Documentation / git-fast-import.txton commit Merge git://git.kernel.org/pub/scm/gitk/gitk (a41acc6)
   1git-fast-import(1)
   2==================
   3
   4NAME
   5----
   6git-fast-import - Backend for fast Git data importers
   7
   8
   9SYNOPSIS
  10--------
  11frontend | 'git-fast-import' [options]
  12
  13DESCRIPTION
  14-----------
  15This program is usually not what the end user wants to run directly.
  16Most end users want to use one of the existing frontend programs,
  17which parses a specific type of foreign source and feeds the contents
  18stored there to git-fast-import.
  19
  20fast-import reads a mixed command/data stream from standard input and
  21writes one or more packfiles directly into the current repository.
  22When EOF is received on standard input, fast import writes out
  23updated branch and tag refs, fully updating the current repository
  24with the newly imported data.
  25
  26The fast-import backend itself can import into an empty repository (one that
  27has already been initialized by linkgit:git-init[1]) or incrementally
  28update an existing populated repository.  Whether or not incremental
  29imports are supported from a particular foreign source depends on
  30the frontend program in use.
  31
  32
  33OPTIONS
  34-------
  35--date-format=<fmt>::
  36        Specify the type of dates the frontend will supply to
  37        fast-import within `author`, `committer` and `tagger` commands.
  38        See ``Date Formats'' below for details about which formats
  39        are supported, and their syntax.
  40
  41--force::
  42        Force updating modified existing branches, even if doing
  43        so would cause commits to be lost (as the new commit does
  44        not contain the old commit).
  45
  46--max-pack-size=<n>::
  47        Maximum size of each output packfile, expressed in MiB.
  48        The default is 4096 (4 GiB) as that is the maximum allowed
  49        packfile size (due to file format limitations). Some
  50        importers may wish to lower this, such as to ensure the
  51        resulting packfiles fit on CDs.
  52
  53--depth=<n>::
  54        Maximum delta depth, for blob and tree deltification.
  55        Default is 10.
  56
  57--active-branches=<n>::
  58        Maximum number of branches to maintain active at once.
  59        See ``Memory Utilization'' below for details.  Default is 5.
  60
  61--export-marks=<file>::
  62        Dumps the internal marks table to <file> when complete.
  63        Marks are written one per line as `:markid SHA-1`.
  64        Frontends can use this file to validate imports after they
  65        have been completed, or to save the marks table across
  66        incremental runs.  As <file> is only opened and truncated
  67        at checkpoint (or completion) the same path can also be
  68        safely given to \--import-marks.
  69
  70--import-marks=<file>::
  71        Before processing any input, load the marks specified in
  72        <file>.  The input file must exist, must be readable, and
  73        must use the same format as produced by \--export-marks.
  74        Multiple options may be supplied to import more than one
  75        set of marks.  If a mark is defined to different values,
  76        the last file wins.
  77
  78--export-pack-edges=<file>::
  79        After creating a packfile, print a line of data to
  80        <file> listing the filename of the packfile and the last
  81        commit on each branch that was written to that packfile.
  82        This information may be useful after importing projects
  83        whose total object set exceeds the 4 GiB packfile limit,
  84        as these commits can be used as edge points during calls
  85        to linkgit:git-pack-objects[1].
  86
  87--quiet::
  88        Disable all non-fatal output, making fast-import silent when it
  89        is successful.  This option disables the output shown by
  90        \--stats.
  91
  92--stats::
  93        Display some basic statistics about the objects fast-import has
  94        created, the packfiles they were stored into, and the
  95        memory used by fast-import during this run.  Showing this output
  96        is currently the default, but can be disabled with \--quiet.
  97
  98
  99Performance
 100-----------
 101The design of fast-import allows it to import large projects in a minimum
 102amount of memory usage and processing time.  Assuming the frontend
 103is able to keep up with fast-import and feed it a constant stream of data,
 104import times for projects holding 10+ years of history and containing
 105100,000+ individual commits are generally completed in just 1-2
 106hours on quite modest (~$2,000 USD) hardware.
 107
 108Most bottlenecks appear to be in foreign source data access (the
 109source just cannot extract revisions fast enough) or disk IO (fast-import
 110writes as fast as the disk will take the data).  Imports will run
 111faster if the source data is stored on a different drive than the
 112destination Git repository (due to less IO contention).
 113
 114
 115Development Cost
 116----------------
 117A typical frontend for fast-import tends to weigh in at approximately 200
 118lines of Perl/Python/Ruby code.  Most developers have been able to
 119create working importers in just a couple of hours, even though it
 120is their first exposure to fast-import, and sometimes even to Git.  This is
 121an ideal situation, given that most conversion tools are throw-away
 122(use once, and never look back).
 123
 124
 125Parallel Operation
 126------------------
 127Like `git-push` or `git-fetch`, imports handled by fast-import are safe to
 128run alongside parallel `git repack -a -d` or `git gc` invocations,
 129or any other Git operation (including `git prune`, as loose objects
 130are never used by fast-import).
 131
 132fast-import does not lock the branch or tag refs it is actively importing.
 133After the import, during its ref update phase, fast-import tests each
 134existing branch ref to verify the update will be a fast-forward
 135update (the commit stored in the ref is contained in the new
 136history of the commit to be written).  If the update is not a
 137fast-forward update, fast-import will skip updating that ref and instead
 138prints a warning message.  fast-import will always attempt to update all
 139branch refs, and does not stop on the first failure.
 140
 141Branch updates can be forced with \--force, but its recommended that
 142this only be used on an otherwise quiet repository.  Using \--force
 143is not necessary for an initial import into an empty repository.
 144
 145
 146Technical Discussion
 147--------------------
 148fast-import tracks a set of branches in memory.  Any branch can be created
 149or modified at any point during the import process by sending a
 150`commit` command on the input stream.  This design allows a frontend
 151program to process an unlimited number of branches simultaneously,
 152generating commits in the order they are available from the source
 153data.  It also simplifies the frontend programs considerably.
 154
 155fast-import does not use or alter the current working directory, or any
 156file within it.  (It does however update the current Git repository,
 157as referenced by `GIT_DIR`.)  Therefore an import frontend may use
 158the working directory for its own purposes, such as extracting file
 159revisions from the foreign source.  This ignorance of the working
 160directory also allows fast-import to run very quickly, as it does not
 161need to perform any costly file update operations when switching
 162between branches.
 163
 164Input Format
 165------------
 166With the exception of raw file data (which Git does not interpret)
 167the fast-import input format is text (ASCII) based.  This text based
 168format simplifies development and debugging of frontend programs,
 169especially when a higher level language such as Perl, Python or
 170Ruby is being used.
 171
 172fast-import is very strict about its input.  Where we say SP below we mean
 173*exactly* one space.  Likewise LF means one (and only one) linefeed.
 174Supplying additional whitespace characters will cause unexpected
 175results, such as branch names or file names with leading or trailing
 176spaces in their name, or early termination of fast-import when it encounters
 177unexpected input.
 178
 179Stream Comments
 180~~~~~~~~~~~~~~~
 181To aid in debugging frontends fast-import ignores any line that
 182begins with `#` (ASCII pound/hash) up to and including the line
 183ending `LF`.  A comment line may contain any sequence of bytes
 184that does not contain an LF and therefore may be used to include
 185any detailed debugging information that might be specific to the
 186frontend and useful when inspecting a fast-import data stream.
 187
 188Date Formats
 189~~~~~~~~~~~~
 190The following date formats are supported.  A frontend should select
 191the format it will use for this import by passing the format name
 192in the \--date-format=<fmt> command line option.
 193
 194`raw`::
 195        This is the Git native format and is `<time> SP <offutc>`.
 196        It is also fast-import's default format, if \--date-format was
 197        not specified.
 198+
 199The time of the event is specified by `<time>` as the number of
 200seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is
 201written as an ASCII decimal integer.
 202+
 203The local offset is specified by `<offutc>` as a positive or negative
 204offset from UTC.  For example EST (which is 5 hours behind UTC)
 205would be expressed in `<tz>` by ``-0500'' while UTC is ``+0000''.
 206The local offset does not affect `<time>`; it is used only as an
 207advisement to help formatting routines display the timestamp.
 208+
 209If the local offset is not available in the source material, use
 210``+0000'', or the most common local offset.  For example many
 211organizations have a CVS repository which has only ever been accessed
 212by users who are located in the same location and timezone.  In this
 213case a reasonable offset from UTC could be assumed.
 214+
 215Unlike the `rfc2822` format, this format is very strict.  Any
 216variation in formatting will cause fast-import to reject the value.
 217
 218`rfc2822`::
 219        This is the standard email format as described by RFC 2822.
 220+
 221An example value is ``Tue Feb 6 11:22:18 2007 -0500''.  The Git
 222parser is accurate, but a little on the lenient side.  It is the
 223same parser used by linkgit:git-am[1] when applying patches
 224received from email.
 225+
 226Some malformed strings may be accepted as valid dates.  In some of
 227these cases Git will still be able to obtain the correct date from
 228the malformed string.  There are also some types of malformed
 229strings which Git will parse wrong, and yet consider valid.
 230Seriously malformed strings will be rejected.
 231+
 232Unlike the `raw` format above, the timezone/UTC offset information
 233contained in an RFC 2822 date string is used to adjust the date
 234value to UTC prior to storage.  Therefore it is important that
 235this information be as accurate as possible.
 236+
 237If the source material uses RFC 2822 style dates,
 238the frontend should let fast-import handle the parsing and conversion
 239(rather than attempting to do it itself) as the Git parser has
 240been well tested in the wild.
 241+
 242Frontends should prefer the `raw` format if the source material
 243already uses UNIX-epoch format, can be coaxed to give dates in that
 244format, or its format is easily convertible to it, as there is no
 245ambiguity in parsing.
 246
 247`now`::
 248        Always use the current time and timezone.  The literal
 249        `now` must always be supplied for `<when>`.
 250+
 251This is a toy format.  The current time and timezone of this system
 252is always copied into the identity string at the time it is being
 253created by fast-import.  There is no way to specify a different time or
 254timezone.
 255+
 256This particular format is supplied as its short to implement and
 257may be useful to a process that wants to create a new commit
 258right now, without needing to use a working directory or
 259linkgit:git-update-index[1].
 260+
 261If separate `author` and `committer` commands are used in a `commit`
 262the timestamps may not match, as the system clock will be polled
 263twice (once for each command).  The only way to ensure that both
 264author and committer identity information has the same timestamp
 265is to omit `author` (thus copying from `committer`) or to use a
 266date format other than `now`.
 267
 268Commands
 269~~~~~~~~
 270fast-import accepts several commands to update the current repository
 271and control the current import process.  More detailed discussion
 272(with examples) of each command follows later.
 273
 274`commit`::
 275        Creates a new branch or updates an existing branch by
 276        creating a new commit and updating the branch to point at
 277        the newly created commit.
 278
 279`tag`::
 280        Creates an annotated tag object from an existing commit or
 281        branch.  Lightweight tags are not supported by this command,
 282        as they are not recommended for recording meaningful points
 283        in time.
 284
 285`reset`::
 286        Reset an existing branch (or a new branch) to a specific
 287        revision.  This command must be used to change a branch to
 288        a specific revision without making a commit on it.
 289
 290`blob`::
 291        Convert raw file data into a blob, for future use in a
 292        `commit` command.  This command is optional and is not
 293        needed to perform an import.
 294
 295`checkpoint`::
 296        Forces fast-import to close the current packfile, generate its
 297        unique SHA-1 checksum and index, and start a new packfile.
 298        This command is optional and is not needed to perform
 299        an import.
 300
 301`progress`::
 302        Causes fast-import to echo the entire line to its own
 303        standard output.  This command is optional and is not needed
 304        to perform an import.
 305
 306`commit`
 307~~~~~~~~
 308Create or update a branch with a new commit, recording one logical
 309change to the project.
 310
 311....
 312        'commit' SP <ref> LF
 313        mark?
 314        ('author' SP <name> SP LT <email> GT SP <when> LF)?
 315        'committer' SP <name> SP LT <email> GT SP <when> LF
 316        data
 317        ('from' SP <committish> LF)?
 318        ('merge' SP <committish> LF)?
 319        (filemodify | filedelete | filecopy | filerename | filedeleteall)*
 320        LF?
 321....
 322
 323where `<ref>` is the name of the branch to make the commit on.
 324Typically branch names are prefixed with `refs/heads/` in
 325Git, so importing the CVS branch symbol `RELENG-1_0` would use
 326`refs/heads/RELENG-1_0` for the value of `<ref>`.  The value of
 327`<ref>` must be a valid refname in Git.  As `LF` is not valid in
 328a Git refname, no quoting or escaping syntax is supported here.
 329
 330A `mark` command may optionally appear, requesting fast-import to save a
 331reference to the newly created commit for future use by the frontend
 332(see below for format).  It is very common for frontends to mark
 333every commit they create, thereby allowing future branch creation
 334from any imported commit.
 335
 336The `data` command following `committer` must supply the commit
 337message (see below for `data` command syntax).  To import an empty
 338commit message use a 0 length data.  Commit messages are free-form
 339and are not interpreted by Git.  Currently they must be encoded in
 340UTF-8, as fast-import does not permit other encodings to be specified.
 341
 342Zero or more `filemodify`, `filedelete`, `filecopy`, `filerename`
 343and `filedeleteall` commands
 344may be included to update the contents of the branch prior to
 345creating the commit.  These commands may be supplied in any order.
 346However it is recommended that a `filedeleteall` command precede
 347all `filemodify`, `filecopy` and `filerename` commands in the same
 348commit, as `filedeleteall`
 349wipes the branch clean (see below).
 350
 351The `LF` after the command is optional (it used to be required).
 352
 353`author`
 354^^^^^^^^
 355An `author` command may optionally appear, if the author information
 356might differ from the committer information.  If `author` is omitted
 357then fast-import will automatically use the committer's information for
 358the author portion of the commit.  See below for a description of
 359the fields in `author`, as they are identical to `committer`.
 360
 361`committer`
 362^^^^^^^^^^^
 363The `committer` command indicates who made this commit, and when
 364they made it.
 365
 366Here `<name>` is the person's display name (for example
 367``Com M Itter'') and `<email>` is the person's email address
 368(``cm@example.com'').  `LT` and `GT` are the literal less-than (\x3c)
 369and greater-than (\x3e) symbols.  These are required to delimit
 370the email address from the other fields in the line.  Note that
 371`<name>` is free-form and may contain any sequence of bytes, except
 372`LT` and `LF`.  It is typically UTF-8 encoded.
 373
 374The time of the change is specified by `<when>` using the date format
 375that was selected by the \--date-format=<fmt> command line option.
 376See ``Date Formats'' above for the set of supported formats, and
 377their syntax.
 378
 379`from`
 380^^^^^^
 381The `from` command is used to specify the commit to initialize
 382this branch from.  This revision will be the first ancestor of the
 383new commit.
 384
 385Omitting the `from` command in the first commit of a new branch
 386will cause fast-import to create that commit with no ancestor. This
 387tends to be desired only for the initial commit of a project.
 388Omitting the `from` command on existing branches is usually desired,
 389as the current commit on that branch is automatically assumed to
 390be the first ancestor of the new commit.
 391
 392As `LF` is not valid in a Git refname or SHA-1 expression, no
 393quoting or escaping syntax is supported within `<committish>`.
 394
 395Here `<committish>` is any of the following:
 396
 397* The name of an existing branch already in fast-import's internal branch
 398  table.  If fast-import doesn't know the name, its treated as a SHA-1
 399  expression.
 400
 401* A mark reference, `:<idnum>`, where `<idnum>` is the mark number.
 402+
 403The reason fast-import uses `:` to denote a mark reference is this character
 404is not legal in a Git branch name.  The leading `:` makes it easy
 405to distinguish between the mark 42 (`:42`) and the branch 42 (`42`
 406or `refs/heads/42`), or an abbreviated SHA-1 which happened to
 407consist only of base-10 digits.
 408+
 409Marks must be declared (via `mark`) before they can be used.
 410
 411* A complete 40 byte or abbreviated commit SHA-1 in hex.
 412
 413* Any valid Git SHA-1 expression that resolves to a commit.  See
 414  ``SPECIFYING REVISIONS'' in linkgit:git-rev-parse[1] for details.
 415
 416The special case of restarting an incremental import from the
 417current branch value should be written as:
 418----
 419        from refs/heads/branch^0
 420----
 421The `{caret}0` suffix is necessary as fast-import does not permit a branch to
 422start from itself, and the branch is created in memory before the
 423`from` command is even read from the input.  Adding `{caret}0` will force
 424fast-import to resolve the commit through Git's revision parsing library,
 425rather than its internal branch table, thereby loading in the
 426existing value of the branch.
 427
 428`merge`
 429^^^^^^^
 430Includes one additional ancestor commit, and makes the current
 431commit a merge commit.  An unlimited number of `merge` commands per
 432commit are permitted by fast-import, thereby establishing an n-way merge.
 433However Git's other tools never create commits with more than 15
 434additional ancestors (forming a 16-way merge).  For this reason
 435it is suggested that frontends do not use more than 15 `merge`
 436commands per commit.
 437
 438Here `<committish>` is any of the commit specification expressions
 439also accepted by `from` (see above).
 440
 441`filemodify`
 442^^^^^^^^^^^^
 443Included in a `commit` command to add a new file or change the
 444content of an existing file.  This command has two different means
 445of specifying the content of the file.
 446
 447External data format::
 448        The data content for the file was already supplied by a prior
 449        `blob` command.  The frontend just needs to connect it.
 450+
 451....
 452        'M' SP <mode> SP <dataref> SP <path> LF
 453....
 454+
 455Here `<dataref>` can be either a mark reference (`:<idnum>`)
 456set by a prior `blob` command, or a full 40-byte SHA-1 of an
 457existing Git blob object.
 458
 459Inline data format::
 460        The data content for the file has not been supplied yet.
 461        The frontend wants to supply it as part of this modify
 462        command.
 463+
 464....
 465        'M' SP <mode> SP 'inline' SP <path> LF
 466        data
 467....
 468+
 469See below for a detailed description of the `data` command.
 470
 471In both formats `<mode>` is the type of file entry, specified
 472in octal.  Git only supports the following modes:
 473
 474* `100644` or `644`: A normal (not-executable) file.  The majority
 475  of files in most projects use this mode.  If in doubt, this is
 476  what you want.
 477* `100755` or `755`: A normal, but executable, file.
 478* `120000`: A symlink, the content of the file will be the link target.
 479
 480In both formats `<path>` is the complete path of the file to be added
 481(if not already existing) or modified (if already existing).
 482
 483A `<path>` string must use UNIX-style directory separators (forward
 484slash `/`), may contain any byte other than `LF`, and must not
 485start with double quote (`"`).
 486
 487If an `LF` or double quote must be encoded into `<path>` shell-style
 488quoting should be used, e.g. `"path/with\n and \" in it"`.
 489
 490The value of `<path>` must be in canonical form. That is it must not:
 491
 492* contain an empty directory component (e.g. `foo//bar` is invalid),
 493* end with a directory separator (e.g. `foo/` is invalid),
 494* start with a directory separator (e.g. `/foo` is invalid),
 495* contain the special component `.` or `..` (e.g. `foo/./bar` and
 496  `foo/../bar` are invalid).
 497
 498It is recommended that `<path>` always be encoded using UTF-8.
 499
 500`filedelete`
 501^^^^^^^^^^^^
 502Included in a `commit` command to remove a file or recursively
 503delete an entire directory from the branch.  If the file or directory
 504removal makes its parent directory empty, the parent directory will
 505be automatically removed too.  This cascades up the tree until the
 506first non-empty directory or the root is reached.
 507
 508....
 509        'D' SP <path> LF
 510....
 511
 512here `<path>` is the complete path of the file or subdirectory to
 513be removed from the branch.
 514See `filemodify` above for a detailed description of `<path>`.
 515
 516`filecopy`
 517^^^^^^^^^^^^
 518Recursively copies an existing file or subdirectory to a different
 519location within the branch.  The existing file or directory must
 520exist.  If the destination exists it will be completely replaced
 521by the content copied from the source.
 522
 523....
 524        'C' SP <path> SP <path> LF
 525....
 526
 527here the first `<path>` is the source location and the second
 528`<path>` is the destination.  See `filemodify` above for a detailed
 529description of what `<path>` may look like.  To use a source path
 530that contains SP the path must be quoted.
 531
 532A `filecopy` command takes effect immediately.  Once the source
 533location has been copied to the destination any future commands
 534applied to the source location will not impact the destination of
 535the copy.
 536
 537`filerename`
 538^^^^^^^^^^^^
 539Renames an existing file or subdirectory to a different location
 540within the branch.  The existing file or directory must exist. If
 541the destination exists it will be replaced by the source directory.
 542
 543....
 544        'R' SP <path> SP <path> LF
 545....
 546
 547here the first `<path>` is the source location and the second
 548`<path>` is the destination.  See `filemodify` above for a detailed
 549description of what `<path>` may look like.  To use a source path
 550that contains SP the path must be quoted.
 551
 552A `filerename` command takes effect immediately.  Once the source
 553location has been renamed to the destination any future commands
 554applied to the source location will create new files there and not
 555impact the destination of the rename.
 556
 557Note that a `filerename` is the same as a `filecopy` followed by a
 558`filedelete` of the source location.  There is a slight performance
 559advantage to using `filerename`, but the advantage is so small
 560that it is never worth trying to convert a delete/add pair in
 561source material into a rename for fast-import.  This `filerename`
 562command is provided just to simplify frontends that already have
 563rename information and don't want bother with decomposing it into a
 564`filecopy` followed by a `filedelete`.
 565
 566`filedeleteall`
 567^^^^^^^^^^^^^^^
 568Included in a `commit` command to remove all files (and also all
 569directories) from the branch.  This command resets the internal
 570branch structure to have no files in it, allowing the frontend
 571to subsequently add all interesting files from scratch.
 572
 573....
 574        'deleteall' LF
 575....
 576
 577This command is extremely useful if the frontend does not know
 578(or does not care to know) what files are currently on the branch,
 579and therefore cannot generate the proper `filedelete` commands to
 580update the content.
 581
 582Issuing a `filedeleteall` followed by the needed `filemodify`
 583commands to set the correct content will produce the same results
 584as sending only the needed `filemodify` and `filedelete` commands.
 585The `filedeleteall` approach may however require fast-import to use slightly
 586more memory per active branch (less than 1 MiB for even most large
 587projects); so frontends that can easily obtain only the affected
 588paths for a commit are encouraged to do so.
 589
 590`mark`
 591~~~~~~
 592Arranges for fast-import to save a reference to the current object, allowing
 593the frontend to recall this object at a future point in time, without
 594knowing its SHA-1.  Here the current object is the object creation
 595command the `mark` command appears within.  This can be `commit`,
 596`tag`, and `blob`, but `commit` is the most common usage.
 597
 598....
 599        'mark' SP ':' <idnum> LF
 600....
 601
 602where `<idnum>` is the number assigned by the frontend to this mark.
 603The value of `<idnum>` is expressed as an ASCII decimal integer.
 604The value 0 is reserved and cannot be used as
 605a mark.  Only values greater than or equal to 1 may be used as marks.
 606
 607New marks are created automatically.  Existing marks can be moved
 608to another object simply by reusing the same `<idnum>` in another
 609`mark` command.
 610
 611`tag`
 612~~~~~
 613Creates an annotated tag referring to a specific commit.  To create
 614lightweight (non-annotated) tags see the `reset` command below.
 615
 616....
 617        'tag' SP <name> LF
 618        'from' SP <committish> LF
 619        'tagger' SP <name> SP LT <email> GT SP <when> LF
 620        data
 621....
 622
 623where `<name>` is the name of the tag to create.
 624
 625Tag names are automatically prefixed with `refs/tags/` when stored
 626in Git, so importing the CVS branch symbol `RELENG-1_0-FINAL` would
 627use just `RELENG-1_0-FINAL` for `<name>`, and fast-import will write the
 628corresponding ref as `refs/tags/RELENG-1_0-FINAL`.
 629
 630The value of `<name>` must be a valid refname in Git and therefore
 631may contain forward slashes.  As `LF` is not valid in a Git refname,
 632no quoting or escaping syntax is supported here.
 633
 634The `from` command is the same as in the `commit` command; see
 635above for details.
 636
 637The `tagger` command uses the same format as `committer` within
 638`commit`; again see above for details.
 639
 640The `data` command following `tagger` must supply the annotated tag
 641message (see below for `data` command syntax).  To import an empty
 642tag message use a 0 length data.  Tag messages are free-form and are
 643not interpreted by Git.  Currently they must be encoded in UTF-8,
 644as fast-import does not permit other encodings to be specified.
 645
 646Signing annotated tags during import from within fast-import is not
 647supported.  Trying to include your own PGP/GPG signature is not
 648recommended, as the frontend does not (easily) have access to the
 649complete set of bytes which normally goes into such a signature.
 650If signing is required, create lightweight tags from within fast-import with
 651`reset`, then create the annotated versions of those tags offline
 652with the standard linkgit:git-tag[1] process.
 653
 654`reset`
 655~~~~~~~
 656Creates (or recreates) the named branch, optionally starting from
 657a specific revision.  The reset command allows a frontend to issue
 658a new `from` command for an existing branch, or to create a new
 659branch from an existing commit without creating a new commit.
 660
 661....
 662        'reset' SP <ref> LF
 663        ('from' SP <committish> LF)?
 664        LF?
 665....
 666
 667For a detailed description of `<ref>` and `<committish>` see above
 668under `commit` and `from`.
 669
 670The `LF` after the command is optional (it used to be required).
 671
 672The `reset` command can also be used to create lightweight
 673(non-annotated) tags.  For example:
 674
 675====
 676        reset refs/tags/938
 677        from :938
 678====
 679
 680would create the lightweight tag `refs/tags/938` referring to
 681whatever commit mark `:938` references.
 682
 683`blob`
 684~~~~~~
 685Requests writing one file revision to the packfile.  The revision
 686is not connected to any commit; this connection must be formed in
 687a subsequent `commit` command by referencing the blob through an
 688assigned mark.
 689
 690....
 691        'blob' LF
 692        mark?
 693        data
 694....
 695
 696The mark command is optional here as some frontends have chosen
 697to generate the Git SHA-1 for the blob on their own, and feed that
 698directly to `commit`.  This is typically more work than its worth
 699however, as marks are inexpensive to store and easy to use.
 700
 701`data`
 702~~~~~~
 703Supplies raw data (for use as blob/file content, commit messages, or
 704annotated tag messages) to fast-import.  Data can be supplied using an exact
 705byte count or delimited with a terminating line.  Real frontends
 706intended for production-quality conversions should always use the
 707exact byte count format, as it is more robust and performs better.
 708The delimited format is intended primarily for testing fast-import.
 709
 710Comment lines appearing within the `<raw>` part of `data` commands
 711are always taken to be part of the body of the data and are therefore
 712never ignored by fast-import.  This makes it safe to import any
 713file/message content whose lines might start with `#`.
 714
 715Exact byte count format::
 716        The frontend must specify the number of bytes of data.
 717+
 718....
 719        'data' SP <count> LF
 720        <raw> LF?
 721....
 722+
 723where `<count>` is the exact number of bytes appearing within
 724`<raw>`.  The value of `<count>` is expressed as an ASCII decimal
 725integer.  The `LF` on either side of `<raw>` is not
 726included in `<count>` and will not be included in the imported data.
 727+
 728The `LF` after `<raw>` is optional (it used to be required) but
 729recommended.  Always including it makes debugging a fast-import
 730stream easier as the next command always starts in column 0
 731of the next line, even if `<raw>` did not end with an `LF`.
 732
 733Delimited format::
 734        A delimiter string is used to mark the end of the data.
 735        fast-import will compute the length by searching for the delimiter.
 736        This format is primarily useful for testing and is not
 737        recommended for real data.
 738+
 739....
 740        'data' SP '<<' <delim> LF
 741        <raw> LF
 742        <delim> LF
 743        LF?
 744....
 745+
 746where `<delim>` is the chosen delimiter string.  The string `<delim>`
 747must not appear on a line by itself within `<raw>`, as otherwise
 748fast-import will think the data ends earlier than it really does.  The `LF`
 749immediately trailing `<raw>` is part of `<raw>`.  This is one of
 750the limitations of the delimited format, it is impossible to supply
 751a data chunk which does not have an LF as its last byte.
 752+
 753The `LF` after `<delim> LF` is optional (it used to be required).
 754
 755`checkpoint`
 756~~~~~~~~~~~~
 757Forces fast-import to close the current packfile, start a new one, and to
 758save out all current branch refs, tags and marks.
 759
 760....
 761        'checkpoint' LF
 762        LF?
 763....
 764
 765Note that fast-import automatically switches packfiles when the current
 766packfile reaches \--max-pack-size, or 4 GiB, whichever limit is
 767smaller.  During an automatic packfile switch fast-import does not update
 768the branch refs, tags or marks.
 769
 770As a `checkpoint` can require a significant amount of CPU time and
 771disk IO (to compute the overall pack SHA-1 checksum, generate the
 772corresponding index file, and update the refs) it can easily take
 773several minutes for a single `checkpoint` command to complete.
 774
 775Frontends may choose to issue checkpoints during extremely large
 776and long running imports, or when they need to allow another Git
 777process access to a branch.  However given that a 30 GiB Subversion
 778repository can be loaded into Git through fast-import in about 3 hours,
 779explicit checkpointing may not be necessary.
 780
 781The `LF` after the command is optional (it used to be required).
 782
 783`progress`
 784~~~~~~~~~~
 785Causes fast-import to print the entire `progress` line unmodified to
 786its standard output channel (file descriptor 1) when the command is
 787processed from the input stream.  The command otherwise has no impact
 788on the current import, or on any of fast-import's internal state.
 789
 790....
 791        'progress' SP <any> LF
 792        LF?
 793....
 794
 795The `<any>` part of the command may contain any sequence of bytes
 796that does not contain `LF`.  The `LF` after the command is optional.
 797Callers may wish to process the output through a tool such as sed to
 798remove the leading part of the line, for example:
 799
 800====
 801        frontend | git-fast-import | sed 's/^progress //'
 802====
 803
 804Placing a `progress` command immediately after a `checkpoint` will
 805inform the reader when the `checkpoint` has been completed and it
 806can safely access the refs that fast-import updated.
 807
 808Tips and Tricks
 809---------------
 810The following tips and tricks have been collected from various
 811users of fast-import, and are offered here as suggestions.
 812
 813Use One Mark Per Commit
 814~~~~~~~~~~~~~~~~~~~~~~~
 815When doing a repository conversion, use a unique mark per commit
 816(`mark :<n>`) and supply the \--export-marks option on the command
 817line.  fast-import will dump a file which lists every mark and the Git
 818object SHA-1 that corresponds to it.  If the frontend can tie
 819the marks back to the source repository, it is easy to verify the
 820accuracy and completeness of the import by comparing each Git
 821commit to the corresponding source revision.
 822
 823Coming from a system such as Perforce or Subversion this should be
 824quite simple, as the fast-import mark can also be the Perforce changeset
 825number or the Subversion revision number.
 826
 827Freely Skip Around Branches
 828~~~~~~~~~~~~~~~~~~~~~~~~~~~
 829Don't bother trying to optimize the frontend to stick to one branch
 830at a time during an import.  Although doing so might be slightly
 831faster for fast-import, it tends to increase the complexity of the frontend
 832code considerably.
 833
 834The branch LRU builtin to fast-import tends to behave very well, and the
 835cost of activating an inactive branch is so low that bouncing around
 836between branches has virtually no impact on import performance.
 837
 838Handling Renames
 839~~~~~~~~~~~~~~~~
 840When importing a renamed file or directory, simply delete the old
 841name(s) and modify the new name(s) during the corresponding commit.
 842Git performs rename detection after-the-fact, rather than explicitly
 843during a commit.
 844
 845Use Tag Fixup Branches
 846~~~~~~~~~~~~~~~~~~~~~~
 847Some other SCM systems let the user create a tag from multiple
 848files which are not from the same commit/changeset.  Or to create
 849tags which are a subset of the files available in the repository.
 850
 851Importing these tags as-is in Git is impossible without making at
 852least one commit which ``fixes up'' the files to match the content
 853of the tag.  Use fast-import's `reset` command to reset a dummy branch
 854outside of your normal branch space to the base commit for the tag,
 855then commit one or more file fixup commits, and finally tag the
 856dummy branch.
 857
 858For example since all normal branches are stored under `refs/heads/`
 859name the tag fixup branch `TAG_FIXUP`.  This way it is impossible for
 860the fixup branch used by the importer to have namespace conflicts
 861with real branches imported from the source (the name `TAG_FIXUP`
 862is not `refs/heads/TAG_FIXUP`).
 863
 864When committing fixups, consider using `merge` to connect the
 865commit(s) which are supplying file revisions to the fixup branch.
 866Doing so will allow tools such as linkgit:git-blame[1] to track
 867through the real commit history and properly annotate the source
 868files.
 869
 870After fast-import terminates the frontend will need to do `rm .git/TAG_FIXUP`
 871to remove the dummy branch.
 872
 873Import Now, Repack Later
 874~~~~~~~~~~~~~~~~~~~~~~~~
 875As soon as fast-import completes the Git repository is completely valid
 876and ready for use.  Typically this takes only a very short time,
 877even for considerably large projects (100,000+ commits).
 878
 879However repacking the repository is necessary to improve data
 880locality and access performance.  It can also take hours on extremely
 881large projects (especially if -f and a large \--window parameter is
 882used).  Since repacking is safe to run alongside readers and writers,
 883run the repack in the background and let it finish when it finishes.
 884There is no reason to wait to explore your new Git project!
 885
 886If you choose to wait for the repack, don't try to run benchmarks
 887or performance tests until repacking is completed.  fast-import outputs
 888suboptimal packfiles that are simply never seen in real use
 889situations.
 890
 891Repacking Historical Data
 892~~~~~~~~~~~~~~~~~~~~~~~~~
 893If you are repacking very old imported data (e.g. older than the
 894last year), consider expending some extra CPU time and supplying
 895\--window=50 (or higher) when you run linkgit:git-repack[1].
 896This will take longer, but will also produce a smaller packfile.
 897You only need to expend the effort once, and everyone using your
 898project will benefit from the smaller repository.
 899
 900Include Some Progress Messages
 901~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 902Every once in a while have your frontend emit a `progress` message
 903to fast-import.  The contents of the messages are entirely free-form,
 904so one suggestion would be to output the current month and year
 905each time the current commit date moves into the next month.
 906Your users will feel better knowing how much of the data stream
 907has been processed.
 908
 909
 910Packfile Optimization
 911---------------------
 912When packing a blob fast-import always attempts to deltify against the last
 913blob written.  Unless specifically arranged for by the frontend,
 914this will probably not be a prior version of the same file, so the
 915generated delta will not be the smallest possible.  The resulting
 916packfile will be compressed, but will not be optimal.
 917
 918Frontends which have efficient access to all revisions of a
 919single file (for example reading an RCS/CVS ,v file) can choose
 920to supply all revisions of that file as a sequence of consecutive
 921`blob` commands.  This allows fast-import to deltify the different file
 922revisions against each other, saving space in the final packfile.
 923Marks can be used to later identify individual file revisions during
 924a sequence of `commit` commands.
 925
 926The packfile(s) created by fast-import do not encourage good disk access
 927patterns.  This is caused by fast-import writing the data in the order
 928it is received on standard input, while Git typically organizes
 929data within packfiles to make the most recent (current tip) data
 930appear before historical data.  Git also clusters commits together,
 931speeding up revision traversal through better cache locality.
 932
 933For this reason it is strongly recommended that users repack the
 934repository with `git repack -a -d` after fast-import completes, allowing
 935Git to reorganize the packfiles for faster data access.  If blob
 936deltas are suboptimal (see above) then also adding the `-f` option
 937to force recomputation of all deltas can significantly reduce the
 938final packfile size (30-50% smaller can be quite typical).
 939
 940
 941Memory Utilization
 942------------------
 943There are a number of factors which affect how much memory fast-import
 944requires to perform an import.  Like critical sections of core
 945Git, fast-import uses its own memory allocators to amortize any overheads
 946associated with malloc.  In practice fast-import tends to amortize any
 947malloc overheads to 0, due to its use of large block allocations.
 948
 949per object
 950~~~~~~~~~~
 951fast-import maintains an in-memory structure for every object written in
 952this execution.  On a 32 bit system the structure is 32 bytes,
 953on a 64 bit system the structure is 40 bytes (due to the larger
 954pointer sizes).  Objects in the table are not deallocated until
 955fast-import terminates.  Importing 2 million objects on a 32 bit system
 956will require approximately 64 MiB of memory.
 957
 958The object table is actually a hashtable keyed on the object name
 959(the unique SHA-1).  This storage configuration allows fast-import to reuse
 960an existing or already written object and avoid writing duplicates
 961to the output packfile.  Duplicate blobs are surprisingly common
 962in an import, typically due to branch merges in the source.
 963
 964per mark
 965~~~~~~~~
 966Marks are stored in a sparse array, using 1 pointer (4 bytes or 8
 967bytes, depending on pointer size) per mark.  Although the array
 968is sparse, frontends are still strongly encouraged to use marks
 969between 1 and n, where n is the total number of marks required for
 970this import.
 971
 972per branch
 973~~~~~~~~~~
 974Branches are classified as active and inactive.  The memory usage
 975of the two classes is significantly different.
 976
 977Inactive branches are stored in a structure which uses 96 or 120
 978bytes (32 bit or 64 bit systems, respectively), plus the length of
 979the branch name (typically under 200 bytes), per branch.  fast-import will
 980easily handle as many as 10,000 inactive branches in under 2 MiB
 981of memory.
 982
 983Active branches have the same overhead as inactive branches, but
 984also contain copies of every tree that has been recently modified on
 985that branch.  If subtree `include` has not been modified since the
 986branch became active, its contents will not be loaded into memory,
 987but if subtree `src` has been modified by a commit since the branch
 988became active, then its contents will be loaded in memory.
 989
 990As active branches store metadata about the files contained on that
 991branch, their in-memory storage size can grow to a considerable size
 992(see below).
 993
 994fast-import automatically moves active branches to inactive status based on
 995a simple least-recently-used algorithm.  The LRU chain is updated on
 996each `commit` command.  The maximum number of active branches can be
 997increased or decreased on the command line with \--active-branches=.
 998
 999per active tree
1000~~~~~~~~~~~~~~~
1001Trees (aka directories) use just 12 bytes of memory on top of the
1002memory required for their entries (see ``per active file'' below).
1003The cost of a tree is virtually 0, as its overhead amortizes out
1004over the individual file entries.
1005
1006per active file entry
1007~~~~~~~~~~~~~~~~~~~~~
1008Files (and pointers to subtrees) within active trees require 52 or 64
1009bytes (32/64 bit platforms) per entry.  To conserve space, file and
1010tree names are pooled in a common string table, allowing the filename
1011``Makefile'' to use just 16 bytes (after including the string header
1012overhead) no matter how many times it occurs within the project.
1013
1014The active branch LRU, when coupled with the filename string pool
1015and lazy loading of subtrees, allows fast-import to efficiently import
1016projects with 2,000+ branches and 45,114+ files in a very limited
1017memory footprint (less than 2.7 MiB per active branch).
1018
1019
1020Author
1021------
1022Written by Shawn O. Pearce <spearce@spearce.org>.
1023
1024Documentation
1025--------------
1026Documentation by Shawn O. Pearce <spearce@spearce.org>.
1027
1028GIT
1029---
1030Part of the linkgit:git[7] suite