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GIT-FAST-IMPORT(1) Git Manual GIT-FAST-IMPORT(1)
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 contents stored there to git fast-import.
fast-import reads a mixed command/data stream from standard input and writes one or more packfiles directly into the
current repository. When EOF is received on standard input, fast import writes out updated branch and tag refs, fully
updating the current repository with the newly imported data.
The fast-import backend itself can import into an empty repository (one that has already been initialized by git init) or
incrementally update an existing populated repository. Whether or not incremental imports are supported from a particular
foreign source depends on the frontend program in use.
OPTIONS
--date-format=<fmt>
Specify the type of dates the frontend will supply to fast-import within author, committer and tagger commands. See
"Date Formats" below for details about which formats are supported, and their syntax.
--force
Force updating modified existing branches, even if doing so would cause commits to be lost (as the new commit does
not contain the old commit).
--max-pack-size=<n>
Maximum size of each output packfile. The default is unlimited.
--big-file-threshold=<n>
Maximum size of a blob that fast-import will attempt to create a delta for, expressed in bytes. The default is 512m
(512 MiB). Some importers may wish to lower this on systems with constrained memory.
--depth=<n>
Maximum delta depth, for blob and tree deltification. Default is 10.
--active-branches=<n>
Maximum number of branches to maintain active at once. See "Memory Utilization" below for details. Default is 5.
--export-marks=<file>
Dumps the internal marks table to <file> when complete. Marks are written one per line as :markid SHA-1. Frontends
can use this file to validate imports after they have been completed, or to save the marks table across incremental
runs. As <file> is only opened and truncated at checkpoint (or completion) the same path can also be safely given to
--import-marks.
--import-marks=<file>
Before processing any input, load the marks specified in <file>. The input file must exist, must be readable, and
must use the same format as produced by --export-marks. Multiple options may be supplied to import more than one set
of marks. If a mark is defined to different values, the last file wins.
--relative-marks
After specifying --relative-marks= the paths specified with --import-marks= and --export-marks= are relative to an
internal directory in the current repository. In git-fast-import this means that the paths are relative to the
.git/info/fast-import directory. However, other importers may use a different location.
--no-relative-marks
Negates a previous --relative-marks. Allows for combining relative and non-relative marks by interweaving
--(no-)-relative-marks= with the --(import|export)-marks= options.
--cat-blob-fd=<fd>
Specify the file descriptor that will be written to when the cat-blob command is encountered in the stream. The
default behaviour is to write to stdout.
--export-pack-edges=<file>
After creating a packfile, print a line of data to <file> listing the filename of the packfile and the last commit on
each branch that was written to that packfile. This information may be useful after importing projects whose total
object set exceeds the 4 GiB packfile limit, as these commits can be used as edge points during calls to git
pack-objects.
--quiet
Disable all non-fatal output, making fast-import silent when it is successful. This option disables the output shown
by --stats.
--stats
Display some basic statistics about the objects fast-import has created, the packfiles they were stored into, and the
memory used by fast-import during this run. Showing this output is currently the default, but can be disabled with
--quiet.
PERFORMANCE
The design of fast-import allows it to import large projects in a minimum amount of memory usage and processing time.
Assuming the frontend is 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 containing 100,000+ individual commits are generally completed in just 1-2
hours on quite modest (~$2,000 USD) hardware.
Most bottlenecks appear to be in foreign source data access (the source just cannot extract revisions fast enough) or
disk IO (fast-import writes as fast as the disk will take the data). Imports will run faster if the source data is stored
on a different drive than the destination Git repository (due to less IO contention).
DEVELOPMENT COST
A typical frontend for fast-import tends to weigh in at approximately 200 lines of Perl/Python/Ruby code. Most developers
have been able to create working importers in just a couple of hours, even though it is their first exposure to
fast-import, and sometimes even to Git. This is an 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 to run alongside parallel git repack -a -d or git gc
invocations, or any other Git operation (including git prune, as loose objects are 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 each existing branch ref to verify the update will be a fast-forward update (the commit stored in the
ref is contained in the new history of the commit to be written). If the update is not a fast-forward update, fast-import
will skip updating that ref and instead prints a warning message. fast-import will always attempt to update all branch
refs, and does not stop on the first failure.
Branch updates can be forced with --force, but it's recommended that this only be used on an otherwise quiet repository.
Using --force is 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 created or modified at any point during the import
process by sending a commit command on the input stream. This design allows a frontend program to process an unlimited
number of branches simultaneously, generating commits in the order they are available from the source data. It also
simplifies the frontend programs considerably.
fast-import does not use or alter the current working directory, or any file within it. (It does however update the
current Git repository, as referenced by GIT_DIR.) Therefore an import frontend may use the working directory for its own
purposes, such as extracting file revisions from the foreign source. This ignorance of the working directory also allows
fast-import to run very quickly, as it does not need to perform any costly file update operations when switching between
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 based format simplifies development and debugging of frontend programs, especially when a higher level language
such as Perl, Python or Ruby is being used.
fast-import is very strict about its input. Where we say SP below we mean exactly one space. Likewise LF means one (and
only one) linefeed. Supplying additional whitespace characters will cause unexpected results, such as branch names or
file names with leading or trailing spaces in their name, or early termination of fast-import when it encounters
unexpected input.
Stream Comments
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.
Date Formats
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.
raw
This is the Git native format and is <time> SP <offutc>. It is also fast-import's default format, if --date-format
was not specified.
The time of the event is specified by <time> as the number of seconds since the UNIX epoch (midnight, Jan 1, 1970,
UTC) and is written as an ASCII decimal integer.
The local offset is specified by <offutc> as a positive or negative offset 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 an advisement 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 many organizations have a CVS repository which has only ever been accessed by users who are located in the
same location and timezone. In this case a reasonable offset from UTC could be assumed.
Unlike the rfc2822 format, this format is very strict. Any variation 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 Git parser is accurate, but a little on the lenient side. It
is the same parser used by git am when applying patches received from email.
Some malformed strings may be accepted as valid dates. In some of these cases Git will still be able to obtain the
correct date from the malformed string. There are also some types of malformed strings which Git will parse wrong,
and yet consider valid. Seriously malformed strings will be rejected.
Unlike the raw format above, the timezone/UTC offset information contained in an RFC 2822 date string is used to
adjust the date value to UTC prior to storage. Therefore it is important that this 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 has been well tested in the wild.
Frontends should prefer the raw format if the source material already uses UNIX-epoch format, can be coaxed to give
dates in that format, or its format is easily convertible to it, as there is no ambiguity in parsing.
now
Always use the current time and timezone. The literal now must always be supplied for <when>.
This is a toy format. The current time and timezone of this system is always copied into the identity string at the
time it is being created by fast-import. There is no way to specify a different time or timezone.
This particular format is supplied as it's short to implement and may be useful to a process that wants to create a
new commit right now, without needing to use a working directory or git update-index.
If separate author and committer commands are used in a commit the timestamps may not match, as the system clock will
be polled twice (once for each command). The only way to ensure that both author and committer identity information
has the same timestamp is to omit author (thus copying from committer) or to use a date format other than now.
Commands
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.
commit
Creates a new branch or updates an existing branch by creating a new commit and updating the branch to point at the
newly created commit.
tag
Creates an annotated tag object from an existing commit or branch. Lightweight tags are not supported by this
command, as they are not recommended for recording meaningful points in time.
reset
Reset an existing branch (or a new branch) to a specific revision. This command must be used to change a branch to a
specific revision without making a commit on it.
blob
Convert raw file data into a blob, for future use in a commit command. This command is optional and is not needed to
perform an import.
checkpoint
Forces fast-import to close the current packfile, generate its unique SHA-1 checksum and index, and start a new
packfile. This command is optional and is not needed to perform an import.
progress
Causes fast-import to echo the entire line to its own standard output. This command is optional and is not needed to
perform an import.
cat-blob
Causes fast-import to print a blob in cat-file --batch format to the file descriptor set with --cat-blob-fd or stdout
if unspecified.
feature
Require that fast-import supports the specified feature, or abort if it does not.
option
Specify any of the options listed under OPTIONS that do not change stream semantic to suit the frontend's needs. This
command is optional and is not needed to perform an import.
commit
Create or update a branch with a new commit, recording one logical change to the project.
'commit' SP <ref> LF
mark?
('author' (SP <name>)? SP LT <email> GT SP <when> LF)?
'committer' (SP <name>)? SP LT <email> GT SP <when> LF
data
('from' SP <committish> LF)?
('merge' SP <committish> 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/ 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).
author
An author command may optionally appear, if the author information might differ from the committer information. If
author is omitted then fast-import will automatically use the committer's information for the author portion of the
commit. See below for a description of the fields in author, as they are identical to committer.
committer
The committer command indicates who made this commit, and when they made it.
Here <name> is the person's display name (for example "Com M Itter") and <email> is the person's email address
("cmATexample.com[1]"). LT and GT are the literal less-than (\x3c) and greater-than (\x3e) symbols. These are required
to delimit the email address from the other fields in the line. Note that <name> is free-form and may contain any
sequence of bytes, except LT and LF. It is typically UTF-8 encoded.
The time of the change is specified by <when> using the date format that was selected by the --date-format=<fmt>
command line option. See "Date Formats" above for the set of supported formats, and their syntax.
from
The from command is used to specify the commit to initialize this branch from. This revision will be the first
ancestor of the new commit.
Omitting the from command in the first commit of a new branch will cause fast-import to create that commit with no
ancestor. This tends to be desired only for the initial commit of a project. If the frontend creates all files from
scratch when making a new branch, a merge command may be used instead of from to start the 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 to be the first ancestor of the new commit.
As LF is not valid in a Git refname or SHA-1 expression, no quoting or escaping syntax is supported within
<committish>.
Here <committish> is any of the following:
o The name of an existing branch already in fast-import's internal branch table. If fast-import doesn't know the
name, it's treated as a SHA-1 expression.
o A mark reference, :<idnum>, where <idnum> is the mark number.
The reason fast-import uses : to denote a mark reference is this character is not legal in a Git branch name. The
leading : makes it easy to distinguish between the mark 42 (:42) and the branch 42 (42 or refs/heads/42), or an
abbreviated SHA-1 which happened to consist only of base-10 digits.
Marks must be declared (via mark) before they can be used.
o A complete 40 byte or abbreviated commit SHA-1 in hex.
o Any valid Git SHA-1 expression that resolves to a commit. See "SPECIFYING REVISIONS" in gitrevisions(7) for
details.
The special case of restarting an incremental import from the current branch value should be written as:
from refs/heads/branch^0
The ^0 suffix is necessary as fast-import does not permit a branch to start from itself, and the branch is created in
memory before the from command is even read from the input. Adding ^0 will force fast-import to resolve the commit
through Git's revision parsing library, rather than its internal branch table, thereby loading in the existing value
of the branch.
merge
Includes one additional ancestor commit. If the from command is omitted when creating a new branch, the first merge
commit will be the first ancestor of the current commit, and the branch will start out with no files. An unlimited
number of merge commands per commit are permitted by fast-import, thereby establishing an n-way merge. However Git's
other tools never create commits with more than 15 additional ancestors (forming a 16-way merge). For this reason it
is suggested that frontends do not use more than 15 merge commands per commit; 16, if starting a new, empty branch.
Here <committish> is any of the commit specification expressions also accepted by from (see above).
filemodify
Included in a commit command to add a new file or change the content of an existing file. This command has two
different means of specifying the content of the file.
External data format
The data content for the file was already supplied by a prior blob 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 an existing Git blob object. If <mode> is 040000` then <dataref> must be the full 40-byte SHA-1 of an
existing Git 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 modify
command.
'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, specified in octal. Git only supports the following modes:
o 100644 or 644: A normal (not-executable) file. The majority of files in most projects use this mode. If in
doubt, this is what you want.
o 100755 or 755: A normal, but executable, file.
o 120000: A symlink, the content of the file will be the link target.
o 160000: A gitlink, SHA-1 of the object refers to a commit in another repository. Git links can only be specified
by SHA or through a commit mark. They are used to implement submodules.
o 040000: A subdirectory. Subdirectories can only be specified by SHA 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 (forward slash /), may contain any byte other than LF, and
must not start with double quote (").
If an LF or double quote must be encoded into <path> shell-style quoting should be used, e.g. "path/with\n and \" in
it".
The value of <path> must be in canonical form. That is it must not:
o contain an empty directory component (e.g. foo//bar is invalid),
o end with a directory separator (e.g. foo/ is invalid),
o start with a directory separator (e.g. /foo is invalid),
o contain the special component . or .. (e.g. foo/./bar and foo/../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 recursively delete an entire directory from the branch. If the file
or directory removal makes its parent directory empty, the parent directory will be automatically removed too. This
cascades up the tree until the first non-empty directory or the root is reached.
'D' SP <path> LF
here <path> is the complete path of the file or subdirectory to be removed from the branch. See filemodify above for
a detailed description of <path>.
filecopy
Recursively copies an existing file or subdirectory to a different location within the branch. The existing file or
directory must exist. If the destination exists it will be completely replaced by 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
detailed description of what <path> may look like. To use a source path that contains SP the path must be quoted.
A filecopy command takes effect immediately. Once the source location has been copied to the destination any future
commands applied to the source location will not impact the destination of the copy.
filerename
Renames an existing file or subdirectory to a different location within the branch. The existing file or directory
must exist. If the 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
detailed description of what <path> may look like. To use a source path that contains SP the path must be quoted.
A filerename command takes effect immediately. Once the source location has been renamed to the destination any
future commands applied to the source location will create new files there and not impact the destination of the
rename.
Note that a filerename is the same as a filecopy followed by a filedelete of the source location. There is a slight
performance advantage to using filerename, but the advantage is so small that it is never worth trying to convert a
delete/add pair in source material into a rename for fast-import. This filerename command is provided just to
simplify frontends that already have rename information and don't want bother with decomposing it into a filecopy
followed by a filedelete.
filedeleteall
Included in a commit command to remove all files (and also all directories) from the branch. This command resets the
internal branch structure to have no files in it, allowing the frontend to 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 to update the content.
Issuing a filedeleteall followed by the needed filemodify commands to set the correct content will produce the same
results as sending only the needed filemodify and filedelete commands. The filedeleteall approach may however require
fast-import to use slightly more memory per active branch (less than 1 MiB for even most large projects); so
frontends that can easily obtain only the affected paths for a commit are encouraged to do so.
notemodify
Included in a commit command to add a new note (annotating a given commit) or change the content of an existing note.
This command has two different means of specifying the content of the note.
External data format
The data content for the note was already supplied by a prior blob command. The frontend just needs to connect it
to the commit that is to be annotated.
'N' SP <dataref> SP <committish> LF
Here <dataref> can be either a mark reference (:<idnum>) set by a prior blob command, or a full 40-byte SHA-1 of
an existing 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 modify
command.
'N' SP 'inline' SP <committish> LF
data
See below for a detailed description of the data command.
In both formats <committish> is any of the commit specification expressions also accepted by from (see above).
mark
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.
'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 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.
tag
Creates an annotated tag referring to a specific commit. To create lightweight (non-annotated) tags see the reset command
below.
'tag' SP <name> LF
'from' SP <committish> LF
'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 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.
reset
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.
'reset' SP <ref> LF
('from' SP <committish> LF)?
LF?
For a detailed description of <ref> and <committish> 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:
reset refs/tags/938
from :938
would create the lightweight tag refs/tags/938 referring to whatever commit mark :938 references.
blob
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.
'blob' LF
mark?
data
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.
data
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 #.
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
decimal integer. The LF on either side of <raw> is not included in <count> and will not be included in the imported
data.
The LF after <raw> is optional (it used to be required) but recommended. Always including it makes debugging a
fast-import stream easier as the next command always starts in column 0 of 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 not recommended 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
otherwise fast-import will think the data ends earlier than it really does. The LF immediately trailing <raw> is part
of <raw>. This is one of the limitations of the delimited format, it is impossible to supply a 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 to save out all current branch refs, tags and
marks.
'checkpoint' LF
LF?
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).
progress
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.
'progress' SP <any> LF
LF?
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:
frontend | git fast-import | sed 's/^progress //'
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.
cat-blob
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.
'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
or ready 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 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.
feature
Require that fast-import supports the specified feature, or abort if it 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 with a leading -- was passed on the command line (see OPTIONS,
above).
import-marks
Like --import-marks except in two respects: first, only one "feature import-marks" command is allowed per stream;
second, an --import-marks= command-line option overrides any "feature import-marks" command in the stream.
cat-blob
Ignored. Versions of fast-import not supporting the "cat-blob" command will 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 import before
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 exit with a message indicating so.
option
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.
'option' SP <option> LF
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 commandline options change import semantics and may therefore not be passed as option:
o date-format
o import-marks
o export-marks
o cat-blob-fd
o force
CRASH REPORTS
If fast-import is supplied invalid input it will terminate with a non-zero exit status and create a crash report in the
top level of the Git repository it was importing into. Crash reports contain a snapshot of the internal fast-import state
as well as the most recent commands that lead up to the crash.
All recent commands (including stream comments, file changes and progress commands) are shown in the command history
within the crash report, but raw file data and commit messages are excluded from the crash report. This exclusion saves
space within the report file and reduces the amount of buffering that fast-import must perform during execution.
After writing a crash report fast-import will close the current packfile and export the marks table. This allows the
frontend developer to inspect the repository state and resume the import from the point where it crashed. The modified
branches and tags are not updated during a crash, as the import did not complete successfully. Branch and tag information
can be found in the crash report and must 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
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 various users 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
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.
Freely Skip Around Branches
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.
Handling Renames
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.
Use Tag Fixup Branches
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.
Import Now, Repack Later
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.
Repacking Historical Data
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.
Include Some Progress Messages
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.
PACKFILE OPTIMIZATION
When packing a blob fast-import always attempts to deltify against the last blob written. Unless specifically arranged
for by the frontend, this will probably not be a prior version of the same file, so the generated delta will not be the
smallest possible. The resulting packfile will be compressed, but will not be optimal.
Frontends which have efficient access to all revisions of a single file (for example reading an RCS/CVS ,v file) can
choose to supply all revisions of that file as a sequence of consecutive blob commands. This allows fast-import to
deltify the different file revisions against each other, saving space in the final packfile. Marks can be used to later
identify individual file revisions during a sequence of commit commands.
The packfile(s) created by fast-import do not encourage good disk access patterns. This is caused by fast-import writing
the data in the order it is received on standard input, while Git typically organizes data within packfiles to make the
most recent (current tip) data appear 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 the repository with git repack -a -d after fast-import
completes, allowing Git to reorganize the packfiles for faster data access. If blob deltas are suboptimal (see above)
then also adding the -f option to force recomputation of all deltas can significantly reduce the final packfile size
(30-50% smaller can be quite typical).
MEMORY UTILIZATION
There are a number of factors which affect how much memory fast-import requires to perform an import. Like critical
sections of core Git, fast-import uses its own memory allocators to amortize any overheads associated with malloc. In
practice fast-import tends to amortize any malloc overheads to 0, due to its use of large block allocations.
per object
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.
per mark
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.
per branch
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=.
per active tree
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.
per active file entry
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).
SIGNALS
Sending SIGUSR1 to the git fast-import process ends the current packfile early, simulating a checkpoint command. The
impatient operator can use this facility to peek at the objects and refs from an import in progress, at the cost of some
added running time and worse compression.
AUTHOR
Written by Shawn O. Pearce <spearceATspearce.org[2]>.
DOCUMENTATION
Documentation by Shawn O. Pearce <spearceATspearce.org[2]>.
GIT
Part of the git(1) suite
NOTES
1. cmATexample.com
mailto:cmATexample.com
2. spearceATspearce.org
mailto:spearceATspearce.org
Git 1.7.4.4 04/11/2011 GIT-FAST-IMPORT(1)
