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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
--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).
--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.
Options for Frontends
--cat-blob-fd=<fd>
Write responses to cat-blob and ls queries to the file descriptor <fd> instead of
stdout. Allows progress output intended for the end-user to be separated from other
output.
--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.
--done
Terminate with error if there is no done command at the end of the stream. This option
might be useful for detecting errors that cause the frontend to terminate before it
has started to write a stream.
Locations of Marks Files
--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.
--import-marks-if-exists=<file>
Like --import-marks but instead of erroring out, silently skips the file if it does
not exist.
--[no-]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.
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=<n>
Maximum number of branches to maintain active at once. See "Memory Utilization" below
for details. Default is 5.
--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.
--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.
--max-pack-size=<n>
Maximum size of each output packfile. The default is unlimited.
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 and HT one (and only one) horizontal
tab. 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.
done
Marks the end of the stream. This command is optional unless the done feature was
requested using the --done command line option or feature done command.
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.
ls
Causes fast-import to print a line describing a directory entry in ls-tree 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 ("cm AT example.com"). 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> and <email> are free-form
and may contain any sequence of bytes, except LT, GT and LF. <name> 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. The state of the tree built at
this commit will begin with the state at the from commit, and be altered by the
content modifications in this 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. The additional ancestry link does not change
the way the tree state is built at this 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 (").
A path can use C-style string quoting; this is accepted in all cases and mandatory if
the filename starts with double quote or contains LF. In C-style quoting, the complete
name should be surrounded with double quotes, and any LF, backslash, or double quote
characters must 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:
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 <notes_ref> command to add a new note annotating a <committish>
or change this annotation contents. Internally it is similar to filemodify 100644 on
<committish> path (maybe split into subdirectories). It's not advised to use any other
commands to write to the <notes_ref> tree except filedeleteall to delete all existing
notes in this tree. 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.
See "Responses To Commands" below for details about how to read this output safely.
ls
Prints information about the object at a path to a file descriptor previously arranged
with the --cat-blob-fd argument. This allows printing a blob from the active commit (with
cat-blob) or copying a blob or tree from a previous commit for use in the current one
(with filemodify).
The ls command can be used anywhere in the stream that comments are accepted, including
the middle of a commit.
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's active 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 the full 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 tree named 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 cat-blob, filemodify, or ls commands.
If there is no file or subtree at that path, git fast-import will instead report
missing SP <path> LF
See "Responses To Commands" below for details about how to read this output safely.
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, 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 overrides any of these
"feature" commands in the stream; third, "feature import-marks-if-exists" like a
corresponding command-line option silently skips a nonexistent file.
cat-blob, ls
Require that the backend support the cat-blob or ls command. Versions of fast-import
not supporting the specified 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.
done
Error out if the stream ends without a done command. Without this feature, errors
causing the frontend to end abruptly at a convenient point in the stream can go
undetected. This may occur, for example, if an import front end dies in mid-operation
without emitting SIGTERM or SIGKILL at its subordinate git fast-import instance.
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
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 is in use, the done command is
mandatory and marks the end of the stream.
RESPONSES TO COMMANDS
New objects written by fast-import are not available immediately. Most fast-import
commands have no visible effect until the next checkpoint (or completion). The frontend
can send commands to fill fast-import's input pipe without worrying about how quickly they
will take effect, which improves performance by simplifying scheduling.
For some frontends, though, it is useful to be able to read back data from the current
repository as it is being updated (for example when the source material describes objects
in terms of patches to be applied to previously imported objects). This can be
accomplished by connecting the frontend and fast-import via bidirectional pipes:
mkfifo fast-import-output
frontend <fast-import-output |
git fast-import >fast-import-output
A frontend set up this way can use progress, ls, and cat-blob commands to read information
from the import in progress.
To avoid deadlock, such frontends must completely consume any pending output from
progress, ls, and cat-blob before performing writes to fast-import that might block.
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.
GIT
Part of the git(1) suite
Git 1.8.3.1 07/30/2024 GIT-FAST-IMPORT(1)
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