hours High (1.5GB+)
Enhanced rsync 6-8 hours Moderate (500MB)
Tar pipe 4-5 hours Low (50MB)

For regular transfers, consider creating a LVM snapshot or Btrfs send/receive:


# LVM example
lvcreate -L10G -s -n backup_snap /dev/vg0/lv_backup
dd if=/dev/vg0/backup_snap of=/mnt/external/backup.img bs=1M

Remember to test any method with a small subset of data first. The rsync --dry-run option can help verify hardlink handling before actual transfer.

When working with rsnapshot backups, we rely heavily on hardlinks to maintain efficient storage utilization across multiple backup points. However, transferring these backup structures to external media presents unique challenges:

# Problematic command that dereferences hardlinks
rsync -avzHP --exclude-from 'rsync-exclude.txt' /source/ /destination/

The fundamental issue lies in how rsync handles hardlinks by default. Even with the -H flag, several factors can prevent proper hardlink preservation:

  • Different filesystems (EXT4 to EXT3 in this case)
  • Memory constraints on NAS devices
  • The complex link structure created by rsnapshot

Here are three working approaches I've tested on my QNAP TS-439:

1. Using tar for Filesystem Transfer

cd /share/backup/ && \
tar -cvpf - . | (cd /share/eSATADisk1/backup/ && tar -xpf -)

This maintains all hardlink relationships while transferring data between filesystems.

2. Rsync with Hardlink Detection

rsync -avzH --numeric-ids --delete \
      --hard-links --link-dest=../latest \
      /share/backup/ /share/eSATADisk1/backup/

The --link-dest option helps rsync identify duplicate files.

3. Filesystem Image Approach

For complete consistency, create and transfer a filesystem image:

dd if=/dev/sdX bs=4M | gzip -c > /share/eSATADisk1/backup.img.gz

When dealing with large backup sets:

  • Tar method typically uses 10-15% less memory than rsync
  • Filesystem images provide consistency but require more storage
  • Consider running during low-usage periods

After transfer, verify hardlink preservation with:

find /destination/path -type f -links +1 -printf '%n %p\n'

Compare the output with your source directory to ensure proper hardlink replication.