To aid in debugging frontends fast-import ignores any line that begins with # (ASCII pound/hash) up to and including the line ending LF. A comment line may contain any sequence of bytes that does not contain an LF and therefore may be used to include any detailed debugging information that might be specific to the frontend and useful when inspecting a fast-import data stream.

The following date formats are supported. A frontend should select the format it will use for this import by passing the format name in the --date-format=<fmt> command-line option.

fast-import accepts several commands to update the current repository and control the current import process. More detailed discussion (with examples) of each command follows later.

Create or update a branch with a new commit, recording one logical change to the project.

where <ref> is the name of the branch to make the commit on. Typically branch names are prefixed with refs/heads/ in Git, so importing the CVS branch symbol RELENG-1_0 would use refs/heads/RELENG-1_0 for the value of <ref>. The value of <ref> must be a valid refname in Git. As LF is not valid in a Git refname, no quoting or escaping syntax is supported here.

A mark command may optionally appear, requesting fast-import to save a reference to the newly created commit for future use by the frontend (see below for format). It is very common for frontends to mark every commit they create, thereby allowing future branch creation from any imported commit.

The data command following committer must supply the commit message (see below for data command syntax). To import an empty commit message use a 0 length data. Commit messages are free-form and are not interpreted by Git. Currently they must be encoded in UTF-8, as fast-import does not permit other encodings to be specified.

Zero or more filemodify, filedelete, filecopy, filerename, filedeleteall and notemodify commands may be included to update the contents of the branch prior to creating the commit. These commands may be supplied in any order. However it is recommended that a filedeleteall command precede all filemodify, filecopy, filerename and notemodify commands in the same commit, as filedeleteall wipes the branch clean (see below).

The LF after the command is optional (it used to be required).

Arranges for fast-import to save a reference to the current object, allowing the frontend to recall this object at a future point in time, without knowing its SHA-1. Here the current object is the object creation command the mark command appears within. This can be commit, tag, and blob, but commit is the most common usage.

where <idnum> is the number assigned by the frontend to this mark. The value of <idnum> is expressed as an ASCII decimal integer. The value 0 is reserved and cannot be used as a mark. Only values greater than or equal to 1 may be used as marks.

New marks are created automatically. Existing marks can be moved to another object simply by reusing the same <idnum> in another mark command.

Creates an annotated tag referring to a specific commit. To create lightweight (non-annotated) tags see the reset command below.

where <name> is the name of the tag to create.

Tag names are automatically prefixed with refs/tags/ when stored in Git, so importing the CVS branch symbol RELENG-1_0-FINAL would use just RELENG-1_0-FINAL for <name>, and fast-import will write the corresponding ref as refs/tags/RELENG-1_0-FINAL.

The value of <name> must be a valid refname in Git and therefore may contain forward slashes. As LF is not valid in a Git refname, no quoting or escaping syntax is supported here.

The from command is the same as in the commit command; see above for details.

The tagger command uses the same format as committer within commit; again see above for details.

The data command following tagger must supply the annotated tag message (see below for data command syntax). To import an empty tag message use a 0 length data. Tag messages are free-form and are not interpreted by Git. Currently they must be encoded in UTF-8, as fast-import does not permit other encodings to be specified.

Signing annotated tags during import from within fast-import is not supported. Trying to include your own PGP/GPG signature is not recommended, as the frontend does not (easily) have access to the complete set of bytes which normally goes into such a signature. If signing is required, create lightweight tags from within fast-import with reset, then create the annotated versions of those tags offline with the standard git tag process.

Creates (or recreates) the named branch, optionally starting from a specific revision. The reset command allows a frontend to issue a new from command for an existing branch, or to create a new branch from an existing commit without creating a new commit.

For a detailed description of <ref> and <commit-ish> see above under commit and from.

The LF after the command is optional (it used to be required).

The reset command can also be used to create lightweight (non-annotated) tags. For example:

would create the lightweight tag refs/tags/938 referring to whatever commit mark :938 references.

Requests writing one file revision to the packfile. The revision is not connected to any commit; this connection must be formed in a subsequent commit command by referencing the blob through an assigned mark.

The mark command is optional here as some frontends have chosen to generate the Git SHA-1 for the blob on their own, and feed that directly to commit. This is typically more work than it’s worth however, as marks are inexpensive to store and easy to use.

Supplies raw data (for use as blob/file content, commit messages, or annotated tag messages) to fast-import. Data can be supplied using an exact byte count or delimited with a terminating line. Real frontends intended for production-quality conversions should always use the exact byte count format, as it is more robust and performs better. The delimited format is intended primarily for testing fast-import.

Comment lines appearing within the <raw> part of data commands are always taken to be part of the body of the data and are therefore never ignored by fast-import. This makes it safe to import any file/message content whose lines might start with #.

Forces fast-import to close the current packfile, start a new one, and to save out all current branch refs, tags and marks.

Note that fast-import automatically switches packfiles when the current packfile reaches --max-pack-size, or 4 GiB, whichever limit is smaller. During an automatic packfile switch fast-import does not update the branch refs, tags or marks.

As a checkpoint can require a significant amount of CPU time and disk IO (to compute the overall pack SHA-1 checksum, generate the corresponding index file, and update the refs) it can easily take several minutes for a single checkpoint command to complete.

Frontends may choose to issue checkpoints during extremely large and long running imports, or when they need to allow another Git process access to a branch. However given that a 30 GiB Subversion repository can be loaded into Git through fast-import in about 3 hours, explicit checkpointing may not be necessary.

The LF after the command is optional (it used to be required).

Causes fast-import to print the entire progress line unmodified to its standard output channel (file descriptor 1) when the command is processed from the input stream. The command otherwise has no impact on the current import, or on any of fast-import’s internal state.

The <any> part of the command may contain any sequence of bytes that does not contain LF. The LF after the command is optional. Callers may wish to process the output through a tool such as sed to remove the leading part of the line, for example:

Placing a progress command immediately after a checkpoint will inform the reader when the checkpoint has been completed and it can safely access the refs that fast-import updated.

Causes fast-import to print the SHA-1 corresponding to a mark to stdout or to the file descriptor previously arranged with the --cat-blob-fd argument. The command otherwise has no impact on the current import; its purpose is to retrieve SHA-1s that later commits might want to refer to in their commit messages.

This command can be used anywhere in the stream that comments are accepted. In particular, the get-mark command can be used in the middle of a commit but not in the middle of a data command.

See “Responses To Commands” below for details about how to read this output safely.

Causes fast-import to print a blob to a file descriptor previously arranged with the --cat-blob-fd argument. The command otherwise has no impact on the current import; its main purpose is to retrieve blobs that may be in fast-import’s memory but not accessible from the target repository.

The <dataref> can be either a mark reference (:<idnum>) set previously or a full 40-byte SHA-1 of a Git blob, preexisting or ready to be written.

Output uses the same format as git cat-file --batch:

This command can be used anywhere in the stream that comments are accepted. In particular, the cat-blob command can be used in the middle of a commit but not in the middle of a data command.

See “Responses To Commands” below for details about how to read this output safely.

Prints information about the object at a path to a file descriptor previously arranged with the --cat-blob-fd argument. This allows printing a blob from the active commit (with cat-blob) or copying a blob or tree from a previous commit for use in the current one (with filemodify).

The ls command can be used anywhere in the stream that comments are accepted, including the middle of a commit.

See filemodify above for a detailed description of <path>.

Output uses the same format as git ls-tree <tree> -- <path>:

The <dataref> represents the blob, tree, or commit object at <path> and can be used in later get-mark, cat-blob, filemodify, or ls commands.

If there is no file or subtree at that path, git fast-import will instead report

See “Responses To Commands” below for details about how to read this output safely.

Require that fast-import supports the specified feature, or abort if it does not.

The <feature> part of the command may be any one of the following:

Processes the specified option so that git fast-import behaves in a way that suits the frontend’s needs. Note that options specified by the frontend are overridden by any options the user may specify to git fast-import itself.

The <option> part of the command may contain any of the options listed in the OPTIONS section that do not change import semantics, without the leading -- and is treated in the same way.

Option commands must be the first commands on the input (not counting feature commands), to give an option command after any non-option command is an error.

The following command-line options change import semantics and may therefore not be passed as option:

If the done feature is not in use, treated as if EOF was read. This can be used to tell fast-import to finish early.

If the --done command-line option or feature done command is in use, the done command is mandatory and marks the end of the stream.

When doing a repository conversion, use a unique mark per commit (mark :<n>) and supply the --export-marks option on the command line. fast-import will dump a file which lists every mark and the Git object SHA-1 that corresponds to it. If the frontend can tie the marks back to the source repository, it is easy to verify the accuracy and completeness of the import by comparing each Git commit to the corresponding source revision.

Coming from a system such as Perforce or Subversion this should be quite simple, as the fast-import mark can also be the Perforce changeset number or the Subversion revision number.

Don’t bother trying to optimize the frontend to stick to one branch at a time during an import. Although doing so might be slightly faster for fast-import, it tends to increase the complexity of the frontend code considerably.

The branch LRU builtin to fast-import tends to behave very well, and the cost of activating an inactive branch is so low that bouncing around between branches has virtually no impact on import performance.

When importing a renamed file or directory, simply delete the old name(s) and modify the new name(s) during the corresponding commit. Git performs rename detection after-the-fact, rather than explicitly during a commit.

Some other SCM systems let the user create a tag from multiple files which are not from the same commit/changeset. Or to create tags which are a subset of the files available in the repository.

Importing these tags as-is in Git is impossible without making at least one commit which “fixes up” the files to match the content of the tag. Use fast-import’s reset command to reset a dummy branch outside of your normal branch space to the base commit for the tag, then commit one or more file fixup commits, and finally tag the dummy branch.

For example since all normal branches are stored under refs/heads/ name the tag fixup branch TAG_FIXUP. This way it is impossible for the fixup branch used by the importer to have namespace conflicts with real branches imported from the source (the name TAG_FIXUP is not refs/heads/TAG_FIXUP).

When committing fixups, consider using merge to connect the commit(s) which are supplying file revisions to the fixup branch. Doing so will allow tools such as git blame to track through the real commit history and properly annotate the source files.

After fast-import terminates the frontend will need to do rm .git/TAG_FIXUP to remove the dummy branch.

As soon as fast-import completes the Git repository is completely valid and ready for use. Typically this takes only a very short time, even for considerably large projects (100,000+ commits).

However repacking the repository is necessary to improve data locality and access performance. It can also take hours on extremely large projects (especially if -f and a large --window parameter is used). Since repacking is safe to run alongside readers and writers, run the repack in the background and let it finish when it finishes. There is no reason to wait to explore your new Git project!

If you choose to wait for the repack, don’t try to run benchmarks or performance tests until repacking is completed. fast-import outputs suboptimal packfiles that are simply never seen in real use situations.

If you are repacking very old imported data (e.g. older than the last year), consider expending some extra CPU time and supplying --window=50 (or higher) when you run git repack. This will take longer, but will also produce a smaller packfile. You only need to expend the effort once, and everyone using your project will benefit from the smaller repository.

Every once in a while have your frontend emit a progress message to fast-import. The contents of the messages are entirely free-form, so one suggestion would be to output the current month and year each time the current commit date moves into the next month. Your users will feel better knowing how much of the data stream has been processed.

fast-import maintains an in-memory structure for every object written in this execution. On a 32 bit system the structure is 32 bytes, on a 64 bit system the structure is 40 bytes (due to the larger pointer sizes). Objects in the table are not deallocated until fast-import terminates. Importing 2 million objects on a 32 bit system will require approximately 64 MiB of memory.

The object table is actually a hashtable keyed on the object name (the unique SHA-1). This storage configuration allows fast-import to reuse an existing or already written object and avoid writing duplicates to the output packfile. Duplicate blobs are surprisingly common in an import, typically due to branch merges in the source.

Marks are stored in a sparse array, using 1 pointer (4 bytes or 8 bytes, depending on pointer size) per mark. Although the array is sparse, frontends are still strongly encouraged to use marks between 1 and n, where n is the total number of marks required for this import.

Branches are classified as active and inactive. The memory usage of the two classes is significantly different.

Inactive branches are stored in a structure which uses 96 or 120 bytes (32 bit or 64 bit systems, respectively), plus the length of the branch name (typically under 200 bytes), per branch. fast-import will easily handle as many as 10,000 inactive branches in under 2 MiB of memory.

Active branches have the same overhead as inactive branches, but also contain copies of every tree that has been recently modified on that branch. If subtree include has not been modified since the branch became active, its contents will not be loaded into memory, but if subtree src has been modified by a commit since the branch became active, then its contents will be loaded in memory.

As active branches store metadata about the files contained on that branch, their in-memory storage size can grow to a considerable size (see below).

fast-import automatically moves active branches to inactive status based on a simple least-recently-used algorithm. The LRU chain is updated on each commit command. The maximum number of active branches can be increased or decreased on the command line with --active-branches=.

Trees (aka directories) use just 12 bytes of memory on top of the memory required for their entries (see “per active file” below). The cost of a tree is virtually 0, as its overhead amortizes out over the individual file entries.

Files (and pointers to subtrees) within active trees require 52 or 64 bytes (32/64 bit platforms) per entry. To conserve space, file and tree names are pooled in a common string table, allowing the filename “Makefile” to use just 16 bytes (after including the string header overhead) no matter how many times it occurs within the project.

The active branch LRU, when coupled with the filename string pool and lazy loading of subtrees, allows fast-import to efficiently import projects with 2,000+ branches and 45,114+ files in a very limited memory footprint (less than 2.7 MiB per active branch).