NAME

git-fast-import - Backend for fast Git data importers

SYNOPSIS

frontend | git fast-import [<options>]

DESCRIPTION

This program is usually not what the end user wants to run directly.Most end users want to use one of the existing frontend programs,which parses a specific type of foreign source and feeds the contentsstored there to git fast-import.

fast-import reads a mixed command/data stream from standard input andwrites one or more packfiles directly into the current repository.When EOF is received on standard input, fast import writes outupdated branch and tag refs, fully updating the current repositorywith the newly imported data.

The fast-import backend itself can import into an empty repository (one thathas already been initialized by git init) or incrementallyupdate an existing populated repository. Whether or not incrementalimports are supported from a particular foreign source depends onthe frontend program in use.

OPTIONS

• —force

• Force updating modified existing branches, even if doingso would cause commits to be lost (as the new commit doesnot contain the old commit).

• —quiet

• Disable the output shown by —stats, making fast-import usuallybe silent when it is successful. However, if the import streamhas directives intended to show user output (e.g. progressdirectives), the corresponding messages will still be shown.

• —stats

• Display some basic statistics about the objects fast-import hascreated, the packfiles they were stored into, and thememory used by fast-import during this run. Showing this outputis currently the default, but can be disabled with —quiet.

Options for Frontends

• —cat-blob-fd=

• Write responses to get-mark, cat-blob, and ls queries to thefile descriptor instead of stdout. Allows progressoutput intended for the end-user to be separated from otheroutput.

• —date-format=

• Specify the type of dates the frontend will supply tofast-import within author, committer and tagger commands.See “Date Formats” below for details about which formatsare supported, and their syntax.

• —done

• Terminate with error if there is no done command at the end ofthe stream. This option might be useful for detecting errorsthat cause the frontend to terminate before it has started towrite a stream.

Locations of Marks Files

• —export-marks=

• Dumps the internal marks table to when complete.Marks are written one per line as :markid SHA-1.Frontends can use this file to validate imports after theyhave been completed, or to save the marks table acrossincremental runs. As is only opened and truncatedat checkpoint (or completion) the same path can also besafely given to —import-marks.

• —import-marks=

• Before processing any input, load the marks specified in. The input file must exist, must be readable, andmust use the same format as produced by —export-marks.Multiple options may be supplied to import more than oneset of marks. If a mark is defined to different values,the last file wins.

• —import-marks-if-exists=

• Like —import-marks but instead of erroring out, silentlyskips the file if it does not exist.

• —[no-]relative-marks

• After specifying —relative-marks the paths specifiedwith —import-marks= and —export-marks= are relativeto an internal directory in the current repository.In git-fast-import this means that the paths are relativeto the .git/info/fast-import directory. However, otherimporters may use a different location.

Relative and non-relative marks may be combined by interweaving—(no-)-relative-marks with the —(import|export)-marks= options.

Performance and Compression Tuning

• —active-branches=

• Maximum number of branches to maintain active at once.See “Memory Utilization” below for details. Default is 5.

• —big-file-threshold=

• Maximum size of a blob that fast-import will attempt tocreate a delta for, expressed in bytes. The default is 512m(512 MiB). Some importers may wish to lower this on systemswith constrained memory.

• —depth=

• Maximum delta depth, for blob and tree deltification.Default is 50.

• —export-pack-edges=

• After creating a packfile, print a line of data to listing the filename of the packfile and the lastcommit on each branch that was written to that packfile.This information may be useful after importing projectswhose total object set exceeds the 4 GiB packfile limit,as these commits can be used as edge points during callsto git pack-objects.

• —max-pack-size=

• Maximum size of each output packfile.The default is unlimited.

• fastimport.unpackLimit

• See git-config[1]

PERFORMANCE

The design of fast-import allows it to import large projects in a minimumamount of memory usage and processing time. Assuming the frontendis able to keep up with fast-import and feed it a constant stream of data,import times for projects holding 10+ years of history and containing100,000+ individual commits are generally completed in just 1-2hours on quite modest (~$2,000 USD) hardware.

Most bottlenecks appear to be in foreign source data access (thesource just cannot extract revisions fast enough) or disk IO (fast-importwrites as fast as the disk will take the data). Imports will runfaster if the source data is stored on a different drive than thedestination Git repository (due to less IO contention).

DEVELOPMENT COST

A typical frontend for fast-import tends to weigh in at approximately 200lines of Perl/Python/Ruby code. Most developers have been able tocreate working importers in just a couple of hours, even though itis their first exposure to fast-import, and sometimes even to Git. This isan ideal situation, given that most conversion tools are throw-away(use once, and never look back).

PARALLEL OPERATION

Like git push or git fetch, imports handled by fast-import are safe torun alongside parallel git repack -a -d or git gc invocations,or any other Git operation (including git prune, as loose objectsare never used by fast-import).

fast-import does not lock the branch or tag refs it is actively importing.After the import, during its ref update phase, fast-import tests eachexisting branch ref to verify the update will be a fast-forwardupdate (the commit stored in the ref is contained in the newhistory of the commit to be written). If the update is not afast-forward update, fast-import will skip updating that ref and insteadprints a warning message. fast-import will always attempt to update allbranch refs, and does not stop on the first failure.

Branch updates can be forced with —force, but it’s recommended thatthis only be used on an otherwise quiet repository. Using —forceis not necessary for an initial import into an empty repository.

TECHNICAL DISCUSSION

fast-import tracks a set of branches in memory. Any branch can be createdor modified at any point during the import process by sending acommit command on the input stream. This design allows a frontendprogram to process an unlimited number of branches simultaneously,generating commits in the order they are available from the sourcedata. It also simplifies the frontend programs considerably.

fast-import does not use or alter the current working directory, or anyfile within it. (It does however update the current Git repository,as referenced by GIT_DIR.) Therefore an import frontend may usethe working directory for its own purposes, such as extracting filerevisions from the foreign source. This ignorance of the workingdirectory also allows fast-import to run very quickly, as it does notneed to perform any costly file update operations when switchingbetween branches.

INPUT FORMAT

With the exception of raw file data (which Git does not interpret)the fast-import input format is text (ASCII) based. This text basedformat simplifies development and debugging of frontend programs,especially when a higher level language such as Perl, Python orRuby is being used.

fast-import is very strict about its input. Where we say SP below we meanexactly one space. Likewise LF means one (and only one) linefeedand HT one (and only one) horizontal tab.Supplying additional whitespace characters will cause unexpectedresults, such as branch names or file names with leading or trailingspaces in their name, or early termination of fast-import when it encountersunexpected input.

Stream Comments

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

Date Formats

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

• raw
• This is the Git native format and is <time> SP <offutc>.It is also fast-import’s default format, if —date-format wasnot specified.

The time of the event is specified by <time> as the number ofseconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and iswritten as an ASCII decimal integer.

The local offset is specified by <offutc> as a positive or negativeoffset from UTC. For example EST (which is 5 hours behind UTC)would be expressed in <tz> by “-0500” while UTC is “+0000”.The local offset does not affect <time>; it is used only as anadvisement to help formatting routines display the timestamp.

If the local offset is not available in the source material, use“+0000”, or the most common local offset. For example manyorganizations have a CVS repository which has only ever been accessedby users who are located in the same location and time zone. In thiscase a reasonable offset from UTC could be assumed.

Unlike the rfc2822 format, this format is very strict. Anyvariation in formatting will cause fast-import to reject the value.

• rfc2822
• This is the standard email format as described by RFC 2822.

An example value is “Tue Feb 6 11:22:18 2007 -0500”. The Gitparser is accurate, but a little on the lenient side. It is thesame parser used by git am when applying patchesreceived from email.

Some malformed strings may be accepted as valid dates. In some ofthese cases Git will still be able to obtain the correct date fromthe malformed string. There are also some types of malformedstrings which Git will parse wrong, and yet consider valid.Seriously malformed strings will be rejected.

Unlike the raw format above, the time zone/UTC offset informationcontained in an RFC 2822 date string is used to adjust the datevalue to UTC prior to storage. Therefore it is important thatthis information be as accurate as possible.

If the source material uses RFC 2822 style dates,the frontend should let fast-import handle the parsing and conversion(rather than attempting to do it itself) as the Git parser hasbeen well tested in the wild.

Frontends should prefer the raw format if the source materialalready uses UNIX-epoch format, can be coaxed to give dates in thatformat, or its format is easily convertible to it, as there is noambiguity in parsing.

• now
• Always use the current time and time zone. The literalnow must always be supplied for <when>.

This is a toy format. The current time and time zone of this systemis always copied into the identity string at the time it is beingcreated by fast-import. There is no way to specify a different time ortime zone.

This particular format is supplied as it’s short to implement andmay be useful to a process that wants to create a new commitright now, without needing to use a working directory orgit update-index.

If separate author and committer commands are used in a committhe timestamps may not match, as the system clock will be polledtwice (once for each command). The only way to ensure that bothauthor and committer identity information has the same timestampis to omit author (thus copying from committer) or to use adate format other than now.

Commands

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

• commit

• Creates a new branch or updates an existing branch bycreating a new commit and updating the branch to point atthe newly created commit.

• tag

• Creates an annotated tag object from an existing commit orbranch. Lightweight tags are not supported by this command,as they are not recommended for recording meaningful pointsin time.

• reset

• Reset an existing branch (or a new branch) to a specificrevision. This command must be used to change a branch toa specific revision without making a commit on it.

• blob

• Convert raw file data into a blob, for future use in acommit command. This command is optional and is notneeded to perform an import.

• checkpoint

• Forces fast-import to close the current packfile, generate itsunique SHA-1 checksum and index, and start a new packfile.This command is optional and is not needed to performan import.

• progress

• Causes fast-import to echo the entire line to its ownstandard output. This command is optional and is not neededto perform an import.

• done

• Marks the end of the stream. This command is optionalunless the done feature was requested using the—done command-line option or feature done command.

• get-mark

• Causes fast-import to print the SHA-1 corresponding to a markto the file descriptor set with —cat-blob-fd, or stdout ifunspecified.

• cat-blob

• Causes fast-import to print a blob in _cat-file —batch_format to the file descriptor set with —cat-blob-fd orstdout if unspecified.

• ls

• Causes fast-import to print a line describing a directoryentry in ls-tree format to the file descriptor set with—cat-blob-fd or stdout if unspecified.

• feature

• Enable the specified feature. This requires that fast-importsupports the specified feature, and aborts if it does not.

• option

• Specify any of the options listed under OPTIONS that do notchange stream semantic to suit the frontend’s needs. Thiscommand is optional and is not needed to perform an import.

commit

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

    'commit' SP <ref> LF
    mark?
    original-oid?
    ('author' (SP <name>)? SP LT <email> GT SP <when> LF)?
    'committer' (SP <name>)? SP LT <email> GT SP <when> LF
    ('encoding' SP <encoding>)?
    data
    ('from' SP <commit-ish> LF)?
    ('merge' SP <commit-ish> LF)?
    (filemodify | filedelete | filecopy | filerename | filedeleteall | notemodify)*
    LF?

where <ref> is the name of the branch to make the commit on.Typically branch names are prefixed with refs/heads/ inGit, so importing the CVS branch symbol RELENG-1_0 would userefs/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 ina Git refname, no quoting or escaping syntax is supported here.

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

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

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

The LF after the command is optional (it used to be required). Notethat for reasons of backward compatibility, if the commit ends with adata command (i.e. it has no from, merge, filemodify,filedelete, filecopy, filerename, filedeleteall ornotemodify commands) then two LF commands may appear at the end ofthe command instead of just one.

author

An author command may optionally appear, if the author informationmight differ from the committer information. If author is omittedthen fast-import will automatically use the committer’s information forthe author portion of the commit. See below for a description ofthe fields in author, as they are identical to committer.

committer

The committer command indicates who made this commit, and whenthey made it.

Here <name> is the person’s display name (for example“Com M Itter”) and <email> is the person’s email address(“cm@example.com”). LT and GT are the literal less-than (\x3c)and greater-than (\x3e) symbols. These are required to delimitthe email address from the other fields in the line. Note that<name> and <email> are free-form and may contain any sequenceof bytes, except LT, GT and LF. <name> is typically UTF-8 encoded.

The time of the change is specified by <when> using the date formatthat was selected by the —date-format=<fmt> command-line option.See “Date Formats” above for the set of supported formats, andtheir syntax.

encoding

The optional encoding command indicates the encoding of the commitmessage. Most commits are UTF-8 and the encoding is omitted, but thisallows importing commit messages into git without first reencoding them.

from

The from command is used to specify the commit to initializethis branch from. This revision will be the first ancestor of thenew commit. The state of the tree built at this commit will beginwith the state at the from commit, and be altered by the contentmodifications in this commit.

Omitting the from command in the first commit of a new branchwill cause fast-import to create that commit with no ancestor. Thistends to be desired only for the initial commit of a project.If the frontend creates all files from scratch when making a newbranch, a merge command may be used instead of from to startthe commit with an empty tree.Omitting the from command on existing branches is usually desired,as the current commit on that branch is automatically assumed tobe the first ancestor of the new commit.

As LF is not valid in a Git refname or SHA-1 expression, noquoting or escaping syntax is supported within <commit-ish>.

Here <commit-ish> is any of the following:

• The name of an existing branch already in fast-import’s internal branchtable. If fast-import doesn’t know the name, it’s treated as a SHA-1expression.

• A mark reference, :<idnum>, where <idnum> is the mark number.

The reason fast-import uses : to denote a mark reference is this characteris not legal in a Git branch name. The leading : makes it easyto distinguish between the mark 42 (:42) and the branch 42 (42or refs/heads/42), or an abbreviated SHA-1 which happened toconsist only of base-10 digits.

Marks must be declared (via mark) before they can be used.

• A complete 40 byte or abbreviated commit SHA-1 in hex.

• Any valid Git SHA-1 expression that resolves to a commit. See“SPECIFYING REVISIONS” in gitrevisions[7] for details.

• The special null SHA-1 (40 zeros) specifies that the branch is to beremoved.

The special case of restarting an incremental import from thecurrent branch value should be written as:

    from refs/heads/branch^0

The ^0 suffix is necessary as fast-import does not permit a branch tostart from itself, and the branch is created in memory before thefrom command is even read from the input. Adding ^0 will forcefast-import to resolve the commit through Git’s revision parsing library,rather than its internal branch table, thereby loading in theexisting value of the branch.

merge

Includes one additional ancestor commit. The additional ancestrylink does not change the way the tree state is built at this commit.If the from command isomitted when creating a new branch, the first merge commit will bethe first ancestor of the current commit, and the branch will startout with no files. An unlimited number of merge commands percommit are permitted by fast-import, thereby establishing an n-way merge.

Here <commit-ish> is any of the commit specification expressionsalso accepted by from (see above).

filemodify

Included in a commit command to add a new file or change thecontent of an existing file. This command has two different meansof specifying the content of the file.

• External data format
• The data content for the file was already supplied by a priorblob command. The frontend just needs to connect it.

    'M' SP <mode> SP <dataref> SP <path> LF

Here usually <dataref> must be either a mark reference (:<idnum>)set by a prior blob command, or a full 40-byte SHA-1 of anexisting Git blob object. If <mode> is 040000` then<dataref> must be the full 40-byte SHA-1 of an existingGit tree object or a mark reference set with —import-marks.

• Inline data format
• The data content for the file has not been supplied yet.The frontend wants to supply it as part of this modifycommand.

    'M' SP <mode> SP 'inline' SP <path> LF
    data

See below for a detailed description of the data command.

In both formats <mode> is the type of file entry, specifiedin octal. Git only supports the following modes:

• 100644 or 644: A normal (not-executable) file. The majorityof files in most projects use this mode. If in doubt, this iswhat you want.

• 100755 or 755: A normal, but executable, file.

• 120000: A symlink, the content of the file will be the link target.

• 160000: A gitlink, SHA-1 of the object refers to a commit inanother repository. Git links can only be specified by SHA or througha commit mark. They are used to implement submodules.

• 040000: A subdirectory. Subdirectories can only be specified bySHA or through a tree mark set with —import-marks.

In both formats <path> is the complete path of the file to be added(if not already existing) or modified (if already existing).

A <path> string must use UNIX-style directory separators (forwardslash /), may contain any byte other than LF, and must notstart with double quote (").

A path can use C-style string quoting; this is accepted in all casesand mandatory if the filename starts with double quote or containsLF. In C-style quoting, the complete name should be surrounded withdouble quotes, and any LF, backslash, or double quote charactersmust be escaped by preceding them with a backslash (e.g.,"path/with\n, \ and \" in it").

The value of <path> must be in canonical form. That is it must not:

• contain an empty directory component (e.g. foo//bar is invalid),

• end with a directory separator (e.g. foo/ is invalid),

• start with a directory separator (e.g. /foo is invalid),

• contain the special component . or .. (e.g. foo/./bar andfoo/../bar are invalid).

The root of the tree can be represented by an empty string as <path>.

It is recommended that <path> always be encoded using UTF-8.

filedelete

Included in a commit command to remove a file or recursivelydelete an entire directory from the branch. If the file or directoryremoval makes its parent directory empty, the parent directory willbe automatically removed too. This cascades up the tree until thefirst non-empty directory or the root is reached.

    'D' SP <path> LF

here <path> is the complete path of the file or subdirectory tobe removed from the branch.See filemodify above for a detailed description of <path>.

filecopy

Recursively copies an existing file or subdirectory to a differentlocation within the branch. The existing file or directory mustexist. If the destination exists it will be completely replacedby the content copied from the source.

    'C' SP <path> SP <path> LF

here the first <path> is the source location and the second<path> is the destination. See filemodify above for a detaileddescription of what <path> may look like. To use a source paththat contains SP the path must be quoted.

A filecopy command takes effect immediately. Once the sourcelocation has been copied to the destination any future commandsapplied to the source location will not impact the destination ofthe copy.

filerename

Renames an existing file or subdirectory to a different locationwithin the branch. The existing file or directory must exist. Ifthe destination exists it will be replaced by the source directory.

    'R' SP <path> SP <path> LF

here the first <path> is the source location and the second<path> is the destination. See filemodify above for a detaileddescription of what <path> may look like. To use a source paththat contains SP the path must be quoted.

A filerename command takes effect immediately. Once the sourcelocation has been renamed to the destination any future commandsapplied to the source location will create new files there and notimpact the destination of the rename.

Note that a filerename is the same as a filecopy followed by afiledelete of the source location. There is a slight performanceadvantage to using filerename, but the advantage is so smallthat it is never worth trying to convert a delete/add pair insource material into a rename for fast-import. This filerenamecommand is provided just to simplify frontends that already haverename information and don’t want bother with decomposing it into afilecopy followed by a filedelete.

filedeleteall

Included in a commit command to remove all files (and also alldirectories) from the branch. This command resets the internalbranch structure to have no files in it, allowing the frontendto subsequently add all interesting files from scratch.

    'deleteall' LF

This command is extremely useful if the frontend does not know(or does not care to know) what files are currently on the branch,and therefore cannot generate the proper filedelete commands toupdate the content.

Issuing a filedeleteall followed by the needed filemodifycommands to set the correct content will produce the same resultsas sending only the needed filemodify and filedelete commands.The filedeleteall approach may however require fast-import to use slightlymore memory per active branch (less than 1 MiB for even most largeprojects); so frontends that can easily obtain only the affectedpaths for a commit are encouraged to do so.

notemodify

Included in a commit <notes_ref> command to add a new noteannotating a <commit-ish> or change this annotation contents.Internally it is similar to filemodify 100644 on <commit-ish>path (maybe split into subdirectories). It’s not advised touse any other commands to write to the <notes_ref> tree exceptfiledeleteall to delete all existing notes in this tree.This command has two different means of specifying the contentof the note.

• External data format
• The data content for the note was already supplied by a priorblob command. The frontend just needs to connect it to thecommit that is to be annotated.

    'N' SP <dataref> SP <commit-ish> LF

Here <dataref> can be either a mark reference (:<idnum>)set by a prior blob command, or a full 40-byte SHA-1 of anexisting Git blob object.

• Inline data format
• The data content for the note has not been supplied yet.The frontend wants to supply it as part of this modifycommand.

    'N' SP 'inline' SP <commit-ish> LF
    data

See below for a detailed description of the data command.

In both formats <commit-ish> is any of the commit specificationexpressions also accepted by from (see above).

mark

Arranges for fast-import to save a reference to the current object, allowingthe frontend to recall this object at a future point in time, withoutknowing its SHA-1. Here the current object is the object creationcommand the mark command appears within. This can be commit,tag, and blob, but commit is the most common usage.

    'mark' SP ':' <idnum> LF

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 asa mark. Only values greater than or equal to 1 may be used as marks.

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

original-oid

Provides the name of the object in the original source control system.fast-import will simply ignore this directive, but filter processeswhich operate on and modify the stream before feeding to fast-importmay have uses for this information

    'original-oid' SP <object-identifier> LF

where <object-identifer> is any string not containing LF.

tag

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

    'tag' SP <name> LF
    'from' SP <commit-ish> LF
    original-oid?
    'tagger' (SP <name>)? SP LT <email> GT SP <when> LF
    data

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

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

The value of <name> must be a valid refname in Git and thereforemay 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; seeabove for details.

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

The data command following tagger must supply the annotated tagmessage (see below for data command syntax). To import an emptytag message use a 0 length data. Tag messages are free-form and arenot 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 notsupported. Trying to include your own PGP/GPG signature is notrecommended, as the frontend does not (easily) have access to thecomplete set of bytes which normally goes into such a signature.If signing is required, create lightweight tags from within fast-import withreset, then create the annotated versions of those tags offlinewith the standard git tag process.

reset

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

    'reset' SP <ref> LF
    ('from' SP <commit-ish> LF)?
    LF?

For a detailed description of <ref> and <commit-ish> see aboveunder 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:

reset refs/tags/938
from :938

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

blob

Requests writing one file revision to the packfile. The revisionis not connected to any commit; this connection must be formed ina subsequent commit command by referencing the blob through anassigned mark.

    'blob' LF
    mark?
    original-oid?
    data

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

data

Supplies raw data (for use as blob/file content, commit messages, orannotated tag messages) to fast-import. Data can be supplied using an exactbyte count or delimited with a terminating line. Real frontendsintended for production-quality conversions should always use theexact 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 commandsare always taken to be part of the body of the data and are thereforenever ignored by fast-import. This makes it safe to import anyfile/message content whose lines might start with #.

• Exact byte count format
• The frontend must specify the number of bytes of data.

    'data' SP <count> LF
    <raw> LF?

where <count> is the exact number of bytes appearing within<raw>. The value of <count> is expressed as an ASCII decimalinteger. The LF on either side of <raw> is notincluded in <count> and will not be included in the imported data.

The LF after <raw> is optional (it used to be required) butrecommended. Always including it makes debugging a fast-importstream easier as the next command always starts in column 0of the next line, even if <raw> did not end with an LF.

• Delimited format
• A delimiter string is used to mark the end of the data.fast-import will compute the length by searching for the delimiter.This format is primarily useful for testing and is notrecommended for real data.

    'data' SP '<<' <delim> LF
    <raw> LF
    <delim> LF
    LF?

where <delim> is the chosen delimiter string. The string <delim>must not appear on a line by itself within <raw>, as otherwisefast-import will think the data ends earlier than it really does. The LFimmediately trailing <raw> is part of <raw>. This is one ofthe limitations of the delimited format, it is impossible to supplya data chunk which does not have an LF as its last byte.

The LF after <delim> LF is optional (it used to be required).

checkpoint

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

    'checkpoint' LF
    LF?

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

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

Frontends may choose to issue checkpoints during extremely largeand long running imports, or when they need to allow another Gitprocess access to a branch. However given that a 30 GiB Subversionrepository 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).

progress

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

    'progress' SP <any> LF
    LF?

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

frontend | git fast-import | sed 's/^progress //'

Placing a progress command immediately after a checkpoint willinform the reader when the checkpoint has been completed and itcan safely access the refs that fast-import updated.

get-mark

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

    'get-mark' SP ':' <idnum> LF

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

cat-blob

Causes fast-import to print a blob to a file descriptor previouslyarranged with the —cat-blob-fd argument. The command otherwisehas no impact on the current import; its main purpose is toretrieve blobs that may be in fast-import’s memory but notaccessible from the target repository.

    'cat-blob' SP <dataref> LF

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

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

<sha1> SP 'blob' SP <size> LF
<contents> LF

This command can be used where a filemodify directive can appear,allowing it to be used in the middle of a commit. For a filemodifyusing an inline directive, it can also appear right before the datadirective.

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

ls

Prints information about the object at a path to a file descriptorpreviously arranged with the —cat-blob-fd argument. This allowsprinting a blob from the active commit (with cat-blob) or copying ablob or tree from a previous commit for use in the current one (withfilemodify).

The ls command can also be used where a filemodify directive canappear, allowing it to be used in the middle of a commit.

• Reading from the active commit
• This form can only be used in the middle of a commit.The path names a directory entry within fast-import’sactive commit. The path must be quoted in this case.

    'ls' SP <path> LF

• Reading from a named tree
• The <dataref> can be a mark reference (:<idnum>) or thefull 40-byte SHA-1 of a Git tag, commit, or tree object,preexisting or waiting to be written.The path is relative to the top level of the treenamed by <dataref>.

    'ls' SP <dataref> SP <path> LF

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

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

<mode> SP ('blob' | 'tree' | 'commit') SP <dataref> HT <path> LF

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

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

missing SP <path> LF

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

feature

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

    'feature' SP <feature> ('=' <argument>)? LF

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

• date-format
• export-marks
• relative-marks
• no-relative-marks
• force

• Act as though the corresponding command-line option witha leading — was passed on the command line(see OPTIONS, above).

• import-marks

• import-marks-if-exists

• Like —import-marks except in two respects: first, only one"feature import-marks" or "feature import-marks-if-exists"command is allowed per stream; second, an —import-marks=or —import-marks-if-exists command-line option overridesany of these "feature" commands in the stream; third,"feature import-marks-if-exists" like a correspondingcommand-line option silently skips a nonexistent file.

• get-mark

• cat-blob
• ls

• Require that the backend support the get-mark, cat-blob,or ls command respectively.Versions of fast-import not supporting the specified commandwill exit with a message indicating so.This lets the import error out early with a clear message,rather than wasting time on the early part of an importbefore the unsupported command is detected.

• notes

• Require that the backend support the notemodify (N)subcommand to the commit command.Versions of fast-import not supporting notes will exitwith a message indicating so.

• done

• Error out if the stream ends without a done command.Without this feature, errors causing the frontend to endabruptly at a convenient point in the stream can goundetected. This may occur, for example, if an importfront end dies in mid-operation without emitting SIGTERMor SIGKILL at its subordinate git fast-import instance.

option

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

    'option' SP <option> LF

The <option> part of the command may contain any of the optionslisted 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 countingfeature commands), to give an option command after any non-optioncommand is an error.

The following command-line options change import semantics and may thereforenot be passed as option:

• date-format

• import-marks

• export-marks

• cat-blob-fd

• force

done

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 isin use, the done command is mandatory and marks the end of thestream.

RESPONSES TO COMMANDS

New objects written by fast-import are not available immediately.Most fast-import commands have no visible effect until the nextcheckpoint (or completion). The frontend can send commands tofill fast-import’s input pipe without worrying about how quicklythey will take effect, which improves performance by simplifyingscheduling.

For some frontends, though, it is useful to be able to read backdata from the current repository as it is being updated (forexample when the source material describes objects in terms ofpatches to be applied to previously imported objects). This canbe accomplished by connecting the frontend and fast-import viabidirectional pipes:

mkfifo fast-import-output
frontend <fast-import-output |
git fast-import >fast-import-output

A frontend set up this way can use progress, get-mark, ls, andcat-blob commands to read information from the import in progress.

To avoid deadlock, such frontends must completely consume anypending output from progress, ls, get-mark, and cat-blob beforeperforming writes to fast-import that might block.

CRASH REPORTS

If fast-import is supplied invalid input it will terminate with anon-zero exit status and create a crash report in the top level ofthe Git repository it was importing into. Crash reports containa snapshot of the internal fast-import state as well as the mostrecent commands that lead up to the crash.

All recent commands (including stream comments, file changes andprogress commands) are shown in the command history within the crashreport, but raw file data and commit messages are excluded from thecrash report. This exclusion saves space within the report fileand reduces the amount of buffering that fast-import must performduring execution.

After writing a crash report fast-import will close the currentpackfile and export the marks table. This allows the frontenddeveloper to inspect the repository state and resume the import fromthe point where it crashed. The modified branches and tags are notupdated during a crash, as the import did not complete successfully.Branch and tag information can be found in the crash report andmust be applied manually if the update is needed.

An example crash:

$ cat >in <<END_OF_INPUT
# my very first test commit
commit refs/heads/master
committer Shawn O. Pearce <spearce> 19283 -0400
# who is that guy anyway?
data <<EOF
this is my commit
EOF
M 644 inline .gitignore
data <<EOF
.gitignore
EOF
M 777 inline bob
END_OF_INPUT

$ git fast-import <in
fatal: Corrupt mode: M 777 inline bob
fast-import: dumping crash report to .git/fast_import_crash_8434

$ cat .git/fast_import_crash_8434
fast-import crash report:
    fast-import process: 8434
    parent process     : 1391
    at Sat Sep 1 00:58:12 2007

fatal: Corrupt mode: M 777 inline bob

Most Recent Commands Before Crash

my very first test commit

commit refs/heads/master committer Shawn O. Pearce <spearce> 19283 -0400

who is that guy anyway?

data <<EOF M 644 inline .gitignore data <<EOF

• M 777 inline bob

Active Branch LRU

active_branches = 1 cur, 5 max</pre>

pos  clock name
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1)      0 refs/heads/master

Inactive Branches

refs/heads/master: status : active loaded dirty tip commit : 0000000000000000000000000000000000000000 old tree : 0000000000000000000000000000000000000000 cur tree : 0000000000000000000000000000000000000000 commit clock: 0 last pack :

-------------------
END OF CRASH REPORT

TIPS AND TRICKS

The following tips and tricks have been collected from varioususers of fast-import, and are offered here as suggestions.

Use One Mark Per Commit

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

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

Freely Skip Around Branches

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

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

Handling Renames

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

Use Tag Fixup Branches

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

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

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

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

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

Import Now, Repack Later

As soon as fast-import completes the Git repository is completely validand 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 datalocality and access performance. It can also take hours on extremelylarge projects (especially if -f and a large —window parameter isused). 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 benchmarksor performance tests until repacking is completed. fast-import outputssuboptimal packfiles that are simply never seen in real usesituations.

Repacking Historical Data

If you are repacking very old imported data (e.g. older than thelast 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 yourproject will benefit from the smaller repository.

Include Some Progress Messages

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

PACKFILE OPTIMIZATION

When packing a blob fast-import always attempts to deltify against the lastblob written. Unless specifically arranged for by the frontend,this will probably not be a prior version of the same file, so thegenerated delta will not be the smallest possible. The resultingpackfile will be compressed, but will not be optimal.

Frontends which have efficient access to all revisions of asingle file (for example reading an RCS/CVS ,v file) can chooseto supply all revisions of that file as a sequence of consecutiveblob commands. This allows fast-import to deltify the different filerevisions against each other, saving space in the final packfile.Marks can be used to later identify individual file revisions duringa sequence of commit commands.

The packfile(s) created by fast-import do not encourage good disk accesspatterns. This is caused by fast-import writing the data in the orderit is received on standard input, while Git typically organizesdata within packfiles to make the most recent (current tip) dataappear before historical data. Git also clusters commits together,speeding up revision traversal through better cache locality.

For this reason it is strongly recommended that users repack therepository with git repack -a -d after fast-import completes, allowingGit to reorganize the packfiles for faster data access. If blobdeltas are suboptimal (see above) then also adding the -f optionto force recomputation of all deltas can significantly reduce thefinal packfile size (30-50% smaller can be quite typical).

Instead of running git repack you can also run git gc—aggressive, which will also optimize other things after an import(e.g. pack loose refs). As noted in the "AGGRESSIVE" section ingit-gc[1] the —aggressive option will find new deltas withthe -f option to git-repack[1]. For the reasons elaboratedon above using —aggressive after a fast-import is one of the fewcases where it’s known to be worthwhile.

MEMORY UTILIZATION

There are a number of factors which affect how much memory fast-importrequires to perform an import. Like critical sections of coreGit, fast-import uses its own memory allocators to amortize any overheadsassociated with malloc. In practice fast-import tends to amortize anymalloc overheads to 0, due to its use of large block allocations.

per object

fast-import maintains an in-memory structure for every object written inthis execution. On a 32 bit system the structure is 32 bytes,on a 64 bit system the structure is 40 bytes (due to the largerpointer sizes). Objects in the table are not deallocated untilfast-import terminates. Importing 2 million objects on a 32 bit systemwill 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 reusean existing or already written object and avoid writing duplicatesto the output packfile. Duplicate blobs are surprisingly commonin an import, typically due to branch merges in the source.

per mark

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

per branch

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

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

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

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

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

per active tree

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

per active file entry

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

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

SIGNALS

Sending SIGUSR1 to the git fast-import process ends the currentpackfile early, simulating a checkpoint command. The impatientoperator can use this facility to peek at the objects and refs from animport in progress, at the cost of some added running time and worsecompression.

SEE ALSO

git-fast-export[1]

GIT

Part of the git[1] suite