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GIT-FAST-IMPORT(1) | Git Manual | GIT-FAST-IMPORT(1) |
NAME¶
git-fast-import - Backend for fast Git data importersSYNOPSIS¶
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¶
--forceForce 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 get-mark, 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.
fastimport.unpackLimit
See git-config(1)
PERFORMANCE¶
The design of fast-import allows it to import large projects in a minimum amount of memory usage and processing time. Assuming the frontend is able to keep up with fast-import and feed it a constant stream of data, import times for projects holding 10+ years of history and containing 100,000+ individual commits are generally completed in just 1-2 hours on quite modest (~$2,000 USD) hardware. Most bottlenecks appear to be in foreign source data access (the source just cannot extract revisions fast enough) or disk IO (fast-import writes as fast as the disk will take the data). Imports will run faster if the source data is stored on a different drive than the destination Git repository (due to less IO contention).DEVELOPMENT COST¶
A typical frontend for fast-import tends to weigh in at approximately 200 lines of Perl/Python/Ruby code. Most developers have been able to create working importers in just a couple of hours, even though it is their first exposure to fast-import, and sometimes even to Git. This is an ideal situation, given that most conversion tools are throw-away (use once, and never look back).PARALLEL OPERATION¶
Like git push or git fetch, imports handled by fast-import are safe to run alongside parallel git repack -a -d or git gc invocations, or any other Git operation (including git prune, as loose objects are never used by fast-import). fast-import does not lock the branch or tag refs it is actively importing. After the import, during its ref update phase, fast-import tests each existing branch ref to verify the update will be a fast-forward update (the commit stored in the ref is contained in the new history of the commit to be written). If the update is not a fast-forward update, fast-import will skip updating that ref and instead prints a warning message. fast-import will always attempt to update all branch refs, and does not stop on the first failure. Branch updates can be forced with --force, but it’s recommended that this only be used on an otherwise quiet repository. Using --force is not necessary for an initial import into an empty repository.TECHNICAL DISCUSSION¶
fast-import tracks a set of branches in memory. Any branch can be created or modified at any point during the import process by sending a commit command on the input stream. This design allows a frontend program to process an unlimited number of branches simultaneously, generating commits in the order they are available from the source data. It also simplifies the frontend programs considerably. fast-import does not use or alter the current working directory, or any file within it. (It does however update the current Git repository, as referenced by GIT_DIR.) Therefore an import frontend may use the working directory for its own purposes, such as extracting file revisions from the foreign source. This ignorance of the working directory also allows fast-import to run very quickly, as it does not need to perform any costly file update operations when switching between branches.INPUT FORMAT¶
With the exception of raw file data (which Git does not interpret) the fast-import input format is text (ASCII) based. This text based format simplifies development and debugging of frontend programs, especially when a higher level language such as Perl, Python or Ruby is being used. fast-import is very strict about its input. Where we say SP below we mean exactly one space. Likewise LF means one (and only one) linefeed and HT one (and only one) horizontal tab. Supplying additional whitespace characters will cause unexpected results, such as branch names or file names with leading or trailing spaces in their name, or early termination of fast-import when it encounters unexpected input.Stream Comments¶
To aid in debugging frontends fast-import ignores any line that begins with # (ASCII pound/hash) up to and including the line ending LF. A comment line may contain any sequence of bytes that does not contain an LF and therefore may be used to include any detailed debugging information that might be specific to the frontend and useful when inspecting a fast-import data stream.Date Formats¶
The following date formats are supported. A frontend should select the format it will use for this import by passing the format name in the --date-format=<fmt> command-line option. rawThis is the Git native format and is <time> SP
<offutc>. It is also fast-import’s default format, if
--date-format was not specified.
The time of the event is specified by <time> as the number of
seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is written as an
ASCII decimal integer.
The local offset is specified by <offutc> as a positive or negative
offset from UTC. For example EST (which is 5 hours behind UTC) would be
expressed in <tz> by “-0500” while UTC is
“+0000”. The local offset does not affect <time>;
it is used only as an advisement to help formatting routines display the
timestamp.
If the local offset is not available in the source material, use
“+0000”, or the most common local offset. For example many
organizations have a CVS repository which has only ever been accessed by users
who are located in the same location and time zone. In this case a reasonable
offset from UTC could be assumed.
Unlike the rfc2822 format, this format is very strict. Any variation in
formatting will cause fast-import to reject the value.
rfc2822
This is the standard email format as described by RFC
2822.
An example value is “Tue Feb 6 11:22:18 2007 -0500”. The Git
parser is accurate, but a little on the lenient side. It is the same parser
used by git am when applying patches received from email.
Some malformed strings may be accepted as valid dates. In some of these cases
Git will still be able to obtain the correct date from the malformed string.
There are also some types of malformed strings which Git will parse wrong, and
yet consider valid. Seriously malformed strings will be rejected.
Unlike the raw format above, the time zone/UTC offset information
contained in an RFC 2822 date string is used to adjust the date value to UTC
prior to storage. Therefore it is important that this information be as
accurate as possible.
If the source material uses RFC 2822 style dates, the frontend should let
fast-import handle the parsing and conversion (rather than attempting to do it
itself) as the Git parser has been well tested in the wild.
Frontends should prefer the raw format if the source material already
uses UNIX-epoch format, can be coaxed to give dates in that format, or its
format is easily convertible to it, as there is no ambiguity in parsing.
now
Always use the current time and time zone. The literal
now must always be supplied for <when>.
This is a toy format. The current time and time zone of this system is always
copied into the identity string at the time it is being created by
fast-import. There is no way to specify a different time or time zone.
This particular format is supplied as it’s short to implement and may be
useful to a process that wants to create a new commit right now, without
needing to use a working directory or git update-index.
If separate author 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. commitCreates 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.
get-mark
Causes fast-import to print the SHA-1 corresponding to a
mark to the file descriptor set with --cat-blob-fd, or stdout if
unspecified.
cat-blob
Causes fast-import to print a blob in cat-file
--batch format to the file descriptor set with --cat-blob-fd 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
Enable the specified feature. This requires that
fast-import supports the specified feature, and aborts if it does not.
option
Specify any of the options listed under OPTIONS that do
not change stream semantic to suit the frontend’s needs. This command
is optional and is not needed to perform an import.
commit¶
Create or update a branch with a new commit, recording one logical change to the project.'commit' SP <ref> LF mark? ('author' (SP <name>)? SP LT <email> GT SP <when> LF)? 'committer' (SP <name>)? SP LT <email> GT SP <when> LF data ('from' SP <commit-ish> LF)? ('merge' SP <commit-ish> LF)? (filemodify | filedelete | filecopy | filerename | filedeleteall | notemodify)* LF?
An author command may optionally appear, if the author information might
differ from the committer information. If author is omitted then
fast-import will automatically use the committer’s information for the
author portion of the commit. See below for a description of the fields in
author, as they are identical to committer.
The committer command indicates who made this commit, and when they made
it.
Here <name> is the person’s display name (for example
“Com M Itter”) and <email> is the person’s
email address (“cm@example.com”). 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.
The from command is used to specify the commit to initialize this branch
from. This revision will be the first ancestor of the new commit. The state of
the tree built at this commit will begin with the state at the from
commit, and be altered by the content modifications in this commit.
Omitting the from command in the first commit of a new branch will cause
fast-import to create that commit with no ancestor. This tends to be desired
only for the initial commit of a project. If the frontend creates all files
from scratch when making a new branch, a merge command may be used
instead of from to start the commit with an empty tree. Omitting the
from command on existing branches is usually desired, as the current
commit on that branch is automatically assumed to be the first ancestor of the
new commit.
As LF is not valid in a Git refname or SHA-1 expression, no quoting or
escaping syntax is supported within <commit-ish>.
Here <commit-ish> is any of the following:
The ^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.
•The name of an existing branch already in
fast-import’s internal branch table. If fast-import doesn’t know
the name, it’s treated as a SHA-1 expression.
•A mark reference, :<idnum>, where
<idnum> is the mark number.
The reason fast-import uses : to denote a mark reference is this
character is not legal in a Git branch name. The leading : makes it
easy to distinguish between the mark 42 ( :42) and the branch 42
(42 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 null SHA-1 (40 zeros) specifies that
the branch is to be removed.
The special case of restarting an incremental import from the current branch
value should be written as:
from refs/heads/branch^0
Includes one additional ancestor commit. The additional ancestry link does not
change the way the tree state is built at this commit. If the from
command is omitted when creating a new branch, the first merge commit
will be the first ancestor of the current commit, and the branch will start
out with no files. An unlimited number of merge commands per commit are
permitted by fast-import, thereby establishing an n-way merge.
Here <commit-ish> is any of the commit specification expressions
also accepted by from (see above).
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.
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
'M' SP <mode> SP <dataref> SP <path> LF
The data content for the file has not been supplied yet.
The frontend wants to supply it as part of this modify command.
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:
'M' SP <mode> SP 'inline' SP <path> LF data
•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.
•100755 or 755: A normal, but
executable, file.
•120000: A symlink, the content of the file
will be the link target.
•160000: A gitlink, SHA-1 of the object
refers to a commit in another repository. Git links can only be specified by
SHA or through a commit mark. They are used to implement submodules.
•040000: A subdirectory. Subdirectories can
only be specified by SHA or through a tree mark set with
--import-marks.
In both formats <path> is the complete path of the file to be added
(if not already existing) or modified (if already existing).
A <path> string must use UNIX-style directory separators (forward
slash /), may contain any byte other than LF, and must not start
with double quote ( ").
A path can use C-style string quoting; this is accepted in all cases and
mandatory if the filename starts with double quote or contains LF. In
C-style quoting, the complete name should be surrounded with double quotes,
and any LF, backslash, or double quote characters must be escaped by
preceding them with a backslash (e.g., "path/with\n, \\ and \" in
it").
The value of <path> must be in canonical form. That is it must not:
•contain an empty directory component (e.g.
foo//bar is invalid),
•end with a directory separator (e.g. foo/
is invalid),
•start with a directory separator (e.g.
/foo is invalid),
•contain the special component . or
.. (e.g. foo/./bar 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.
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.
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>.
'D' SP <path> LF
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.
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.
'C' SP <path> SP <path> LF
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.
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.
'R' SP <path> SP <path> LF
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.
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.
'deleteall' LF
Included in a commit <notes_ref> command to add a new note
annotating a <commit-ish> or change this annotation contents.
Internally it is similar to filemodify 100644 on <commit-ish>
path (maybe split into subdirectories). It’s not advised to use any
other commands to write to the <notes_ref> tree except
filedeleteall to delete all existing notes in this tree. This command
has two different means of specifying the content of the note.
External data format
The data content for the note was already supplied by a
prior blob command. The frontend just needs to connect it to the commit
that is to be annotated.
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
'N' SP <dataref> SP <commit-ish> LF
The data content for the note has not been supplied yet.
The frontend wants to supply it as part of this modify command.
See below for a detailed description of the data command.
In both formats <commit-ish> is any of the commit specification
expressions also accepted by from (see above).'N' SP 'inline' SP <commit-ish> LF data
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
tag¶
Creates an annotated tag referring to a specific commit. To create lightweight (non-annotated) tags see the reset command below.'tag' SP <name> LF 'from' SP <commit-ish> LF 'tagger' (SP <name>)? SP LT <email> GT SP <when> LF data
reset¶
Creates (or recreates) the named branch, optionally starting from a specific revision. The reset command allows a frontend to issue a new from command for an existing branch, or to create a new branch from an existing commit without creating a new commit.'reset' SP <ref> LF ('from' SP <commit-ish> LF)? LF?
reset refs/tags/938 from :938
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
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 formatThe frontend must specify the number of bytes of data.
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
'data' SP <count> LF <raw> LF?
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.
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).
'data' SP '<<' <delim> LF <raw> LF <delim> LF LF?
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?
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?
frontend | git fast-import | sed 's/^progress //'
get-mark¶
Causes fast-import to print the SHA-1 corresponding to a mark to stdout or to the file descriptor previously arranged with the --cat-blob-fd argument. The command otherwise has no impact on the current import; its purpose is to retrieve SHA-1s that later commits might want to refer to in their commit messages.'get-mark' SP ':' <idnum> LF
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
<sha1> SP 'blob' SP <size> LF <contents> LF
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 commitThis 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.
Reading from a named tree
'ls' SP <path> LF
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>.
See filemodify above for a detailed description of <path>.
Output uses the same format as git ls-tree <tree> -- <path>:
'ls' SP <dataref> SP <path> LF
<mode> SP ('blob' | 'tree' | 'commit') SP <dataref> HT <path> LF
missing SP <path> LF
feature¶
Require that fast-import supports the specified feature, or abort if it does not.'feature' SP <feature> ('=' <argument>)? LF
Act as though the corresponding command-line option with
a leading -- was passed on the command line (see OPTIONS, above).
import-marks, import-marks-if-exists
Like --import-marks except in two respects: first, only
one "feature import-marks" or "feature
import-marks-if-exists" command is allowed per stream; second, an
--import-marks= or --import-marks-if-exists command-line option overrides any
of these "feature" commands in the stream; third, "feature
import-marks-if-exists" like a corresponding command-line option silently
skips a nonexistent file.
get-mark, cat-blob, ls
Require that the backend support the get-mark,
cat-blob, or ls command respectively. Versions of fast-import
not supporting the specified command will exit with a message indicating so.
This lets the import error out early with a clear message, rather than wasting
time on the early part of an import before the unsupported command is
detected.
notes
Require that the backend support the notemodify
(N) subcommand to the commit command. Versions of fast-import not
supporting notes will exit with a message indicating so.
done
Error out if the stream ends without a done
command. Without this feature, errors causing the frontend to end abruptly at
a convenient point in the stream can go undetected. This may occur, for
example, if an import front end dies in mid-operation without emitting SIGTERM
or SIGKILL at its subordinate git fast-import instance.
option¶
Processes the specified option so that git fast-import behaves in a way that suits the frontend’s needs. Note that options specified by the frontend are overridden by any options the user may specify to git fast-import itself.'option' SP <option> LF
•date-format
•import-marks
•export-marks
•cat-blob-fd
•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
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.SEE ALSO¶
git-fast-export(1)GIT¶
Part of the git(1) suite05/15/2017 | Git 2.11.0 |