Setup and Config
Getting and Creating Projects
Basic Snapshotting
Branching and Merging
Sharing and Updating Projects
Inspection and Comparison
Patching
Debugging
External Systems
Server Admin
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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.
- --allow-unsafe-features
Many command-line options can be provided as part of the fast-import stream itself by using the
feature
oroption
commands. However, some of these options are unsafe (e.g., allowing fast-import to access the filesystem outside of the repository). These options are disabled by default, but can be allowed by providing this option on the command line. This currently impacts only theexport-marks
,import-marks
, andimport-marks-if-exists
feature commands.Only enable this option if you trust the program generating the fast-import stream! This option is enabled automatically for remote-helpers that use the `import` capability, as they are already trusted to run their own code.
Options for Frontends
- --cat-blob-fd=<fd>
Write responses to
get-mark
,cat-blob
, andls
queries to the file descriptor <fd> instead ofstdout
. Allowsprogress
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
andtagger
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 50.
- --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.
- fastimport.unpackLimit
See git-config[1]
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 time zone. 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 time zone/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 time zone. The literal
now
must always be supplied for<when>
.This is a toy format. The current time and time zone 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 time zone.
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
andcommitter
commands are used in acommit
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 omitauthor
(thus copying fromcommitter
) or to use a date format other thannow
.
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 orfeature done
command.get-mark
Causes fast-import to print the SHA-1 corresponding to a mark to the file descriptor set with
--cat-blob-fd
, orstdout
if unspecified.cat-blob
Causes fast-import to print a blob in cat-file --batch format to the file descriptor set with
--cat-blob-fd
orstdout
if unspecified.ls
Causes fast-import to print a line describing a directory entry in ls-tree format to the file descriptor set with
--cat-blob-fd
orstdout
if unspecified.feature
Enable the specified feature. This requires that fast-import supports the specified feature, and aborts 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 <commit-ish> LF)? ('merge' SP <commit-ish> LF)? (filemodify | filedelete | filecopy | filerename | filedeleteall | notemodify)* LF?
where <ref>
is the name of the branch to make the commit on.
Typically branch names are prefixed with refs/heads/
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@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 <commit-ish>
.
Here <commit-ish>
is any of the following:
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.
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
orrefs/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.A complete 40 byte or abbreviated commit SHA-1 in hex.
Any valid Git SHA-1 expression that resolves to a commit. See “SPECIFYING REVISIONS” in gitrevisions[7] for details.
The special null SHA-1 (40 zeros) specifies that the branch is to be removed.
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.
Here <commit-ish>
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 priorblob
command, or a full 40-byte SHA-1 of an existing Git blob object. If<mode>
is040000`
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:
100644
or644
: A normal (not-executable) file. The majority of files in most projects use this mode. If in doubt, this is what you want.100755
or755
: A normal, but executable, file.120000
: A symlink, the content of the file will be the link target.160000
: A gitlink, SHA-1 of the object refers to a commit in another repository. Git links can only be specified by SHA or through a commit mark. They are used to implement submodules.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:
contain an empty directory component (e.g.
foo//bar
is invalid),end with a directory separator (e.g.
foo/
is invalid),start with a directory separator (e.g.
/foo
is invalid),contain the special component
.
or..
(e.g.foo/./bar
andfoo/../bar
are invalid).
The root of the tree can be represented by an empty string as <path>
.
It is recommended that <path>
always be encoded using UTF-8.
filedelete
Included in a commit
command to remove a file or 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 <commit-ish>
or change this annotation contents.
Internally it is similar to filemodify 100644 on <commit-ish>
path (maybe split into subdirectories). It’s not advised 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 <commit-ish> LF
Here
<dataref>
can be either a mark reference (:<idnum>
) set by a priorblob
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 <commit-ish> LF data
See below for a detailed description of the
data
command.
In both formats <commit-ish>
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 <commit-ish> 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 <commit-ish> LF)? LF?
For a detailed description of <ref>
and <commit-ish>
see above
under commit
and from
.
The LF
after the command is optional (it used to be required).
The reset
command can also be used to create lightweight
(non-annotated) tags. For example:
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. TheLF
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 anLF
.- 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. TheLF
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.
get-mark
Causes fast-import to print the SHA-1 corresponding to a mark to
stdout or to the file descriptor previously arranged with the
--cat-blob-fd
argument. The command otherwise has no impact on the
current import; its purpose is to retrieve SHA-1s that later commits
might want to refer to in their commit messages.
'get-mark' SP ':' <idnum> LF
This command can be used anywhere in the stream that comments are
accepted. In particular, the get-mark
command can be used in the
middle of a commit but not in the middle of a data
command.
See “Responses To Commands” below for details about how to read this output safely.
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 get-mark, 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.
- get-mark
- cat-blob
- ls
Require that the backend support the get-mark, cat-blob, or ls command respectively. Versions of fast-import not supporting the specified 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 command-line options change import semantics and may therefore not be passed as option:
date-format
import-marks
export-marks
cat-blob-fd
force
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
, get-mark
, 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
, get-mark
, 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