GIT-FAST-IMPORT(1)GIT-FAST-IMPORT(1)NAMEgit-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 con-
tain 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 con-
strained memory.
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--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 loca-
tion.
--no-relative-marks
Negates a previous --relative-marks. Allows for combining rela-
tive and non-relative marks by interweaving --(no-)-rela-
tive-marks= with the --(import|export)-marks= options.
--export-pack-edges=<file>
After creating a packfile, print a line of data to <file> list-
ing 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
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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 cur-
rently the default, but can be disabled with --quiet.
PERFORMANCE
The design of fast-import allows it to import large projects in a mini-
mum 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 con-
taining 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 import-
ing. 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
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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 cre-
ated or modified at any point during the import process by sending a
commit command on the input stream. This design allows a frontend pro-
gram to process an unlimited number of branches simultaneously, gener-
ating 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 work-
ing 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 informa-
tion 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
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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 nega-
tive 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 exam-
ple 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 vari-
ation 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 mal-
formed 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.
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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 sys-
tem is always copied into the identity string at the time it is
being created by fast-import. There is no way to specify a dif-
ferent time or timezone.
This particular format is supplied as it’s short to imple-
ment 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 revi-
sion. 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.
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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.
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 inter-
preted 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,
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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).
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.
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@example.com: mailto:cm@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> 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.
The from command is used to specify the commit to initialize this
branch from. This revision will be the first ancestor of the new com-
mit.
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 com-
mand 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>.
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Here <committish> is any of the following:
o The name of an existing branch already in fast-import’s inter-
nal 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:
.ft C
from refs/heads/branch^0
.ft
The {caret}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 {caret}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.
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 addi-
tional ancestors (forming a 16-way merge). For this reason it is sug-
gested that frontends do not use more than 15 merge commands per com-
mit; 16, if starting a new, empty branch.
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Here <committish> is any of the commit specification expressions also
accepted by from (see above).
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
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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 quot-
ing 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).
It is recommended that <path> always be encoded using UTF-8.
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 automat-
ically 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 descrip-
tion of <path>.
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
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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.
Renames an existing file or subdirectory to a different location within
the branch. The existing file or directory must exist. If the destina-
tion 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 destina-
tion 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 advan-
tage 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.
Included in a commit command to remove all files (and also all directo-
ries) from the branch. This command resets the internal branch struc-
ture 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
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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.
Included in a commit command to add a new note (annotating a given com-
mit) 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 com-
mit 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 cre-
ation 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
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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 correspond-
ing 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 quot-
ing 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 recom-
mended, 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 stan-
dard 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
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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-anno-
tated) 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 gen-
erate 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 delim-
ited format is intended primarily for testing fast-import.
Comment lines appearing within the <raw> part of data commands are
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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 rec-
ommended. 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 delim-
iter. 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 impos-
sible 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.
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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 corre-
sponding 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 pro-
cessed 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.
feature
Require that fast-import supports the specified feature, or abort if it
does not.
'feature' SP <feature> LF
The <feature> part of the command may be any string matching
^[a-zA-Z][a-zA-Z-]*$ and should be understood by fast-import.
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Feature work identical as their option counterparts with the exception
of the import-marks feature, see below.
The following features are currently supported:
o date-format
o import-marks
o export-marks
o relative-marks
o no-relative-marks
o force
The import-marks behaves differently from when it is specified as com-
mandline option in that only one "feature import-marks" is allowed per
stream. Also, any --import-marks= specified on the commandline will
override those from the stream (if any).
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 com-
mand is an error.
The following commandline options change import semantics and may
therefore not be passed as option:
o date-format
o import-marks
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o export-marks
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 pack-
file 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 manu-
ally 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
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$ 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 complete-
ness of the import by comparing each Git commit to the corresponding
source revision.
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Coming from a system such as Perforce or Subversion this should be
quite simple, as the fast-import mark can also be the Perforce change-
set 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.
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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 pro-
jects (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 rea-
son 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 gen-
erated 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 revi-
sions 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 com-
mands.
The packfile(s) created by fast-import do not encourage good disk
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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 repos-
itory 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 recompu-
tation 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 ter-
minates. 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
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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 sub-
tree 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 mem-
ory 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 pro-
jects with 2,000+ branches and 45,114+ files in a very limited memory
footprint (less than 2.7 MiB per active branch).
AUTHOR
Written by Shawn O. Pearce <spearce@spearce.org:
mailto:spearce@spearce.org>.
DOCUMENTATION
Documentation by Shawn O. Pearce <spearce@spearce.org:
mailto:spearce@spearce.org>.
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Part of the git(1) suite
25
