GIT-FAST-IMPORT(1) Git Manual GIT-FAST-IMPORT(1)
NAME
git-fast-import - Backend for fast Git data importers
SYNOPSIS
frontend | git fast-import [options]
DESCRIPTION
This program is usually not what the end user wants to run directly.
Most end users want to use one of the existing frontend programs, which
parses a specific type of foreign source and feeds the contents stored
there to git fast-import.
fast-import reads a mixed command/data stream from standard input and
writes one or more packfiles directly into the current repository. When
EOF is received on standard input, fast import writes out updated
branch and tag refs, fully updating the current repository with the
newly imported data.
The fast-import backend itself can import into an empty repository (one
that has already been initialized by git init) or incrementally update
an existing populated repository. Whether or not incremental imports
are supported from a particular foreign source depends on the frontend
program in use.
OPTIONS
--force
Force updating modified existing branches, even if doing so would
cause commits to be lost (as the new commit does not contain the
old commit).
--quiet
Disable all non-fatal output, making fast-import silent when it is
successful. This option disables the output shown by --stats.
--stats
Display some basic statistics about the objects fast-import has
created, the packfiles they were stored into, and the memory used
by fast-import during this run. Showing this output is currently
the default, but can be disabled with --quiet.
Options for Frontends
--cat-blob-fd=<fd>
Write responses to cat-blob and ls queries to the file descriptor
<fd> instead of stdout. Allows progress output intended for the
end-user to be separated from other output.
--date-format=<fmt>
Specify the type of dates the frontend will supply to fast-import
within author, committer and tagger commands. See "Date Formats"
below for details about which formats are supported, and their
syntax.
--done
Terminate with error if there is no done command at the end of the
stream. This option might be useful for detecting errors that cause
the frontend to terminate before it has started to write a stream.
Locations of Marks Files
--export-marks=<file>
Dumps the internal marks table to <file> when complete. Marks are
written one per line as :markid SHA-1. Frontends can use this file
to validate imports after they have been completed, or to save the
marks table across incremental runs. As <file> is only opened and
truncated at checkpoint (or completion) the same path can also be
safely given to --import-marks.
--import-marks=<file>
Before processing any input, load the marks specified in <file>.
The input file must exist, must be readable, and must use the same
format as produced by --export-marks. Multiple options may be
supplied to import more than one set of marks. If a mark is defined
to different values, the last file wins.
--import-marks-if-exists=<file>
Like --import-marks but instead of erroring out, silently skips the
file if it does not exist.
--[no-]relative-marks
After specifying --relative-marks the paths specified with
--import-marks= and --export-marks= are relative to an internal
directory in the current repository. In git-fast-import this means
that the paths are relative to the .git/info/fast-import directory.
However, other importers may use a different location.
Relative and non-relative marks may be combined by interweaving
--(no-)-relative-marks with the --(import|export)-marks= options.
Performance and Compression Tuning
--active-branches=<n>
Maximum number of branches to maintain active at once. See "Memory
Utilization" below for details. Default is 5.
--big-file-threshold=<n>
Maximum size of a blob that fast-import will attempt to create a
delta for, expressed in bytes. The default is 512m (512 MiB). Some
importers may wish to lower this on systems with constrained
memory.
--depth=<n>
Maximum delta depth, for blob and tree deltification. Default is
10.
--export-pack-edges=<file>
After creating a packfile, print a line of data to <file> listing
the filename of the packfile and the last commit on each branch
that was written to that packfile. This information may be useful
after importing projects whose total object set exceeds the 4 GiB
packfile limit, as these commits can be used as edge points during
calls to git pack-objects.
--max-pack-size=<n>
Maximum size of each output packfile. The default is unlimited.
PERFORMANCE
The design of fast-import allows it to import large projects in a
minimum amount of memory usage and processing time. Assuming the
frontend is able to keep up with fast-import and feed it a constant
stream of data, import times for projects holding 10+ years of history
and containing 100,000+ individual commits are generally completed in
just 1-2 hours on quite modest (~$2,000 USD) hardware.
Most bottlenecks appear to be in foreign source data access (the source
just cannot extract revisions fast enough) or disk IO (fast-import
writes as fast as the disk will take the data). Imports will run faster
if the source data is stored on a different drive than the destination
Git repository (due to less IO contention).
DEVELOPMENT COST
A typical frontend for fast-import tends to weigh in at approximately
200 lines of Perl/Python/Ruby code. Most developers have been able to
create working importers in just a couple of hours, even though it is
their first exposure to fast-import, and sometimes even to Git. This is
an ideal situation, given that most conversion tools are throw-away
(use once, and never look back).
PARALLEL OPERATION
Like git push or git fetch, imports handled by fast-import are safe to
run alongside parallel git repack -a -d or git gc invocations, or any
other Git operation (including git prune, as loose objects are never
used by fast-import).
fast-import does not lock the branch or tag refs it is actively
importing. After the import, during its ref update phase, fast-import
tests each existing branch ref to verify the update will be a
fast-forward update (the commit stored in the ref is contained in the
new history of the commit to be written). If the update is not a
fast-forward update, fast-import will skip updating that ref and
instead prints a warning message. fast-import will always attempt to
update all branch refs, and does not stop on the first failure.
Branch updates can be forced with --force, but it's recommended that
this only be used on an otherwise quiet repository. Using --force is
not necessary for an initial import into an empty repository.
TECHNICAL DISCUSSION
fast-import tracks a set of branches in memory. Any branch can be
created or modified at any point during the import process by sending a
commit command on the input stream. This design allows a frontend
program to process an unlimited number of branches simultaneously,
generating commits in the order they are available from the source
data. It also simplifies the frontend programs considerably.
fast-import does not use or alter the current working directory, or any
file within it. (It does however update the current Git repository, as
referenced by GIT_DIR.) Therefore an import frontend may use the
working directory for its own purposes, such as extracting file
revisions from the foreign source. This ignorance of the working
directory also allows fast-import to run very quickly, as it does not
need to perform any costly file update operations when switching
between branches.
INPUT FORMAT
With the exception of raw file data (which Git does not interpret) the
fast-import input format is text (ASCII) based. This text based format
simplifies development and debugging of frontend programs, especially
when a higher level language such as Perl, Python or Ruby is being
used.
fast-import is very strict about its input. Where we say SP below we
mean exactly one space. Likewise LF means one (and only one) linefeed
and HT one (and only one) horizontal tab. Supplying additional
whitespace characters will cause unexpected results, such as branch
names or file names with leading or trailing spaces in their name, or
early termination of fast-import when it encounters unexpected input.
Stream Comments
To aid in debugging frontends fast-import ignores any line that begins
with # (ASCII pound/hash) up to and including the line ending LF. A
comment line may contain any sequence of bytes that does not contain an
LF and therefore may be used to include any detailed debugging
information that might be specific to the frontend and useful when
inspecting a fast-import data stream.
Date Formats
The following date formats are supported. A frontend should select the
format it will use for this import by passing the format name in the
--date-format=<fmt> command line option.
raw
This is the Git native format and is <time> SP <offutc>. It is also
fast-import's default format, if --date-format was not specified.
The time of the event is specified by <time> as the number of
seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is
written as an ASCII decimal integer.
The local offset is specified by <offutc> as a positive or negative
offset from UTC. For example EST (which is 5 hours behind UTC)
would be expressed in <tz> by "-0500" while UTC is "+0000". The
local offset does not affect <time>; it is used only as an
advisement to help formatting routines display the timestamp.
If the local offset is not available in the source material, use
"+0000", or the most common local offset. For example many
organizations have a CVS repository which has only ever been
accessed by users who are located in the same location and
timezone. In this case a reasonable offset from UTC could be
assumed.
Unlike the rfc2822 format, this format is very strict. Any
variation in formatting will cause fast-import to reject the value.
rfc2822
This is the standard email format as described by RFC 2822.
An example value is "Tue Feb 6 11:22:18 2007 -0500". The Git parser
is accurate, but a little on the lenient side. It is the same
parser used by git am when applying patches received from email.
Some malformed strings may be accepted as valid dates. In some of
these cases Git will still be able to obtain the correct date from
the malformed string. There are also some types of malformed
strings which Git will parse wrong, and yet consider valid.
Seriously malformed strings will be rejected.
Unlike the raw format above, the timezone/UTC offset information
contained in an RFC 2822 date string is used to adjust the date
value to UTC prior to storage. Therefore it is important that this
information be as accurate as possible.
If the source material uses RFC 2822 style dates, the frontend
should let fast-import handle the parsing and conversion (rather
than attempting to do it itself) as the Git parser has been well
tested in the wild.
Frontends should prefer the raw format if the source material
already uses UNIX-epoch format, can be coaxed to give dates in that
format, or its format is easily convertible to it, as there is no
ambiguity in parsing.
now
Always use the current time and timezone. The literal now must
always be supplied for <when>.
This is a toy format. The current time and timezone of this system
is always copied into the identity string at the time it is being
created by fast-import. There is no way to specify a different time
or timezone.
This particular format is supplied as it's short to implement and
may be useful to a process that wants to create a new commit right
now, without needing to use a working directory or git
update-index.
If separate author and committer commands are used in a commit the
timestamps may not match, as the system clock will be polled twice
(once for each command). The only way to ensure that both author
and committer identity information has the same timestamp is to
omit author (thus copying from committer) or to use a date format
other than now.
Commands
fast-import accepts several commands to update the current repository
and control the current import process. More detailed discussion (with
examples) of each command follows later.
commit
Creates a new branch or updates an existing branch by creating a
new commit and updating the branch to point at the newly created
commit.
tag
Creates an annotated tag object from an existing commit or branch.
Lightweight tags are not supported by this command, as they are not
recommended for recording meaningful points in time.
reset
Reset an existing branch (or a new branch) to a specific revision.
This command must be used to change a branch to a specific revision
without making a commit on it.
blob
Convert raw file data into a blob, for future use in a commit
command. This command is optional and is not needed to perform an
import.
checkpoint
Forces fast-import to close the current packfile, generate its
unique SHA-1 checksum and index, and start a new packfile. This
command is optional and is not needed to perform an import.
progress
Causes fast-import to echo the entire line to its own standard
output. This command is optional and is not needed to perform an
import.
done
Marks the end of the stream. This command is optional unless the
done feature was requested using the --done command line option or
feature done command.
cat-blob
Causes fast-import to print a blob in cat-file --batch format to
the file descriptor set with --cat-blob-fd or stdout if
unspecified.
ls
Causes fast-import to print a line describing a directory entry in
ls-tree format to the file descriptor set with --cat-blob-fd or
stdout if unspecified.
feature
Require that fast-import supports the specified feature, or abort
if it does not.
option
Specify any of the options listed under OPTIONS that do not change
stream semantic to suit the frontend's needs. This command is
optional and is not needed to perform an import.
commit
Create or update a branch with a new commit, recording one logical
change to the project.
'commit' SP <ref> LF
mark?
('author' (SP <name>)? SP LT <email> GT SP <when> LF)?
'committer' (SP <name>)? SP LT <email> GT SP <when> LF
data
('from' SP <committish> LF)?
('merge' SP <committish> LF)?
(filemodify | filedelete | filecopy | filerename | filedeleteall | notemodify)*
LF?
where <ref> is the name of the branch to make the commit on. Typically
branch names are prefixed with refs/heads/ in Git, so importing the CVS
branch symbol RELENG-1_0 would use refs/heads/RELENG-1_0 for the value
of <ref>. The value of <ref> must be a valid refname in Git. As LF is
not valid in a Git refname, no quoting or escaping syntax is supported
here.
A mark command may optionally appear, requesting fast-import to save a
reference to the newly created commit for future use by the frontend
(see below for format). It is very common for frontends to mark every
commit they create, thereby allowing future branch creation from any
imported commit.
The data command following committer must supply the commit message
(see below for data command syntax). To import an empty commit message
use a 0 length data. Commit messages are free-form and are not
interpreted by Git. Currently they must be encoded in UTF-8, as
fast-import does not permit other encodings to be specified.
Zero or more filemodify, filedelete, filecopy, filerename,
filedeleteall and notemodify commands may be included to update the
contents of the branch prior to creating the commit. These commands may
be supplied in any order. However it is recommended that a
filedeleteall command precede all filemodify, filecopy, filerename and
notemodify commands in the same commit, as filedeleteall wipes the
branch clean (see below).
The LF after the command is optional (it used to be required).
author
An author command may optionally appear, if the author information
might differ from the committer information. If author is omitted
then fast-import will automatically use the committer's information
for the author portion of the commit. See below for a description
of the fields in author, as they are identical to committer.
committer
The committer command indicates who made this commit, and when they
made it.
Here <name> is the person's display name (for example "Com M
Itter") and <email> is the person's email address
("cm AT example.com"). LT and GT are the literal less-than (\x3c) and
greater-than (\x3e) symbols. These are required to delimit the
email address from the other fields in the line. Note that <name>
and <email> are free-form and may contain any sequence of bytes,
except LT, GT and LF. <name> is typically UTF-8 encoded.
The time of the change is specified by <when> using the date format
that was selected by the --date-format=<fmt> command line option.
See "Date Formats" above for the set of supported formats, and
their syntax.
from
The from command is used to specify the commit to initialize this
branch from. This revision will be the first ancestor of the new
commit. The state of the tree built at this commit will begin with
the state at the from commit, and be altered by the content
modifications in this commit.
Omitting the from command in the first commit of a new branch will
cause fast-import to create that commit with no ancestor. This
tends to be desired only for the initial commit of a project. If
the frontend creates all files from scratch when making a new
branch, a merge command may be used instead of from to start the
commit with an empty tree. Omitting the from command on existing
branches is usually desired, as the current commit on that branch
is automatically assumed to be the first ancestor of the new
commit.
As LF is not valid in a Git refname or SHA-1 expression, no quoting
or escaping syntax is supported within <committish>.
Here <committish> is any of the following:
o The name of an existing branch already in fast-import's
internal branch table. If fast-import doesn't know the name,
it's treated as a SHA-1 expression.
o A mark reference, :<idnum>, where <idnum> is the mark number.
The reason fast-import uses : to denote a mark reference is
this character is not legal in a Git branch name. The leading :
makes it easy to distinguish between the mark 42 (:42) and the
branch 42 (42 or refs/heads/42), or an abbreviated SHA-1 which
happened to consist only of base-10 digits.
Marks must be declared (via mark) before they can be used.
o A complete 40 byte or abbreviated commit SHA-1 in hex.
o Any valid Git SHA-1 expression that resolves to a commit. See
"SPECIFYING REVISIONS" in gitrevisions(7) for details.
The special case of restarting an incremental import from the
current branch value should be written as:
from refs/heads/branch^0
The ^0 suffix is necessary as fast-import does not permit a branch
to start from itself, and the branch is created in memory before
the from command is even read from the input. Adding ^0 will force
fast-import to resolve the commit through Git's revision parsing
library, rather than its internal branch table, thereby loading in
the existing value of the branch.
merge
Includes one additional ancestor commit. The additional ancestry
link does not change the way the tree state is built at this
commit. If the from command is omitted when creating a new branch,
the first merge commit will be the first ancestor of the current
commit, and the branch will start out with no files. An unlimited
number of merge commands per commit are permitted by fast-import,
thereby establishing an n-way merge. However Git's other tools
never create commits with more than 15 additional ancestors
(forming a 16-way merge). For this reason it is suggested that
frontends do not use more than 15 merge commands per commit; 16, if
starting a new, empty branch.
Here <committish> is any of the commit specification expressions
also accepted by from (see above).
filemodify
Included in a commit command to add a new file or change the
content of an existing file. This command has two different means
of specifying the content of the file.
External data format
The data content for the file was already supplied by a prior
blob command. The frontend just needs to connect it.
'M' SP <mode> SP <dataref> SP <path> LF
Here usually <dataref> must be either a mark reference
(:<idnum>) set by a prior blob command, or a full 40-byte SHA-1
of an existing Git blob object. If <mode> is 040000` then
<dataref> must be the full 40-byte SHA-1 of an existing Git
tree object or a mark reference set with --import-marks.
Inline data format
The data content for the file has not been supplied yet. The
frontend wants to supply it as part of this modify command.
'M' SP <mode> SP 'inline' SP <path> LF
data
See below for a detailed description of the data command.
In both formats <mode> is the type of file entry, specified in
octal. Git only supports the following modes:
o 100644 or 644: A normal (not-executable) file. The majority of
files in most projects use this mode. If in doubt, this is what
you want.
o 100755 or 755: A normal, but executable, file.
o 120000: A symlink, the content of the file will be the link
target.
o 160000: A gitlink, SHA-1 of the object refers to a commit in
another repository. Git links can only be specified by SHA or
through a commit mark. They are used to implement submodules.
o 040000: A subdirectory. Subdirectories can only be specified by
SHA or through a tree mark set with --import-marks.
In both formats <path> is the complete path of the file to be added
(if not already existing) or modified (if already existing).
A <path> string must use UNIX-style directory separators (forward
slash /), may contain any byte other than LF, and must not start
with double quote (").
A path can use C-style string quoting; this is accepted in all
cases and mandatory if the filename starts with double quote or
contains LF. In C-style quoting, the complete name should be
surrounded with double quotes, and any LF, backslash, or double
quote characters must be escaped by preceding them with a backslash
(e.g., "path/with\n, \\ and \" in it").
The value of <path> must be in canonical form. That is it must not:
o contain an empty directory component (e.g. foo//bar is
invalid),
o end with a directory separator (e.g. foo/ is invalid),
o start with a directory separator (e.g. /foo is invalid),
o contain the special component . or .. (e.g. foo/./bar and
foo/../bar are invalid).
The root of the tree can be represented by an empty string as
<path>.
It is recommended that <path> always be encoded using UTF-8.
filedelete
Included in a commit command to remove a file or recursively delete
an entire directory from the branch. If the file or directory
removal makes its parent directory empty, the parent directory will
be automatically removed too. This cascades up the tree until the
first non-empty directory or the root is reached.
'D' SP <path> LF
here <path> is the complete path of the file or subdirectory to be
removed from the branch. See filemodify above for a detailed
description of <path>.
filecopy
Recursively copies an existing file or subdirectory to a different
location within the branch. The existing file or directory must
exist. If the destination exists it will be completely replaced by
the content copied from the source.
'C' SP <path> SP <path> LF
here the first <path> is the source location and the second <path>
is the destination. See filemodify above for a detailed description
of what <path> may look like. To use a source path that contains SP
the path must be quoted.
A filecopy command takes effect immediately. Once the source
location has been copied to the destination any future commands
applied to the source location will not impact the destination of
the copy.
filerename
Renames an existing file or subdirectory to a different location
within the branch. The existing file or directory must exist. If
the destination exists it will be replaced by the source directory.
'R' SP <path> SP <path> LF
here the first <path> is the source location and the second <path>
is the destination. See filemodify above for a detailed description
of what <path> may look like. To use a source path that contains SP
the path must be quoted.
A filerename command takes effect immediately. Once the source
location has been renamed to the destination any future commands
applied to the source location will create new files there and not
impact the destination of the rename.
Note that a filerename is the same as a filecopy followed by a
filedelete of the source location. There is a slight performance
advantage to using filerename, but the advantage is so small that
it is never worth trying to convert a delete/add pair in source
material into a rename for fast-import. This filerename command is
provided just to simplify frontends that already have rename
information and don't want bother with decomposing it into a
filecopy followed by a filedelete.
filedeleteall
Included in a commit command to remove all files (and also all
directories) from the branch. This command resets the internal
branch structure to have no files in it, allowing the frontend to
subsequently add all interesting files from scratch.
'deleteall' LF
This command is extremely useful if the frontend does not know (or
does not care to know) what files are currently on the branch, and
therefore cannot generate the proper filedelete commands to update
the content.
Issuing a filedeleteall followed by the needed filemodify commands
to set the correct content will produce the same results as sending
only the needed filemodify and filedelete commands. The
filedeleteall approach may however require fast-import to use
slightly more memory per active branch (less than 1 MiB for even
most large projects); so frontends that can easily obtain only the
affected paths for a commit are encouraged to do so.
notemodify
Included in a commit <notes_ref> command to add a new note
annotating a <committish> or change this annotation contents.
Internally it is similar to filemodify 100644 on <committish> path
(maybe split into subdirectories). It's not advised to use any
other commands to write to the <notes_ref> tree except
filedeleteall to delete all existing notes in this tree. This
command has two different means of specifying the content of the
note.
External data format
The data content for the note was already supplied by a prior
blob command. The frontend just needs to connect it to the
commit that is to be annotated.
'N' SP <dataref> SP <committish> LF
Here <dataref> can be either a mark reference (:<idnum>) set by
a prior blob command, or a full 40-byte SHA-1 of an existing
Git blob object.
Inline data format
The data content for the note has not been supplied yet. The
frontend wants to supply it as part of this modify command.
'N' SP 'inline' SP <committish> LF
data
See below for a detailed description of the data command.
In both formats <committish> is any of the commit specification
expressions also accepted by from (see above).
mark
Arranges for fast-import to save a reference to the current object,
allowing the frontend to recall this object at a future point in time,
without knowing its SHA-1. Here the current object is the object
creation command the mark command appears within. This can be commit,
tag, and blob, but commit is the most common usage.
'mark' SP ':' <idnum> LF
where <idnum> is the number assigned by the frontend to this mark. The
value of <idnum> is expressed as an ASCII decimal integer. The value 0
is reserved and cannot be used as a mark. Only values greater than or
equal to 1 may be used as marks.
New marks are created automatically. Existing marks can be moved to
another object simply by reusing the same <idnum> in another mark
command.
tag
Creates an annotated tag referring to a specific commit. To create
lightweight (non-annotated) tags see the reset command below.
'tag' SP <name> LF
'from' SP <committish> LF
'tagger' (SP <name>)? SP LT <email> GT SP <when> LF
data
where <name> is the name of the tag to create.
Tag names are automatically prefixed with refs/tags/ when stored in
Git, so importing the CVS branch symbol RELENG-1_0-FINAL would use just
RELENG-1_0-FINAL for <name>, and fast-import will write the
corresponding ref as refs/tags/RELENG-1_0-FINAL.
The value of <name> must be a valid refname in Git and therefore may
contain forward slashes. As LF is not valid in a Git refname, no
quoting or escaping syntax is supported here.
The from command is the same as in the commit command; see above for
details.
The tagger command uses the same format as committer within commit;
again see above for details.
The data command following tagger must supply the annotated tag message
(see below for data command syntax). To import an empty tag message use
a 0 length data. Tag messages are free-form and are not interpreted by
Git. Currently they must be encoded in UTF-8, as fast-import does not
permit other encodings to be specified.
Signing annotated tags during import from within fast-import is not
supported. Trying to include your own PGP/GPG signature is not
recommended, as the frontend does not (easily) have access to the
complete set of bytes which normally goes into such a signature. If
signing is required, create lightweight tags from within fast-import
with reset, then create the annotated versions of those tags offline
with the standard git tag process.
reset
Creates (or recreates) the named branch, optionally starting from a
specific revision. The reset command allows a frontend to issue a new
from command for an existing branch, or to create a new branch from an
existing commit without creating a new commit.
'reset' SP <ref> LF
('from' SP <committish> LF)?
LF?
For a detailed description of <ref> and <committish> see above under
commit and from.
The LF after the command is optional (it used to be required).
The reset command can also be used to create lightweight
(non-annotated) tags. For example:
reset refs/tags/938
from :938
would create the lightweight tag refs/tags/938 referring to whatever
commit mark :938 references.
blob
Requests writing one file revision to the packfile. The revision is not
connected to any commit; this connection must be formed in a subsequent
commit command by referencing the blob through an assigned mark.
'blob' LF
mark?
data
The mark command is optional here as some frontends have chosen to
generate the Git SHA-1 for the blob on their own, and feed that
directly to commit. This is typically more work than it's worth
however, as marks are inexpensive to store and easy to use.
data
Supplies raw data (for use as blob/file content, commit messages, or
annotated tag messages) to fast-import. Data can be supplied using an
exact byte count or delimited with a terminating line. Real frontends
intended for production-quality conversions should always use the exact
byte count format, as it is more robust and performs better. The
delimited format is intended primarily for testing fast-import.
Comment lines appearing within the <raw> part of data commands are
always taken to be part of the body of the data and are therefore never
ignored by fast-import. This makes it safe to import any file/message
content whose lines might start with #.
Exact byte count format
The frontend must specify the number of bytes of data.
'data' SP <count> LF
<raw> LF?
where <count> is the exact number of bytes appearing within <raw>.
The value of <count> is expressed as an ASCII decimal integer. The
LF on either side of <raw> is not included in <count> and will not
be included in the imported data.
The LF after <raw> is optional (it used to be required) but
recommended. Always including it makes debugging a fast-import
stream easier as the next command always starts in column 0 of the
next line, even if <raw> did not end with an LF.
Delimited format
A delimiter string is used to mark the end of the data. fast-import
will compute the length by searching for the delimiter. This format
is primarily useful for testing and is not recommended for real
data.
'data' SP '<<' <delim> LF
<raw> LF
<delim> LF
LF?
where <delim> is the chosen delimiter string. The string <delim>
must not appear on a line by itself within <raw>, as otherwise
fast-import will think the data ends earlier than it really does.
The LF immediately trailing <raw> is part of <raw>. This is one of
the limitations of the delimited format, it is impossible to supply
a data chunk which does not have an LF as its last byte.
The LF after <delim> LF is optional (it used to be required).
checkpoint
Forces fast-import to close the current packfile, start a new one, and
to save out all current branch refs, tags and marks.
'checkpoint' LF
LF?
Note that fast-import automatically switches packfiles when the current
packfile reaches --max-pack-size, or 4 GiB, whichever limit is smaller.
During an automatic packfile switch fast-import does not update the
branch refs, tags or marks.
As a checkpoint can require a significant amount of CPU time and disk
IO (to compute the overall pack SHA-1 checksum, generate the
corresponding index file, and update the refs) it can easily take
several minutes for a single checkpoint command to complete.
Frontends may choose to issue checkpoints during extremely large and
long running imports, or when they need to allow another Git process
access to a branch. However given that a 30 GiB Subversion repository
can be loaded into Git through fast-import in about 3 hours, explicit
checkpointing may not be necessary.
The LF after the command is optional (it used to be required).
progress
Causes fast-import to print the entire progress line unmodified to its
standard output channel (file descriptor 1) when the command is
processed from the input stream. The command otherwise has no impact on
the current import, or on any of fast-import's internal state.
'progress' SP <any> LF
LF?
The <any> part of the command may contain any sequence of bytes that
does not contain LF. The LF after the command is optional. Callers may
wish to process the output through a tool such as sed to remove the
leading part of the line, for example:
frontend | git fast-import | sed 's/^progress //'
Placing a progress command immediately after a checkpoint will inform
the reader when the checkpoint has been completed and it can safely
access the refs that fast-import updated.
cat-blob
Causes fast-import to print a blob to a file descriptor previously
arranged with the --cat-blob-fd argument. The command otherwise has no
impact on the current import; its main purpose is to retrieve blobs
that may be in fast-import's memory but not accessible from the target
repository.
'cat-blob' SP <dataref> LF
The <dataref> can be either a mark reference (:<idnum>) set previously
or a full 40-byte SHA-1 of a Git blob, preexisting or ready to be
written.
Output uses the same format as git cat-file --batch:
<sha1> SP 'blob' SP <size> LF
<contents> LF
This command can be used anywhere in the stream that comments are
accepted. In particular, the cat-blob command can be used in the middle
of a commit but not in the middle of a data command.
See "Responses To Commands" below for details about how to read this
output safely.
ls
Prints information about the object at a path to a file descriptor
previously arranged with the --cat-blob-fd argument. This allows
printing a blob from the active commit (with cat-blob) or copying a
blob or tree from a previous commit for use in the current one (with
filemodify).
The ls command can be used anywhere in the stream that comments are
accepted, including the middle of a commit.
Reading from the active commit
This form can only be used in the middle of a commit. The path
names a directory entry within fast-import's active commit. The
path must be quoted in this case.
'ls' SP <path> LF
Reading from a named tree
The <dataref> can be a mark reference (:<idnum>) or the full
40-byte SHA-1 of a Git tag, commit, or tree object, preexisting or
waiting to be written. The path is relative to the top level of the
tree named by <dataref>.
'ls' SP <dataref> SP <path> LF
See filemodify above for a detailed description of <path>.
Output uses the same format as git ls-tree <tree> -- <path>:
<mode> SP ('blob' | 'tree' | 'commit') SP <dataref> HT <path> LF
The <dataref> represents the blob, tree, or commit object at <path> and
can be used in later cat-blob, filemodify, or ls commands.
If there is no file or subtree at that path, git fast-import will
instead report
missing SP <path> LF
See "Responses To Commands" below for details about how to read this
output safely.
feature
Require that fast-import supports the specified feature, or abort if it
does not.
'feature' SP <feature> ('=' <argument>)? LF
The <feature> part of the command may be any one of the following:
date-format, export-marks, relative-marks, no-relative-marks, force
Act as though the corresponding command-line option with a leading
-- was passed on the command line (see OPTIONS, above).
import-marks, import-marks-if-exists
Like --import-marks except in two respects: first, only one
"feature import-marks" or "feature import-marks-if-exists" command
is allowed per stream; second, an --import-marks= or
--import-marks-if-exists command-line option overrides any of these
"feature" commands in the stream; third, "feature
import-marks-if-exists" like a corresponding command-line option
silently skips a nonexistent file.
cat-blob, ls
Require that the backend support the cat-blob or ls command.
Versions of fast-import not supporting the specified command will
exit with a message indicating so. This lets the import error out
early with a clear message, rather than wasting time on the early
part of an import before the unsupported command is detected.
notes
Require that the backend support the notemodify (N) subcommand to
the commit command. Versions of fast-import not supporting notes
will exit with a message indicating so.
done
Error out if the stream ends without a done command. Without this
feature, errors causing the frontend to end abruptly at a
convenient point in the stream can go undetected. This may occur,
for example, if an import front end dies in mid-operation without
emitting SIGTERM or SIGKILL at its subordinate git fast-import
instance.
option
Processes the specified option so that git fast-import behaves in a way
that suits the frontend's needs. Note that options specified by the
frontend are overridden by any options the user may specify to git
fast-import itself.
'option' SP <option> LF
The <option> part of the command may contain any of the options listed
in the OPTIONS section that do not change import semantics, without the
leading -- and is treated in the same way.
Option commands must be the first commands on the input (not counting
feature commands), to give an option command after any non-option
command is an error.
The following commandline options change import semantics and may
therefore not be passed as option:
o date-format
o import-marks
o export-marks
o cat-blob-fd
o force
done
If the done feature is not in use, treated as if EOF was read. This can
be used to tell fast-import to finish early.
If the --done command line option or feature done command is in use,
the done command is mandatory and marks the end of the stream.
RESPONSES TO COMMANDS
New objects written by fast-import are not available immediately. Most
fast-import commands have no visible effect until the next checkpoint
(or completion). The frontend can send commands to fill fast-import's
input pipe without worrying about how quickly they will take effect,
which improves performance by simplifying scheduling.
For some frontends, though, it is useful to be able to read back data
from the current repository as it is being updated (for example when
the source material describes objects in terms of patches to be applied
to previously imported objects). This can be accomplished by connecting
the frontend and fast-import via bidirectional pipes:
mkfifo fast-import-output
frontend <fast-import-output |
git fast-import >fast-import-output
A frontend set up this way can use progress, ls, and cat-blob commands
to read information from the import in progress.
To avoid deadlock, such frontends must completely consume any pending
output from progress, ls, and cat-blob before performing writes to
fast-import that might block.
CRASH REPORTS
If fast-import is supplied invalid input it will terminate with a
non-zero exit status and create a crash report in the top level of the
Git repository it was importing into. Crash reports contain a snapshot
of the internal fast-import state as well as the most recent commands
that lead up to the crash.
All recent commands (including stream comments, file changes and
progress commands) are shown in the command history within the crash
report, but raw file data and commit messages are excluded from the
crash report. This exclusion saves space within the report file and
reduces the amount of buffering that fast-import must perform during
execution.
After writing a crash report fast-import will close the current
packfile and export the marks table. This allows the frontend developer
to inspect the repository state and resume the import from the point
where it crashed. The modified branches and tags are not updated during
a crash, as the import did not complete successfully. Branch and tag
information can be found in the crash report and must be applied
manually if the update is needed.
An example crash:
$ cat >in <<END_OF_INPUT
# my very first test commit
commit refs/heads/master
committer Shawn O. Pearce <spearce> 19283 -0400
# who is that guy anyway?
data <<EOF
this is my commit
EOF
M 644 inline .gitignore
data <<EOF
.gitignore
EOF
M 777 inline bob
END_OF_INPUT
$ git fast-import <in
fatal: Corrupt mode: M 777 inline bob
fast-import: dumping crash report to .git/fast_import_crash_8434
$ cat .git/fast_import_crash_8434
fast-import crash report:
fast-import process: 8434
parent process : 1391
at Sat Sep 1 00:58:12 2007
fatal: Corrupt mode: M 777 inline bob
Most Recent Commands Before Crash
---------------------------------
# my very first test commit
commit refs/heads/master
committer Shawn O. Pearce <spearce> 19283 -0400
# who is that guy anyway?
data <<EOF
M 644 inline .gitignore
data <<EOF
* M 777 inline bob
Active Branch LRU
-----------------
active_branches = 1 cur, 5 max
pos clock name
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1) 0 refs/heads/master
Inactive Branches
-----------------
refs/heads/master:
status : active loaded dirty
tip commit : 0000000000000000000000000000000000000000
old tree : 0000000000000000000000000000000000000000
cur tree : 0000000000000000000000000000000000000000
commit clock: 0
last pack :
-------------------
END OF CRASH REPORT
TIPS AND TRICKS
The following tips and tricks have been collected from various users of
fast-import, and are offered here as suggestions.
Use One Mark Per Commit
When doing a repository conversion, use a unique mark per commit (mark
:<n>) and supply the --export-marks option on the command line.
fast-import will dump a file which lists every mark and the Git object
SHA-1 that corresponds to it. If the frontend can tie the marks back to
the source repository, it is easy to verify the accuracy and
completeness of the import by comparing each Git commit to the
corresponding source revision.
Coming from a system such as Perforce or Subversion this should be
quite simple, as the fast-import mark can also be the Perforce
changeset number or the Subversion revision number.
Freely Skip Around Branches
Don't bother trying to optimize the frontend to stick to one branch at
a time during an import. Although doing so might be slightly faster for
fast-import, it tends to increase the complexity of the frontend code
considerably.
The branch LRU builtin to fast-import tends to behave very well, and
the cost of activating an inactive branch is so low that bouncing
around between branches has virtually no impact on import performance.
Handling Renames
When importing a renamed file or directory, simply delete the old
name(s) and modify the new name(s) during the corresponding commit. Git
performs rename detection after-the-fact, rather than explicitly during
a commit.
Use Tag Fixup Branches
Some other SCM systems let the user create a tag from multiple files
which are not from the same commit/changeset. Or to create tags which
are a subset of the files available in the repository.
Importing these tags as-is in Git is impossible without making at least
one commit which "fixes up" the files to match the content of the tag.
Use fast-import's reset command to reset a dummy branch outside of your
normal branch space to the base commit for the tag, then commit one or
more file fixup commits, and finally tag the dummy branch.
For example since all normal branches are stored under refs/heads/ name
the tag fixup branch TAG_FIXUP. This way it is impossible for the fixup
branch used by the importer to have namespace conflicts with real
branches imported from the source (the name TAG_FIXUP is not
refs/heads/TAG_FIXUP).
When committing fixups, consider using merge to connect the commit(s)
which are supplying file revisions to the fixup branch. Doing so will
allow tools such as git blame to track through the real commit history
and properly annotate the source files.
After fast-import terminates the frontend will need to do rm
.git/TAG_FIXUP to remove the dummy branch.
Import Now, Repack Later
As soon as fast-import completes the Git repository is completely valid
and ready for use. Typically this takes only a very short time, even
for considerably large projects (100,000+ commits).
However repacking the repository is necessary to improve data locality
and access performance. It can also take hours on extremely large
projects (especially if -f and a large --window parameter is used).
Since repacking is safe to run alongside readers and writers, run the
repack in the background and let it finish when it finishes. There is
no reason to wait to explore your new Git project!
If you choose to wait for the repack, don't try to run benchmarks or
performance tests until repacking is completed. fast-import outputs
suboptimal packfiles that are simply never seen in real use situations.
Repacking Historical Data
If you are repacking very old imported data (e.g. older than the last
year), consider expending some extra CPU time and supplying --window=50
(or higher) when you run git repack. This will take longer, but will
also produce a smaller packfile. You only need to expend the effort
once, and everyone using your project will benefit from the smaller
repository.
Include Some Progress Messages
Every once in a while have your frontend emit a progress message to
fast-import. The contents of the messages are entirely free-form, so
one suggestion would be to output the current month and year each time
the current commit date moves into the next month. Your users will feel
better knowing how much of the data stream has been processed.
PACKFILE OPTIMIZATION
When packing a blob fast-import always attempts to deltify against the
last blob written. Unless specifically arranged for by the frontend,
this will probably not be a prior version of the same file, so the
generated delta will not be the smallest possible. The resulting
packfile will be compressed, but will not be optimal.
Frontends which have efficient access to all revisions of a single file
(for example reading an RCS/CVS ,v file) can choose to supply all
revisions of that file as a sequence of consecutive blob commands. This
allows fast-import to deltify the different file revisions against each
other, saving space in the final packfile. Marks can be used to later
identify individual file revisions during a sequence of commit
commands.
The packfile(s) created by fast-import do not encourage good disk
access patterns. This is caused by fast-import writing the data in the
order it is received on standard input, while Git typically organizes
data within packfiles to make the most recent (current tip) data appear
before historical data. Git also clusters commits together, speeding up
revision traversal through better cache locality.
For this reason it is strongly recommended that users repack the
repository with git repack -a -d after fast-import completes, allowing
Git to reorganize the packfiles for faster data access. If blob deltas
are suboptimal (see above) then also adding the -f option to force
recomputation of all deltas can significantly reduce the final packfile
size (30-50% smaller can be quite typical).
MEMORY UTILIZATION
There are a number of factors which affect how much memory fast-import
requires to perform an import. Like critical sections of core Git,
fast-import uses its own memory allocators to amortize any overheads
associated with malloc. In practice fast-import tends to amortize any
malloc overheads to 0, due to its use of large block allocations.
per object
fast-import maintains an in-memory structure for every object written
in this execution. On a 32 bit system the structure is 32 bytes, on a
64 bit system the structure is 40 bytes (due to the larger pointer
sizes). Objects in the table are not deallocated until fast-import
terminates. Importing 2 million objects on a 32 bit system will require
approximately 64 MiB of memory.
The object table is actually a hashtable keyed on the object name (the
unique SHA-1). This storage configuration allows fast-import to reuse
an existing or already written object and avoid writing duplicates to
the output packfile. Duplicate blobs are surprisingly common in an
import, typically due to branch merges in the source.
per mark
Marks are stored in a sparse array, using 1 pointer (4 bytes or 8
bytes, depending on pointer size) per mark. Although the array is
sparse, frontends are still strongly encouraged to use marks between 1
and n, where n is the total number of marks required for this import.
per branch
Branches are classified as active and inactive. The memory usage of the
two classes is significantly different.
Inactive branches are stored in a structure which uses 96 or 120 bytes
(32 bit or 64 bit systems, respectively), plus the length of the branch
name (typically under 200 bytes), per branch. fast-import will easily
handle as many as 10,000 inactive branches in under 2 MiB of memory.
Active branches have the same overhead as inactive branches, but also
contain copies of every tree that has been recently modified on that
branch. If subtree include has not been modified since the branch
became active, its contents will not be loaded into memory, but if
subtree src has been modified by a commit since the branch became
active, then its contents will be loaded in memory.
As active branches store metadata about the files contained on that
branch, their in-memory storage size can grow to a considerable size
(see below).
fast-import automatically moves active branches to inactive status
based on a simple least-recently-used algorithm. The LRU chain is
updated on each commit command. The maximum number of active branches
can be increased or decreased on the command line with
--active-branches=.
per active tree
Trees (aka directories) use just 12 bytes of memory on top of the
memory required for their entries (see "per active file" below). The
cost of a tree is virtually 0, as its overhead amortizes out over the
individual file entries.
per active file entry
Files (and pointers to subtrees) within active trees require 52 or 64
bytes (32/64 bit platforms) per entry. To conserve space, file and tree
names are pooled in a common string table, allowing the filename
"Makefile" to use just 16 bytes (after including the string header
overhead) no matter how many times it occurs within the project.
The active branch LRU, when coupled with the filename string pool and
lazy loading of subtrees, allows fast-import to efficiently import
projects with 2,000+ branches and 45,114+ files in a very limited
memory footprint (less than 2.7 MiB per active branch).
SIGNALS
Sending SIGUSR1 to the git fast-import process ends the current
packfile early, simulating a checkpoint command. The impatient operator
can use this facility to peek at the objects and refs from an import in
progress, at the cost of some added running time and worse compression.
GIT
Part of the git(1) suite
Git 1.8.3.1 07/30/2024 GIT-FAST-IMPORT(1)
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