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Rsync’s --link-dest option can make each backup snapshot look like a full copy while sharing unchanged file data with an earlier snapshot. In one small, author-reported test, three snapshots occupied 1,380 KB (about 1.4 MB), compared with 3,540 KB (about 3.5 MB) for three ordinary copies. Those figures describe that example, not a general storage guarantee.
What the 1.4 MB versus 3.5 MB example shows
MonkeyRun reports a source tree and individual complete snapshot size of 1,180 KB. After making three snapshots—with a 200,000-byte file changed before the second and a text edit before the third—the backup root reportedly occupied 1,380 KB. The 3,540 KB comparison is arithmetic: three ordinary copies at 1,180 KB each, not a second measured test. The author reports that the second snapshot added 196 KB and the third added 4 KB. These are small, self-reported results from one example, not a benchmark of typical backup workloads. MonkeyRun’s example and measurements
With hard links, adding the sizes displayed for each apparently complete snapshot can overstate actual storage: the same underlying file data may be referenced by more than one snapshot. Tools such as du may count shared inodes once during a single invocation, so separate per-snapshot totals should not be added as though each copy occupied independent space.
How rsync hard-linked snapshots work
The rsync manual describes --link-dest=DIR as behaving like --copy-dest, except unchanged files are hard-linked from the basis directory. In practice, rsync can reuse a file from that directory when the file matches in preserved attributes. Each snapshot still has its own directory tree and paths; unchanged files can point to the same underlying file data, while changed files need their own data. Rsync project’s rsync(1) manual
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The example uses rsync -a --delete --link-dest=$LATEST and updates a latest symlink after each run. MonkeyRun reports seeing the same inode number and a link count of two for an unchanged file in two snapshots, while a rewritten file had different inode numbers. This illustrates the mechanism; those are the author’s observations, not a guarantee about every run or filesystem.
What rsync compares—and when to use checksums
By default, rsync’s quick check compares file size and modification time to decide whether a file needs transferring. A workflow that deliberately preserves those values while changing file contents can therefore make different content appear unchanged. MonkeyRun demonstrates a same-size, same-mtime mismatch that the default check did not transfer. That is a specific risk when timestamps are preserved, not the expected result of every ordinary edit.
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Adding --checksum makes rsync compare checksums for same-sized files. It can detect content differences that size-and-time checks miss, but requires reading file contents and adds disk I/O. Choose it when the risk of preserved metadata masking a change outweighs that cost. Rsync project’s rsync(1) manual
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What archive mode preserves
The example’s -a flag means archive mode, documented as -rlptgoD. Despite the name, that set does not include every kind of metadata or preserve hard-link relationships within the source tree.
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-a: ACLs, extended attributes, access times and creation times. - Source hard links: archive mode does not preserve the source’s hard-link relationships.
If any of these are important, assess the relevant rsync options and your destination filesystem rather than assuming archive mode covers them. Rsync project’s rsync(1) manual
How deletion and retention behave
In the example, running with --delete removes a deleted source file from the new snapshot, while an older retained snapshot keeps its directory entry. If your situation is “I deleted a folder yesterday,” that older snapshot may let you recover it—provided the snapshot containing it still exists and is usable. MonkeyRun’s example
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Retention counts snapshot directories, not inherently days. In MonkeyRun’s script, the default KEEP=7 means seven snapshot directories; it means seven days only if the job creates one snapshot per day. The article reports that its minute-resolution naming and broad pruning pattern were fixed in v1.6, released 2026-09-30. Those statements describe the author’s script and release; rsync’s manual does not verify them. MonkeyRun’s article and script notes
What this protects against—and what it does not
Older snapshots can provide a local history of file changes and help recover from some accidental deletions or edits. But snapshots stored on the same physical disk do not protect against that disk failing. MonkeyRun puts the distinction this way: “A snapshot tree on the same volume as its source is not a backup against a dead disk – it is an undo history.” A process that can write to the backup root may also alter snapshots, and theft or destructive malware may affect both source and backup. MonkeyRun’s explanation of snapshot limitations
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For protection against failure of the source device, use a genuinely separate destination, such as an external drive or NAS. That separation alone does not make a backup off-site or immutable: those are distinct protections to consider if your risks include theft, fire or malicious deletion.
Is rsync hard-link backup a good fit?
This approach is useful when you want multiple browsable snapshot trees and unchanged files dominate between runs. Its space savings depend on how much data changes; every changed version retained needs its own storage. Before relying on it, decide what metadata your files need, whether default size-and-time checks suit your workflow, how often snapshots run, how many you retain, and whether the destination is isolated from the failures you need to withstand.
Quick Recap
- Good fit: a simple local history with convenient access to older file versions and a relatively low rate of change.
- Needs extra care: workflows that preserve timestamps, require ACLs or extended attributes, or depend on preserving source hard links.
- Not sufficient on its own: protection from failure of the disk holding both source and snapshots, or from an attacker who can modify the backup root.
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