---
name: "bup-margin"
section: "1"
platform: "linux"
package: "bup"
version: "0.34-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/bup-margin"
---

# bup-margin(1)

figure out your deduplication safety margin

## NAME

bup-margin - figure out your deduplication safety margin

## SYNOPSIS

bup margin \[options...\]

## DESCRIPTION

`bup margin` iterates through all objects in your bup repository, calculating the largest number of prefix bits shared between any two entries. This number, `n`, identifies the longest subset of SHA-1 you could use and still encounter a collision between your object ids.

For example, one system that was tested had a collection of 11 million objects (70 GB), and `bup margin` returned 45. That means a 46-bit hash would be sufficient to avoid all collisions among that set of objects; each object in that repository could be uniquely identified by its first 46 bits.

The number of bits needed seems to increase by about 1 or 2 for every doubling of the number of objects. Since SHA-1 hashes have 160 bits, that leaves 115 bits of margin. Of course, because SHA-1 hashes are essentially random, it’s theoretically possible to use many more bits with far fewer objects.

If you’re paranoid about the possibility of SHA-1 collisions, you can monitor your repository by running `bup margin` occasionally to see if you’re getting dangerously close to 160 bits.

## OPTIONS

- \--predict

  Guess the offset into each index file where a particular object will appear, and report the maximum deviation of the correct answer from the guess. This is potentially useful for tuning an interpolation search algorithm.

- \--ignore-midx

  don’t use `.midx` files, use only `.idx` files. This is only really useful when used with `--predict`.

## EXAMPLES

-

  ```
  $ bup margin
  Reading indexes: 100.00% (1612581/1612581), done.
  40
  40 matching prefix bits
  1.94 bits per doubling
  120 bits (61.86 doublings) remaining
  4.19338e+18 times larger is possible
  Everyone on earth could have 625878182 data sets
  like yours, all in one repository, and we would
  expect 1 object collision.
  $ bup margin --predict
  PackIdxList: using 1 index.
  Reading indexes: 100.00% (1612581/1612581), done.
  915 of 1612581 (0.057%)
  ```

## SEE ALSO

[`bup-midx`(1)](/bup-midx/1.md), [`bup-save`(1)](/bup-save/1.md)

## BUP

Part of the [`bup`(1)](/bup/1.md) suite.

## AUTHORS

Avery Pennarun ⟨apenwarr@gmail.com⟩\\c.