bup-config(5) File Formats Manual bup-config(5)

bup-config - bup configuration options

The following options may be set in the relevant git config (git-config(1)). For example:

git --git-dir="$BUP_DIR" config bup.split.trees true

When set, an identifier for the repository which should be unique across all repositories encountered. Because it is currently used in filesystem paths, it must consist of only the characters within double-quotes here: “0123456789_-”, “ABCDEFGHIJKLMNOPQRSTUVWXYZ”, and “abcdefghijklmnopqrstuvwxyz”. Two repo-ids must also not differ only in case (“something” vs “SOMETHING”) unless all all relevant filesystems are case sensitive.
bup init now adds a random identifier when it creates new repositories or refreshes a existing repository that doesn’t have a bup.repo.id, so you can add one to repositories created before this became the norm by re-running bup init. If you do set your own identifier, consider including randomized content to help ensure uniqueness. Note: you should always change the identifier of a copied repository (e.g. locally via cp(1) or across hosts via rsync(1)) since duplicate identifiers can cause significant performance problems, if nothing else.
When true bup-server(1) checks each incoming object against its local index, and if the object already exists, the server suggests to the client that it download the *.idx file that the object was found in so that it can avoid sending duplicate data.
When false the server does not check its local index before writing objects. To avoid writing duplicate objects, the server tells the client to download all of its *.idx files at the start of the session. This mode is useful on more limited server hardware (i.e. routers, slow NAS devices, etc.). If no value is set, and a $BUP_DIR/dumb-server-mode file exists, then bup will act as if this setting were false.
Method used to split data for deduplication, for example by bup save or bup split. Should not normally be changed after adding any data to the repository (see below).
This determines the “granularity” of the deduplication, with larger values producing, on average, larger chunks. The value must be a string like legacy:N where the integer N must be greater than 12 and less than 22. The default of 13 provides backward compatibility, but it is recommended to increase this for larger repositories. N specifies the number of fixed bits in the hash-split algorithm that when all set to one produce a chunk boundary, and thus it determines the average size of the deduplicated objects. This represents a trade-off between the efficiency of the deduplication (fewer bits means better deduplication) as compared to the amount of metadata to keep on disk and the RAM usage during repo operations (more bits means fewer objects, means less metadata space and RAM use). The expected average blob size is 2^bits (1 << bits). A sufficiently small change in a file would cause that much new data to be saved (plus tree metadata). The maximum blob size is four times that. The historical default of legacy:13 is probably small for many current repositories. As mentioned above, setting it higher should decrease the RAM required for many operations by roughly a factor of two per increment while also increasing the repository’s size by some amount (because it exposes less potential deduplication), but the effect depends on the data stored in the repository (file sizes, deduplication rates, etc.). If you have the space and time, you can always test different values for your data by comparing bup get --rewrites to new repositories with different settings. legacy refers to the current split method, which has an unintentional, but harmless quirk. See DESIGN in the source tree for further details. NOTE: Changing this value in an existing repository will duplicate data because it causes the split boundaries to change, so subsequent saves will not deduplicate against the existing data; they will just store the data again. NOTE: As with bup.split.trees below (see NOTE), using the same index for repositories with different bup.split.files settings will result in the index optimizations not working correctly, and so bup save will have to completely re-read files that haven’t been modified, which is expensive.
When this boolean option is set to true, bup will attempt to split trees (directories) when writing to the repository during, for example bup save ..., bup gc .., etc. This can notably decrease the size of the new data added to the repository when large directories have changed (e.g. large active Maildirs). See “Handling large directories” in the DESIGN in the bup source for additional information.
NOTE: Using the same index to save to repositories that have differing values for this option can decrease performance because the index includes hashes for directories that have been saved and changing this option changes the hashes for directories that are affected by splitting. A directory tree’s hash allows bup to avoid traversing the directory if the index indicates that it didn’t otherwise change and the tree object with that hash already exists in the destination repository. Since the the value of this setting changes the hashes of splittable trees, the hash in the index won’t be found in a repository that has a different bup.split.trees value from the one to which that tree was last saved. As a result, any (usually big) directory subject to tree splitting will have to be re-read and its related hashes recalculated.
The default pack file compression level if core.compression isn’t set. If this isn’t set either, the default is 1 (unlike git, which defaults to -1). A compression level given on the command-line overrides this. See also git-config(1).
The default pack file compression level. If not given, falls back to core.compression. See also git-config(1).
Limits the maximum pack size (see git-config(1)) when writing pack files (e.g. via bup save). This setting is relevant when set in the destination repository (which may be remote). The default value is 1e9 bytes, i.e. about 0.93 GiB, and bup may exceed this limit by a chunk. However, setting it to e.g. “2g” (2 GiB) will still mean that all objects in the pack can be addressed by a 31-bit offset, and thus need no large offset in the idx file.

When set, the default repository location, unless overridden by bup -d on the command line.
The preferred cache location.

$XDG_CACHE_HOME/bup/remote

~/.cache/bup/remote

$BUP_DIR/index-cache
The client index cache locations, in order of precedence. Remote repository data is stored here during operations like bup save -r. Currently bup looks for existing data in precedence order, and if none is found for the repository of interest then bup will use $XDG_CACHE_HOME/bup/remote if $XDG_CACHE_HOME is set and ~/.cache/bup/remote otherwise.

git-config(1)

Part of the bup(1) suite.

Rob Browning rlb@defaultvalue.org.

2026-09-01 Bup 0.34+