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diff --git a/Documentation/git-fast-import.txt b/Documentation/git-fast-import.txt new file mode 100644 index 0000000000..445f6b8544 --- /dev/null +++ b/Documentation/git-fast-import.txt @@ -0,0 +1,901 @@ +git-fast-import(1) +================== + +NAME +---- +git-fast-import - Backend for fast Git data importers. + + +SYNOPSIS +-------- +frontend | 'git-fast-import' [options] + +DESCRIPTION +----------- +This program is usually not what the end user wants to run directly. +Most end users want to use one of the existing frontend programs, +which parses a specific type of foreign source and feeds the contents +stored there to git-fast-import. + +fast-import reads a mixed command/data stream from standard input and +writes one or more packfiles directly into the current repository. +When EOF is received on standard input, fast import writes out +updated branch and tag refs, fully updating the current repository +with the newly imported data. + +The fast-import backend itself can import into an empty repository (one that +has already been initialized by gitlink:git-init[1]) 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 +------- +--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, expressed in MiB. + The default is 4096 (4 GiB) as that is the maximum allowed + packfile size (due to file format limitations). Some + importers may wish to lower this, such as to ensure the + resulting packfiles fit on CDs. + +--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. + +--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 gitlink:git-pack-objects[1]. + +--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. + +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 its 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. +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. + +Date Formats +~~~~~~~~~~~~ +The following date formats are supported. A frontend should select +the format it will use for this import by passing the format name +in the \--date-format=<fmt> command line option. + +`raw`:: + This is the Git native format and is `<time> SP <offutc>`. + It is also fast-import's default format, if \--date-format was + not specified. ++ +The time of the event is specified by `<time>` as the number of +seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is +written as an ASCII decimal integer. ++ +The local offset is specified by `<offutc>` as a positive or negative +offset from UTC. For example EST (which is 5 hours behind UTC) +would be expressed in `<tz>` by ``-0500'' while UTC is ``+0000''. +The local offset does not affect `<time>`; it is used only as an +advisement to help formatting routines display the timestamp. ++ +If the local offset is not available in the source material, use +``+0000'', or the most common local offset. For example many +organizations have a CVS repository which has only ever been accessed +by users who are located in the same location and timezone. In this +case a reasonable offset from UTC could be assumed. ++ +Unlike the `rfc2822` format, this format is very strict. Any +variation in formatting will cause fast-import to reject the value. + +`rfc2822`:: + This is the standard email format as described by RFC 2822. ++ +An example value is ``Tue Feb 6 11:22:18 2007 -0500''. The Git +parser is accurate, but a little on the lenient side. It is the +same parser used by gitlink:git-am[1] 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 easiliy 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 its 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 +gitlink:git-update-index[1]. ++ +If separate `author` and `committer` commands are used in a `commit` +the timestamps may not match, as the system clock will be polled +twice (once for each command). The only way to ensure that both +author and committer identity information has the same timestamp +is to omit `author` (thus copying from `committer`) or to use a +date format other than `now`. + +Commands +~~~~~~~~ +fast-import accepts several commands to update the current repository +and control the current import process. More detailed discussion +(with examples) of each command follows later. + +`commit`:: + Creates a new branch or updates an existing branch by + creating a new commit and updating the branch to point at + the newly created commit. + +`tag`:: + Creates an annotated tag object from an existing commit or + branch. Lightweight tags are not supported by this command, + as they are not recommended for recording meaningful points + in time. + +`reset`:: + Reset an existing branch (or a new branch) to a specific + revision. This command must be used to change a branch to + a specific revision without making a commit on it. + +`blob`:: + Convert raw file data into a blob, for future use in a + `commit` command. This command is optional and is not + needed to perform an import. + +`checkpoint`:: + Forces fast-import to close the current packfile, generate its + unique SHA-1 checksum and index, and start a new packfile. + This command is optional and is not needed to perform + an import. + +`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 | filedeleteall)* + LF +.... + +where `<ref>` is the name of the branch to make the commit on. +Typically branch names are prefixed with `refs/heads/` in +Git, so importing the CVS branch symbol `RELENG-1_0` would use +`refs/heads/RELENG-1_0` for the value of `<ref>`. The value of +`<ref>` must be a valid refname in Git. As `LF` is not valid in +a Git refname, no quoting or escaping syntax is supported here. + +A `mark` command may optionally appear, requesting fast-import to save a +reference to the newly created commit for future use by the frontend +(see below for format). It is very common for frontends to mark +every commit they create, thereby allowing future branch creation +from any imported commit. + +The `data` command following `committer` must supply the commit +message (see below for `data` command syntax). To import an empty +commit message use a 0 length data. Commit messages are free-form +and are not interpreted by Git. Currently they must be encoded in +UTF-8, as fast-import does not permit other encodings to be specified. + +Zero or more `filemodify`, `filedelete` and `filedeleteall` commands +may be included to update the contents of the branch prior to +creating the commit. These commands may be supplied in any order. +However it is recommended that a `filedeleteall` command preceed +all `filemodify` commands in the same commit, as `filedeleteall` +wipes the branch clean (see below). + +`author` +^^^^^^^^ +An `author` command may optionally appear, if the author information +might differ from the committer information. If `author` is omitted +then fast-import will automatically use the committer's information for +the author portion of the commit. See below for a description of +the fields in `author`, as they are identical to `committer`. + +`committer` +^^^^^^^^^^^ +The `committer` command indicates who made this commit, and when +they made it. + +Here `<name>` is the person's display name (for example +``Com M Itter'') and `<email>` is the person's email address +(``cm@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>` is free-form and may contain any sequence of bytes, except +`LT` and `LF`. It is typically UTF-8 encoded. + +The time of the change is specified by `<when>` using the date format +that was selected by the \--date-format=<fmt> command line option. +See ``Date Formats'' above for the set of supported formats, and +their syntax. + +`from` +^^^^^^ +The `from` command is used to specify the commit to initialize +this branch from. This revision will be the first ancestor of the +new commit. + +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. +Omitting the `from` command on existing branches is usually desired, +as the current commit on that branch is automatically assumed to +be the first ancestor of the new commit. + +As `LF` is not valid in a Git refname or SHA-1 expression, no +quoting or escaping syntax is supported within `<committish>`. + +Here `<committish>` is any of the following: + +* The name of an existing branch already in fast-import's internal branch + table. If fast-import doesn't know the name, its 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 distingush 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 gitlink:git-rev-parse[1] for details. + +The special case of restarting an incremental import from the +current branch value should be written as: +---- + from refs/heads/branch^0 +---- +The `{caret}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 `{caret}0` will force +fast-import to resolve the commit through Git's revision parsing library, +rather than its internal branch table, thereby loading in the +existing value of the branch. + +`merge` +^^^^^^^ +Includes one additional ancestor commit, and makes the current +commit a merge commit. An unlimited number of `merge` commands per +commit are permitted by fast-import, thereby establishing an n-way merge. +However Git's other tools never create commits with more than 15 +additional ancestors (forming a 16-way merge). For this reason +it is suggested that frontends do not use more than 15 `merge` +commands per commit. + +Here `<committish>` is any of the commit specification expressions +also accepted by `from` (see above). + +`filemodify` +^^^^^^^^^^^^ +Included in a `commit` command to add a new file or change the +content of an existing file. This command has two different means +of specifying the content of the file. + +External data format:: + The data content for the file was already supplied by a prior + `blob` command. The frontend just needs to connect it. ++ +.... + 'M' SP <mode> SP <dataref> SP <path> LF +.... ++ +Here `<dataref>` can be either a mark reference (`:<idnum>`) +set by a prior `blob` command, or a full 40-byte SHA-1 of an +existing Git blob object. + +Inline data format:: + The data content for the file has not been supplied yet. + The frontend wants to supply it as part of this modify + command. ++ +.... + 'M' SP <mode> SP 'inline' SP <path> LF + data +.... ++ +See below for a detailed description of the `data` command. + +In both formats `<mode>` is the type of file entry, specified +in octal. Git only supports the following modes: + +* `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. + +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 seperators (forward +slash `/`), may contain any byte other than `LF`, and must not +start with double quote (`"`). + +If an `LF` or double quote must be encoded into `<path>` shell-style +quoting should be used, e.g. `"path/with\n and \" in it"`. + +The value of `<path>` must be in canoncial form. That is it must not: + +* contain an empty directory component (e.g. `foo//bar` is invalid), +* end with a directory seperator (e.g. `foo/` is invalid), +* start with a directory seperator (e.g. `/foo` is invalid), +* contain the special component `.` or `..` (e.g. `foo/./bar` and + `foo/../bar` are invalid). + +It is recommended that `<path>` always be encoded using UTF-8. + +`filedelete` +^^^^^^^^^^^^ +Included in a `commit` command to remove a file from the branch. +If the file removal makes its directory empty, the directory will +be automatically removed too. This cascades up the tree until the +first non-empty directory or the root is reached. + +.... + 'D' SP <path> LF +.... + +here `<path>` is the complete path of the file to be removed. +See `filemodify` above for a detailed description of `<path>`. + +`filedeleteall` +^^^^^^^^^^^^^^^ +Included in a `commit` command to remove all files (and also all +directories) from the branch. This command resets the internal +branch structure to have no files in it, allowing the frontend +to subsequently add all interesting files from scratch. + +.... + 'deleteall' LF +.... + +This command is extremely useful if the frontend does not know +(or does not care to know) what files are currently on the branch, +and therefore cannot generate the proper `filedelete` commands to +update the content. + +Issuing a `filedeleteall` followed by the needed `filemodify` +commands to set the correct content will produce the same results +as sending only the needed `filemodify` and `filedelete` commands. +The `filedeleteall` approach may however require fast-import to use slightly +more memory per active branch (less than 1 MiB for even most large +projects); so frontends that can easily obtain only the affected +paths for a commit are encouraged to do so. + +`mark` +~~~~~~ +Arranges for fast-import to save a reference to the current object, allowing +the frontend to recall this object at a future point in time, without +knowing its SHA-1. Here the current object is the object creation +command the `mark` command appears within. This can be `commit`, +`tag`, and `blob`, but `commit` is the most common usage. + +.... + 'mark' SP ':' <idnum> LF +.... + +where `<idnum>` is the number assigned by the frontend to this mark. +The value of `<idnum>` is expressed as an ASCII decimal integer. +The value 0 is reserved and cannot be used as +a mark. Only values greater than or equal to 1 may be used as marks. + +New marks are created automatically. Existing marks can be moved +to another object simply by reusing the same `<idnum>` in another +`mark` command. + +`tag` +~~~~~ +Creates an annotated tag referring to a specific commit. To create +lightweight (non-annotated) tags see the `reset` command below. + +.... + 'tag' SP <name> LF + 'from' SP <committish> LF + 'tagger' SP <name> SP LT <email> GT SP <when> LF + data + LF +.... + +where `<name>` is the name of the tag to create. + +Tag names are automatically prefixed with `refs/tags/` when stored +in Git, so importing the CVS branch symbol `RELENG-1_0-FINAL` would +use just `RELENG-1_0-FINAL` for `<name>`, and fast-import will write the +corresponding ref as `refs/tags/RELENG-1_0-FINAL`. + +The value of `<name>` must be a valid refname in Git and therefore +may contain forward slashes. As `LF` is not valid in a Git refname, +no quoting or escaping syntax is supported here. + +The `from` command is the same as in the `commit` command; see +above for details. + +The `tagger` command uses the same format as `committer` within +`commit`; again see above for details. + +The `data` command following `tagger` must supply the annotated tag +message (see below for `data` command syntax). To import an empty +tag message use a 0 length data. Tag messages are free-form and are +not interpreted by Git. Currently they must be encoded in UTF-8, +as fast-import does not permit other encodings to be specified. + +Signing annotated tags during import from within fast-import is not +supported. Trying to include your own PGP/GPG signature is not +recommended, as the frontend does not (easily) have access to the +complete set of bytes which normally goes into such a signature. +If signing is required, create lightweight tags from within fast-import with +`reset`, then create the annotated versions of those tags offline +with the standard gitlink:git-tag[1] process. + +`reset` +~~~~~~~ +Creates (or recreates) the named branch, optionally starting from +a specific revision. The reset command allows a frontend to issue +a new `from` command for an existing branch, or to create a new +branch from an existing commit without creating a new commit. + +.... + 'reset' SP <ref> LF + ('from' SP <committish> LF)? + LF +.... + +For a detailed description of `<ref>` and `<committish>` see above +under `commit` and `from`. + +The `reset` command can also be used to create lightweight +(non-annotated) tags. For example: + +==== + reset refs/tags/938 + from :938 +==== + +would create the lightweight tag `refs/tags/938` referring to +whatever commit mark `:938` references. + +`blob` +~~~~~~ +Requests writing one file revision to the packfile. The revision +is not connected to any commit; this connection must be formed in +a subsequent `commit` command by referencing the blob through an +assigned mark. + +.... + 'blob' LF + mark? + data +.... + +The mark command is optional here as some frontends have chosen +to generate the Git SHA-1 for the blob on their own, and feed that +directly to `commit`. This is typically more work than its worth +however, as marks are inexpensive to store and easy to use. + +`data` +~~~~~~ +Supplies raw data (for use as blob/file content, commit messages, or +annotated tag messages) to fast-import. Data can be supplied using an exact +byte count or delimited with a terminating line. Real frontends +intended for production-quality conversions should always use the +exact byte count format, as it is more robust and performs better. +The delimited format is intended primarily for testing fast-import. + +Exact byte count format:: + The frontend must specify the number of bytes of data. ++ +.... + 'data' SP <count> LF + <raw> LF +.... ++ +where `<count>` is the exact number of bytes appearing within +`<raw>`. The value of `<count>` is expressed as an ASCII decimal +integer. The `LF` on either side of `<raw>` is not +included in `<count>` and will not be included in the imported data. + +Delimited format:: + A delimiter string is used to mark the end of the data. + fast-import will compute the length by searching for the delimiter. + This format is primarly useful for testing and is not + recommended for real data. ++ +.... + 'data' SP '<<' <delim> LF + <raw> LF + <delim> LF +.... ++ +where `<delim>` is the chosen delimiter string. The string `<delim>` +must not appear on a line by itself within `<raw>`, as otherwise +fast-import will think the data ends earlier than it really does. The `LF` +immediately trailing `<raw>` is part of `<raw>`. This is one of +the limitations of the delimited format, it is impossible to supply +a data chunk which does not have an LF as its last byte. + +`checkpoint` +~~~~~~~~~~~~ +Forces fast-import to close the current packfile, start a new one, and to +save out all current branch refs, tags and marks. + +.... + 'checkpoint' LF + LF +.... + +Note that fast-import automatically switches packfiles when the current +packfile reaches \--max-pack-size, or 4 GiB, whichever limit is +smaller. During an automatic packfile switch fast-import does not update +the branch refs, tags or marks. + +As a `checkpoint` can require a significant amount of CPU time and +disk IO (to compute the overall pack SHA-1 checksum, generate the +corresponding index file, and update the refs) it can easily take +several minutes for a single `checkpoint` command to complete. + +Frontends may choose to issue checkpoints during extremely large +and long running imports, or when they need to allow another Git +process access to a branch. However given that a 30 GiB Subversion +repository can be loaded into Git through fast-import in about 3 hours, +explicit checkpointing may not be necessary. + + +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 gitlink:git-blame[1] 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. Typicallly 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 gitlink:git-repack[1]. +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. + + +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 ammortize any overheads +associated with malloc. In practice fast-import tends to ammoritize 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 ammortizes 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). + + +Author +------ +Written by Shawn O. Pearce <spearce@spearce.org>. + +Documentation +-------------- +Documentation by Shawn O. Pearce <spearce@spearce.org>. + +GIT +--- +Part of the gitlink:git[7] suite + |