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1 Git Sparse-Index Design Document
2 ================================
4 The sparse-checkout feature allows users to focus a working directory on
5 a subset of the files at HEAD. The cone mode patterns, enabled by
6 `core.sparseCheckoutCone`, allow for very fast pattern matching to
7 discover which files at HEAD belong in the sparse-checkout cone.
9 Three important scale dimensions for a Git working directory are:
11 * `HEAD`: How many files are present at `HEAD`?
13 * Populated: How many files are within the sparse-checkout cone.
15 * Modified: How many files has the user modified in the working directory?
17 We will use big-O notation -- O(X) -- to denote how expensive certain
18 operations are in terms of these dimensions.
20 These dimensions are ordered by their magnitude: users (typically) modify
21 fewer files than are populated, and we can only populate files at `HEAD`.
23 Problems occur if there is an extreme imbalance in these dimensions. For
24 example, if `HEAD` contains millions of paths but the populated set has
25 only tens of thousands, then commands like `git status` and `git add` can
26 be dominated by operations that require O(`HEAD`) operations instead of
27 O(Populated). Primarily, the cost is in parsing and rewriting the index,
28 which is filled primarily with files at `HEAD` that are marked with the
29 `SKIP_WORKTREE` bit.
31 The sparse-index intends to take these commands that read and modify the
32 index from O(`HEAD`) to O(Populated). To do this, we need to modify the
33 index format in a significant way: add "sparse directory" entries.
35 With cone mode patterns, it is possible to detect when an entire
36 directory will have its contents outside of the sparse-checkout definition.
37 Instead of listing all of the files it contains as individual entries, a
38 sparse-index contains an entry with the directory name, referencing the
39 object ID of the tree at `HEAD` and marked with the `SKIP_WORKTREE` bit.
40 If we need to discover the details for paths within that directory, we
41 can parse trees to find that list.
43 At time of writing, sparse-directory entries violate expectations about the
44 index format and its in-memory data structure. There are many consumers in
45 the codebase that expect to iterate through all of the index entries and
46 see only files. In fact, these loops expect to see a reference to every
47 staged file. One way to handle this is to parse trees to replace a
48 sparse-directory entry with all of the files within that tree as the index
49 is loaded. However, parsing trees is slower than parsing the index format,
50 so that is a slower operation than if we left the index alone. The plan is
51 to make all of these integrations "sparse aware" so this expansion through
52 tree parsing is unnecessary and they use fewer resources than when using a
53 full index.
55 The implementation plan below follows four phases to slowly integrate with
56 the sparse-index. The intention is to incrementally update Git commands to
57 interact safely with the sparse-index without significant slowdowns. This
58 may not always be possible, but the hope is that the primary commands that
59 users need in their daily work are dramatically improved.
61 Phase I: Format and initial speedups
62 ------------------------------------
64 During this phase, Git learns to enable the sparse-index and safely parse
65 one. Protections are put in place so that every consumer of the in-memory
66 data structure can operate with its current assumption of every file at
67 `HEAD`.
69 At first, every index parse will call a helper method,
70 `ensure_full_index()`, which scans the index for sparse-directory entries
71 (pointing to trees) and replaces them with the full list of paths (with
72 blob contents) by parsing tree objects. This will be slower in all cases.
73 The only noticeable change in behavior will be that the serialized index
74 file contains sparse-directory entries.
76 To start, we use a new required index extension, `sdir`, to allow
77 inserting sparse-directory entries into indexes with file format
78 versions 2, 3, and 4. This prevents Git versions that do not understand
79 the sparse-index from operating on one, while allowing tools that do not
80 understand the sparse-index to operate on repositories as long as they do
81 not interact with the index. A new format, index v5, will be introduced
82 that includes sparse-directory entries by default. It might also
83 introduce other features that have been considered for improving the
84 index, as well.
86 Next, consumers of the index will be guarded against operating on a
87 sparse-index by inserting calls to `ensure_full_index()` or
88 `expand_index_to_path()`. If a specific path is requested, then those will
89 be protected from within the `index_file_exists()` and `index_name_pos()`
90 API calls: they will call `ensure_full_index()` if necessary. The
91 intention here is to preserve existing behavior when interacting with a
92 sparse-checkout. We don't want a change to happen by accident, without
93 tests. Many of these locations may not need any change before removing the
94 guards, but we should not do so without tests to ensure the expected
95 behavior happens.
97 It may be desirable to _change_ the behavior of some commands in the
98 presence of a sparse index or more generally in any sparse-checkout
99 scenario. In such cases, these should be carefully communicated and
100 tested. No such behavior changes are intended during this phase.
102 During a scan of the codebase, not every iteration of the cache entries
103 needs an `ensure_full_index()` check. The basic reasons include:
105 1. The loop is scanning for entries with non-zero stage. These entries
106 are not collapsed into a sparse-directory entry.
108 2. The loop is scanning for submodules. These entries are not collapsed
109 into a sparse-directory entry.
111 3. The loop is part of the index API, especially around reading or
112 writing the format.
114 4. The loop is checking for correct order of cache entries and that is
115 correct if and only if the sparse-directory entries are in the correct
116 location.
118 5. The loop ignores entries with the `SKIP_WORKTREE` bit set, or is
119 otherwise already aware of sparse directory entries.
121 6. The sparse-index is disabled at this point when using the split-index
122 feature, so no effort is made to protect the split-index API.
124 Even after inserting these guards, we will keep expanding sparse-indexes
125 for most Git commands using the `command_requires_full_index` repository
126 setting. This setting will be on by default and disabled one builtin at a
127 time until we have sufficient confidence that all of the index operations
128 are properly guarded.
130 To complete this phase, the commands `git status` and `git add` will be
131 integrated with the sparse-index so that they operate with O(Populated)
132 performance. They will be carefully tested for operations within and
133 outside the sparse-checkout definition.
135 Phase II: Careful integrations
136 ------------------------------
138 This phase focuses on ensuring that all index extensions and APIs work
139 well with a sparse-index. This requires significant increases to our test
140 coverage, especially for operations that interact with the working
141 directory outside of the sparse-checkout definition. Some of these
142 behaviors may not be the desirable ones, such as some tests already
143 marked for failure in `t1092-sparse-checkout-compatibility.sh`.
145 The index extensions that may require special integrations are:
147 * FS Monitor
148 * Untracked cache
150 While integrating with these features, we should look for patterns that
151 might lead to better APIs for interacting with the index. Coalescing
152 common usage patterns into an API call can reduce the number of places
153 where sparse-directories need to be handled carefully.
155 Phase III: Important command speedups
156 -------------------------------------
158 At this point, the patterns for testing and implementing sparse-directory
159 logic should be relatively stable. This phase focuses on updating some of
160 the most common builtins that use the index to operate as O(Populated).
161 Here is a potential list of commands that could be valuable to integrate
162 at this point:
164 * `git commit`
165 * `git checkout`
166 * `git merge`
167 * `git rebase`
169 Hopefully, commands such as `git merge` and `git rebase` can benefit
170 instead from merge algorithms that do not use the index as a data
171 structure, such as the merge-ORT strategy. As these topics mature, we
172 may enable the ORT strategy by default for repositories using the
173 sparse-index feature.
175 Along with `git status` and `git add`, these commands cover the majority
176 of users' interactions with the working directory. In addition, we can
177 integrate with these commands:
179 * `git grep`
180 * `git rm`
182 These have been proposed as some whose behavior could change when in a
183 repo with a sparse-checkout definition. It would be good to include this
184 behavior automatically when using a sparse-index. Some clarity is needed
185 to make the behavior switch clear to the user.
187 This phase is the first where parallel work might be possible without too
188 much conflicts between topics.
190 Phase IV: The long tail
191 -----------------------
193 This last phase is less a "phase" and more "the new normal" after all of
194 the previous work.
196 To start, the `command_requires_full_index` option could be removed in
197 favor of expanding only when hitting an API guard.
199 There are many Git commands that could use special attention to operate as
200 O(Populated), while some might be so rare that it is acceptable to leave
201 them with additional overhead when a sparse-index is present.
203 Here are some commands that might be useful to update:
205 * `git sparse-checkout set`
206 * `git am`
207 * `git clean`
208 * `git stash`