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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.OPTIONS¶
--date-format=<fmt>Specify the type of dates the frontend will
supply to fast-import within author, committer and tagger commands. See
“Date Formats” below for details about which formats are
supported, and their syntax.
--force
Force updating modified existing branches,
even if doing so would cause commits to be lost (as the new commit does not
contain the old commit).
--max-pack-size=<n>
Maximum size of each output packfile. The
default is unlimited.
--big-file-threshold=<n>
Maximum size of a blob that fast-import will
attempt to create a delta for, expressed in bytes. The default is 512m (512
MiB). Some importers may wish to lower this on systems with constrained
memory.
--depth=<n>
Maximum delta depth, for blob and tree
deltification. Default is 10.
--active-branches=<n>
Maximum number of branches to maintain active
at once. See “Memory Utilization” below for details. Default is
5.
--export-marks=<file>
Dumps the internal marks table to <file>
when complete. Marks are written one per line as :markid SHA-1. Frontends can
use this file to validate imports after they have been completed, or to save
the marks table across incremental runs. As <file> is only opened and
truncated at checkpoint (or completion) the same path can also be safely given
to --import-marks.
--import-marks=<file>
Before processing any input, load the marks
specified in <file>. The input file must exist, must be readable, and
must use the same format as produced by --export-marks. Multiple options may
be supplied to import more than one set of marks. If a mark is defined to
different values, the last file wins.
--import-marks-if-exists=<file>
Like --import-marks but instead of erroring
out, silently skips the file if it does not exist.
--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.
--no-relative-marks
Negates a previous --relative-marks. Allows
for combining relative and non-relative marks by interweaving
--(no-)-relative-marks with the --(import|export)-marks= options.
--cat-blob-fd=<fd>
Specify the file descriptor that will be
written to when the cat-blob command is encountered in the stream. The default
behaviour is to write to stdout.
--done
Require a 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.
--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.
--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.
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.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).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.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.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 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. 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.
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?
•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.
from refs/heads/branch^0
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.
'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.
•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).
'D' SP <path> LF
'C' SP <path> SP <path> LF
'R' SP <path> SP <path> LF
'deleteall' LF
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 <committish> 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.
'N' SP 'inline' SP <committish> 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 <committish> 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 <committish> 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.The 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 //'
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).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.
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>.
'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.
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.
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.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.$ 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.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.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.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).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.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.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.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.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) suiteNOTES¶
- 1.
- cm@example.com
mailto:cm@example.com
03/19/2016 | Git 1.7.10.4 |