GIT-FAST-IMPORT(1) Git Manual GIT-FAST-IMPORT(1)
NAME
git-fast-import - Backend for fast Git data importers
SYNOPSIS
frontend | git fast-import [options]
DESCRIPTION
This program is usually not what the end user wants to run directly. Most end users want to use one of the
existing frontend programs, which parses a specific type of foreign source and feeds the contents stored there
to git fast-import.
fast-import reads a mixed command/data stream from standard input and writes one or more packfiles directly
into the current repository. When EOF is received on standard input, fast import writes out updated branch and
tag refs, fully updating the current repository with the newly imported data.
The fast-import backend itself can import into an empty repository (one that has already been initialized by
git init) or incrementally update an existing populated repository. Whether or not incremental imports are
supported from a particular foreign source depends on the frontend program in use.
OPTIONS
--force
Force updating modified existing branches, even if doing so would cause commits to be lost (as the new
commit does not contain the old commit).
--quiet
Disable all non-fatal output, making fast-import silent when it is successful. This option disables the
output shown by --stats.
--stats
Display some basic statistics about the objects fast-import has created, the packfiles they were stored
into, and the memory used by fast-import during this run. Showing this output is currently the default,
but can be disabled with --quiet.
Options for Frontends
--cat-blob-fd=<fd>
Write responses to cat-blob and ls queries to the file descriptor <fd> instead of stdout. Allows progress
output intended for the end-user to be separated from other output.
--date-format=<fmt>
Specify the type of dates the frontend will supply to fast-import within author, committer and tagger
commands. See “Date Formats” below for details about which formats are supported, and their syntax.
--done
Terminate with error if there is no done command at the end of the stream. This option might be useful for
detecting errors that cause the frontend to terminate before it has started to write a stream.
Locations of Marks Files
--export-marks=<file>
Dumps the internal marks table to <file> when complete. Marks are written one per line as :markid SHA-1.
Frontends can use this file to validate imports after they have been completed, or to save the marks table
across incremental runs. As <file> is only opened and truncated at checkpoint (or completion) the same
path can also be safely given to --import-marks.
--import-marks=<file>
Before processing any input, load the marks specified in <file>. The input file must exist, must be
Relative and non-relative marks may be combined by interweaving --(no-)-relative-marks with the
--(import|export)-marks= options.
Performance and Compression Tuning
--active-branches=<n>
Maximum number of branches to maintain active at once. See “Memory Utilization” below for details. Default
is 5.
--big-file-threshold=<n>
Maximum size of a blob that fast-import will attempt to create a delta for, expressed in bytes. The
default is 512m (512 MiB). Some importers may wish to lower this on systems with constrained memory.
--depth=<n>
Maximum delta depth, for blob and tree deltification. Default is 10.
--export-pack-edges=<file>
After creating a packfile, print a line of data to <file> listing the filename of the packfile and the
last commit on each branch that was written to that packfile. This information may be useful after
importing projects whose total object set exceeds the 4 GiB packfile limit, as these commits can be used
as edge points during calls to git pack-objects.
--max-pack-size=<n>
Maximum size of each output packfile. The default is unlimited.
PERFORMANCE
The design of fast-import allows it to import large projects in a minimum amount of memory usage and
processing time. Assuming the frontend is able to keep up with fast-import and feed it a constant stream of
data, import times for projects holding 10+ years of history and containing 100,000+ individual commits are
generally completed in just 1-2 hours on quite modest (~$2,000 USD) hardware.
Most bottlenecks appear to be in foreign source data access (the source just cannot extract revisions fast
enough) or disk IO (fast-import writes as fast as the disk will take the data). Imports will run faster if the
source data is stored on a different drive than the destination Git repository (due to less IO contention).
DEVELOPMENT COST
A typical frontend for fast-import tends to weigh in at approximately 200 lines of Perl/Python/Ruby code. Most
developers have been able to create working importers in just a couple of hours, even though it is their first
exposure to fast-import, and sometimes even to Git. This is an ideal situation, given that most conversion
tools are throw-away (use once, and never look back).
PARALLEL OPERATION
Like git push or git fetch, imports handled by fast-import are safe to run alongside parallel git repack -a -d
or git gc invocations, or any other Git operation (including git prune, as loose objects are never used by
fast-import).
fast-import does not lock the branch or tag refs it is actively importing. After the import, during its ref
update phase, fast-import tests each existing branch ref to verify the update will be a fast-forward update
(the commit stored in the ref is contained in the new history of the commit to be written). If the update is
not a fast-forward update, fast-import will skip updating that ref and instead prints a warning message.
fast-import will always attempt to update all branch refs, and does not stop on the first failure.
Branch updates can be forced with --force, but it’s recommended that this only be used on an otherwise quiet
repository. Using --force is not necessary for an initial import into an empty repository.
INPUT FORMAT
With the exception of raw file data (which Git does not interpret) the fast-import input format is text
(ASCII) based. This text based format simplifies development and debugging of frontend programs, especially
when a higher level language such as Perl, Python or Ruby is being used.
fast-import is very strict about its input. Where we say SP below we mean exactly one space. Likewise LF means
one (and only one) linefeed and HT one (and only one) horizontal tab. Supplying additional whitespace
characters will cause unexpected results, such as branch names or file names with leading or trailing spaces
in their name, or early termination of fast-import when it encounters unexpected input.
Stream Comments
To aid in debugging frontends fast-import ignores any line that begins with # (ASCII pound/hash) up to and
including the line ending LF. A comment line may contain any sequence of bytes that does not contain an LF and
therefore may be used to include any detailed debugging information that might be specific to the frontend and
useful when inspecting a fast-import data stream.
Date Formats
The following date formats are supported. A frontend should select the format it will use for this import by
passing the format name in the --date-format=<fmt> command line option.
raw
This is the Git native format and is <time> SP <offutc>. It is also fast-import’s default format, if
--date-format was not specified.
The time of the event is specified by <time> as the number of seconds since the UNIX epoch (midnight, Jan
1, 1970, UTC) and is written as an ASCII decimal integer.
The local offset is specified by <offutc> as a positive or negative offset from UTC. For example EST
(which is 5 hours behind UTC) would be expressed in <tz> by “-0500” while UTC is “+0000”. The local offset
does not affect <time>; it is used only as an advisement to help formatting routines display the
timestamp.
If the local offset is not available in the source material, use “+0000”, or the most common local offset.
For example many organizations have a CVS repository which has only ever been accessed by users who are
located in the same location and timezone. In this case a reasonable offset from UTC could be assumed.
Unlike the rfc2822 format, this format is very strict. Any variation in formatting will cause fast-import
to reject the value.
rfc2822
This is the standard email format as described by RFC 2822.
An example value is “Tue Feb 6 11:22:18 2007 -0500”. The Git parser is accurate, but a little on the
lenient side. It is the same parser used by git am when applying patches received from email.
Some malformed strings may be accepted as valid dates. In some of these cases Git will still be able to
obtain the correct date from the malformed string. There are also some types of malformed strings which
Git will parse wrong, and yet consider valid. Seriously malformed strings will be rejected.
Unlike the raw format above, the timezone/UTC offset information contained in an RFC 2822 date string is
used to adjust the date value to UTC prior to storage. Therefore it is important that this information be
as accurate as possible.
If the source material uses RFC 2822 style dates, the frontend should let fast-import handle the parsing
timezone.
This particular format is supplied as it’s short to implement and may be useful to a process that wants to
create a new commit right now, without needing to use a working directory or git update-index.
If separate author and committer commands are used in a commit the timestamps may not match, as the system
clock will be polled twice (once for each command). The only way to ensure that both author and committer
identity information has the same timestamp is to omit author (thus copying from committer) or to use a
date format other than now.
Commands
fast-import accepts several commands to update the current repository and control the current import process.
More detailed discussion (with examples) of each command follows later.
commit
Creates a new branch or updates an existing branch by creating a new commit and updating the branch to
point at the newly created commit.
tag
Creates an annotated tag object from an existing commit or branch. Lightweight tags are not supported by
this command, as they are not recommended for recording meaningful points in time.
reset
Reset an existing branch (or a new branch) to a specific revision. This command must be used to change a
branch to a specific revision without making a commit on it.
blob
Convert raw file data into a blob, for future use in a commit command. This command is optional and is not
needed to perform an import.
checkpoint
Forces fast-import to close the current packfile, generate its unique SHA-1 checksum and index, and start
a new packfile. This command is optional and is not needed to perform an import.
progress
Causes fast-import to echo the entire line to its own standard output. This command is optional and is not
needed to perform an import.
done
Marks the end of the stream. This command is optional unless the done feature was requested using the
--done command line option or feature done command.
cat-blob
Causes fast-import to print a blob in cat-file --batch format to the file descriptor set with
--cat-blob-fd or stdout if unspecified.
ls
Causes fast-import to print a line describing a directory entry in ls-tree format to the file descriptor
set with --cat-blob-fd or stdout if unspecified.
feature
Require that fast-import supports the specified feature, or abort if it does not.
option
('merge' SP <committish> LF)?
(filemodify | filedelete | filecopy | filerename | filedeleteall | notemodify)*
LF?
where <ref> is the name of the branch to make the commit on. Typically branch names are prefixed with
refs/heads/ in Git, so importing the CVS branch symbol RELENG-1_0 would use refs/heads/RELENG-1_0 for the
value of <ref>. The value of <ref> must be a valid refname in Git. As LF is not valid in a Git refname, no
quoting or escaping syntax is supported here.
A mark command may optionally appear, requesting fast-import to save a reference to the newly created commit
for future use by the frontend (see below for format). It is very common for frontends to mark every commit
they create, thereby allowing future branch creation from any imported commit.
The data command following committer must supply the commit message (see below for data command syntax). To
import an empty commit message use a 0 length data. Commit messages are free-form and are not interpreted by
Git. Currently they must be encoded in UTF-8, as fast-import does not permit other encodings to be specified.
Zero or more filemodify, filedelete, filecopy, filerename, filedeleteall and notemodify commands may be
included to update the contents of the branch prior to creating the commit. These commands may be supplied in
any order. However it is recommended that a filedeleteall command precede all filemodify, filecopy, filerename
and notemodify commands in the same commit, as filedeleteall wipes the branch clean (see below).
The LF after the command is optional (it used to be required).
author
An author command may optionally appear, if the author information might differ from the committer
information. If author is omitted then fast-import will automatically use the committer’s information for
the author portion of the commit. See below for a description of the fields in author, as they are
identical to committer.
committer
The committer command indicates who made this commit, and when they made it.
Here <name> is the person’s display name (for example “Com M Itter”) and <email> is the person’s email
address (“[email protected]”). 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.
(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.
· 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 case of restarting an incremental import from the current branch value should be written as:
from refs/heads/branch^0
The ^0 suffix is necessary as fast-import does not permit a branch to start from itself, and the branch is
created in memory before the from command is even read from the input. Adding ^0 will force fast-import to
resolve the commit through Git’s revision parsing library, rather than its internal branch table, thereby
loading in the existing value of the branch.
merge
Includes one additional ancestor commit. The additional ancestry link does not change the way the tree
state is built at this commit. If the from command is omitted when creating a new branch, the first merge
commit will be the first ancestor of the current commit, and the branch will start out with no files. An
unlimited number of merge commands per commit are permitted by fast-import, thereby establishing an n-way
merge. However Git’s other tools never create commits with more than 15 additional ancestors (forming a
16-way merge). For this reason it is suggested that frontends do not use more than 15 merge commands per
commit; 16, if starting a new, empty branch.
Here <committish> is any of the commit specification expressions also accepted by from (see above).
filemodify
Included in a commit command to add a new file or change the content of an existing file. This command has
two different means of specifying the content of the file.
External data format
The data content for the file was already supplied by a prior blob command. The frontend just needs to
connect it.
'M' SP <mode> SP <dataref> SP <path> LF
Here usually <dataref> must be either a mark reference (:<idnum>) set by a prior blob command, or a
full 40-byte SHA-1 of an existing Git blob object. If <mode> is 040000` then <dataref> must be the
full 40-byte SHA-1 of an existing Git tree object or a mark reference set with --import-marks.
Inline data format
The data content for the file has not been supplied yet. The frontend wants to supply it as part of
this modify command.
'M' SP <mode> SP 'inline' SP <path> LF
data
See below for a detailed description of the data command.
In both formats <mode> is the type of file entry, specified in octal. Git only supports the following
· 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 and foo/../bar are invalid).
The root of the tree can be represented by an empty string as <path>.
It is recommended that <path> always be encoded using UTF-8.
filedelete
Included in a commit command to remove a file or recursively delete an entire directory from the branch.
If the file or directory removal makes its parent directory empty, the parent directory will be
automatically removed too. This cascades up the tree until the first non-empty directory or the root is
reached.
'D' SP <path> LF
here <path> is the complete path of the file or subdirectory to be removed from the branch. See filemodify
above for a detailed description of <path>.
filecopy
Recursively copies an existing file or subdirectory to a different location within the branch. The
existing file or directory must exist. If the destination exists it will be completely replaced by the
content copied from the source.
'C' SP <path> SP <path> LF
here the first <path> is the source location and the second <path> is the destination. See filemodify
above for a detailed description of what <path> may look like. To use a source path that contains SP the
path must be quoted.
A filecopy command takes effect immediately. Once the source location has been copied to the destination
any future commands applied to the source location will not impact the destination of the copy.
filerename
Note that a filerename is the same as a filecopy followed by a filedelete of the source location. There is
a slight performance advantage to using filerename, but the advantage is so small that it is never worth
trying to convert a delete/add pair in source material into a rename for fast-import. This filerename
command is provided just to simplify frontends that already have rename information and don’t want bother
with decomposing it into a filecopy followed by a filedelete.
filedeleteall
Included in a commit command to remove all files (and also all directories) from the branch. This command
resets the internal branch structure to have no files in it, allowing the frontend to subsequently add all
interesting files from scratch.
'deleteall' LF
This command is extremely useful if the frontend does not know (or does not care to know) what files are
currently on the branch, and therefore cannot generate the proper filedelete commands to update the
content.
Issuing a filedeleteall followed by the needed filemodify commands to set the correct content will produce
the same results as sending only the needed filemodify and filedelete commands. The filedeleteall approach
may however require fast-import to use slightly more memory per active branch (less than 1 MiB for even
most large projects); so frontends that can easily obtain only the affected paths for a commit are
encouraged to do so.
notemodify
Included in a commit <notes_ref> command to add a new note annotating a <committish> or change this
annotation contents. Internally it is similar to filemodify 100644 on <committish> path (maybe split into
subdirectories). It’s not advised to use any other commands to write to the <notes_ref> tree except
filedeleteall to delete all existing notes in this tree. This command has two different means of
specifying the content of the note.
External data format
The data content for the note was already supplied by a prior blob command. The frontend just needs to
connect it to the commit that is to be annotated.
'N' SP <dataref> SP <committish> LF
Here <dataref> can be either a mark reference (:<idnum>) set by a prior blob command, or a full
40-byte SHA-1 of an existing Git blob object.
Inline data format
The data content for the note has not been supplied yet. The frontend wants to supply it as part of
this modify command.
'N' SP 'inline' SP <committish> LF
data
See below for a detailed description of the data command.
In both formats <committish> is any of the commit specification expressions also accepted by from (see
above).
mark
Arranges for fast-import to save a reference to the current object, allowing the frontend to recall this
tag
Creates an annotated tag referring to a specific commit. To create lightweight (non-annotated) tags see the
reset command below.
'tag' SP <name> LF
'from' SP <committish> LF
'tagger' (SP <name>)? SP LT <email> GT SP <when> LF
data
where <name> is the name of the tag to create.
Tag names are automatically prefixed with refs/tags/ when stored in Git, so importing the CVS branch symbol
RELENG-1_0-FINAL would use just RELENG-1_0-FINAL for <name>, and fast-import will write the corresponding ref
as refs/tags/RELENG-1_0-FINAL.
The value of <name> must be a valid refname in Git and therefore may contain forward slashes. As LF is not
valid in a Git refname, no quoting or escaping syntax is supported here.
The from command is the same as in the commit command; see above for details.
The tagger command uses the same format as committer within commit; again see above for details.
The data command following tagger must supply the annotated tag message (see below for data command syntax).
To import an empty tag message use a 0 length data. Tag messages are free-form and are not interpreted by Git.
Currently they must be encoded in UTF-8, as fast-import does not permit other encodings to be specified.
Signing annotated tags during import from within fast-import is not supported. Trying to include your own
PGP/GPG signature is not recommended, as the frontend does not (easily) have access to the complete set of
bytes which normally goes into such a signature. If signing is required, create lightweight tags from within
fast-import with reset, then create the annotated versions of those tags offline with the standard git tag
process.
reset
Creates (or recreates) the named branch, optionally starting from a specific revision. The reset command
allows a frontend to issue a new from command for an existing branch, or to create a new branch from an
existing commit without creating a new commit.
'reset' SP <ref> LF
('from' SP <committish> LF)?
LF?
For a detailed description of <ref> and <committish> see above under commit and from.
The LF after the command is optional (it used to be required).
The reset command can also be used to create lightweight (non-annotated) tags. For example:
reset refs/tags/938
from :938
would create the lightweight tag refs/tags/938 referring to whatever commit mark :938 references.
blob
Supplies raw data (for use as blob/file content, commit messages, or annotated tag messages) to fast-import.
Data can be supplied using an exact byte count or delimited with a terminating line. Real frontends intended
for production-quality conversions should always use the exact byte count format, as it is more robust and
performs better. The delimited format is intended primarily for testing fast-import.
Comment lines appearing within the <raw> part of data commands are always taken to be part of the body of the
data and are therefore never ignored by fast-import. This makes it safe to import any file/message content
whose lines might start with #.
Exact byte count format
The frontend must specify the number of bytes of data.
'data' SP <count> LF
<raw> LF?
where <count> is the exact number of bytes appearing within <raw>. The value of <count> is expressed as an
ASCII decimal integer. The LF on either side of <raw> is not included in <count> and will not be included
in the imported data.
The LF after <raw> is optional (it used to be required) but recommended. Always including it makes
debugging a fast-import stream easier as the next command always starts in column 0 of the next line, even
if <raw> did not end with an LF.
Delimited format
A delimiter string is used to mark the end of the data. fast-import will compute the length by searching
for the delimiter. This format is primarily useful for testing and is not recommended for real data.
'data' SP '<<' <delim> LF
<raw> LF
<delim> LF
LF?
where <delim> is the chosen delimiter string. The string <delim> must not appear on a line by itself
within <raw>, as otherwise fast-import will think the data ends earlier than it really does. The LF
immediately trailing <raw> is part of <raw>. This is one of the limitations of the delimited format, it is
impossible to supply a data chunk which does not have an LF as its last byte.
The LF after <delim> LF is optional (it used to be required).
checkpoint
Forces fast-import to close the current packfile, start a new one, and to save out all current branch refs,
tags and marks.
'checkpoint' LF
LF?
Note that fast-import automatically switches packfiles when the current packfile reaches --max-pack-size, or 4
GiB, whichever limit is smaller. During an automatic packfile switch fast-import does not update the branch
refs, tags or marks.
As a checkpoint can require a significant amount of CPU time and disk IO (to compute the overall pack SHA-1
checksum, generate the corresponding index file, and update the refs) it can easily take several minutes for a
single checkpoint command to complete.
LF?
The <any> part of the command may contain any sequence of bytes that does not contain LF. The LF after the
command is optional. Callers may wish to process the output through a tool such as sed to remove the leading
part of the line, for example:
frontend | git fast-import | sed 's/^progress //'
Placing a progress command immediately after a checkpoint will inform the reader when the checkpoint has been
completed and it can safely access the refs that fast-import updated.
cat-blob
Causes fast-import to print a blob to a file descriptor previously arranged with the --cat-blob-fd argument.
The command otherwise has no impact on the current import; its main purpose is to retrieve blobs that may be
in fast-import’s memory but not accessible from the target repository.
'cat-blob' SP <dataref> LF
The <dataref> can be either a mark reference (:<idnum>) set previously or a full 40-byte SHA-1 of a Git blob,
preexisting or ready to be written.
Output uses the same format as git cat-file --batch:
<sha1> SP 'blob' SP <size> LF
<contents> LF
This command can be used anywhere in the stream that comments are accepted. In particular, the cat-blob
command can be used in the middle of a commit but not in the middle of a data command.
See “Responses To Commands” below for details about how to read this output safely.
ls
Prints information about the object at a path to a file descriptor previously arranged with the --cat-blob-fd
argument. This allows printing a blob from the active commit (with cat-blob) or copying a blob or tree from a
previous commit for use in the current one (with filemodify).
The ls command can be used anywhere in the stream that comments are accepted, including the middle of a
commit.
Reading from the active commit
This form can only be used in the middle of a commit. The path names a directory entry within
fast-import’s active commit. The path must be quoted in this case.
'ls' SP <path> LF
Reading from a named tree
The <dataref> can be a mark reference (:<idnum>) or the full 40-byte SHA-1 of a Git tag, commit, or tree
object, preexisting or waiting to be written. The path is relative to the top level of the tree named by
<dataref>.
'ls' SP <dataref> SP <path> LF
See filemodify above for a detailed description of <path>.
feature
Require that fast-import supports the specified feature, or abort if it does not.
'feature' SP <feature> ('=' <argument>)? LF
The <feature> part of the command may be any one of the following:
date-format, export-marks, relative-marks, no-relative-marks, force
Act as though the corresponding command-line option with a leading -- was passed on the command line (see
OPTIONS, above).
import-marks, import-marks-if-exists
Like --import-marks except in two respects: first, only one "feature import-marks" or "feature
import-marks-if-exists" command is allowed per stream; second, an --import-marks= or
--import-marks-if-exists command-line option overrides any of these "feature" commands in the stream;
third, "feature import-marks-if-exists" like a corresponding command-line option silently skips a
nonexistent file.
cat-blob, ls
Require that the backend support the cat-blob or ls command. Versions of fast-import not supporting the
specified command will exit with a message indicating so. This lets the import error out early with a
clear message, rather than wasting time on the early part of an import before the unsupported command is
detected.
notes
Require that the backend support the notemodify (N) subcommand to the commit command. Versions of
fast-import not supporting notes will exit with a message indicating so.
done
Error out if the stream ends without a done command. Without this feature, errors causing the frontend to
end abruptly at a convenient point in the stream can go undetected. This may occur, for example, if an
import front end dies in mid-operation without emitting SIGTERM or SIGKILL at its subordinate git
fast-import instance.
option
Processes the specified option so that git fast-import behaves in a way that suits the frontend’s needs. Note
that options specified by the frontend are overridden by any options the user may specify to git fast-import
itself.
'option' SP <option> LF
The <option> part of the command may contain any of the options listed in the OPTIONS section that do not
change import semantics, without the leading -- and is treated in the same way.
Option commands must be the first commands on the input (not counting feature commands), to give an option
command after any non-option command is an error.
The following commandline options change import semantics and may therefore not be passed as option:
· date-format
· import-marks
RESPONSES TO COMMANDS
New objects written by fast-import are not available immediately. Most fast-import commands have no visible
effect until the next checkpoint (or completion). The frontend can send commands to fill fast-import’s input
pipe without worrying about how quickly they will take effect, which improves performance by simplifying
scheduling.
For some frontends, though, it is useful to be able to read back data from the current repository as it is
being updated (for example when the source material describes objects in terms of patches to be applied to
previously imported objects). This can be accomplished by connecting the frontend and fast-import via
bidirectional pipes:
mkfifo fast-import-output
frontend <fast-import-output |
git fast-import >fast-import-output
A frontend set up this way can use progress, ls, and cat-blob commands to read information from the import in
progress.
To avoid deadlock, such frontends must completely consume any pending output from progress, ls, and cat-blob
before performing writes to fast-import that might block.
CRASH REPORTS
If fast-import is supplied invalid input it will terminate with a non-zero exit status and create a crash
report in the top level of the Git repository it was importing into. Crash reports contain a snapshot of the
internal fast-import state as well as the most recent commands that lead up to the crash.
All recent commands (including stream comments, file changes and progress commands) are shown in the command
history within the crash report, but raw file data and commit messages are excluded from the crash report.
This exclusion saves space within the report file and reduces the amount of buffering that fast-import must
perform during execution.
After writing a crash report fast-import will close the current packfile and export the marks table. This
allows the frontend developer to inspect the repository state and resume the import from the point where it
crashed. The modified branches and tags are not updated during a crash, as the import did not complete
successfully. Branch and tag information can be found in the crash report and must be applied manually if the
update is needed.
An example crash:
$ cat >in <<END_OF_INPUT
# my very first test commit
commit refs/heads/master
committer Shawn O. Pearce <spearce> 19283 -0400
# who is that guy anyway?
data <<EOF
this is my commit
EOF
M 644 inline .gitignore
data <<EOF
.gitignore
EOF
M 777 inline bob
END_OF_INPUT
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.
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
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
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