diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/config.txt | 19 | ||||
| -rw-r--r-- | Documentation/git-pack-objects.txt | 97 | ||||
| -rw-r--r-- | Documentation/git-repack.txt | 5 |
3 files changed, 121 insertions, 0 deletions
diff --git a/Documentation/config.txt b/Documentation/config.txt index 6ecd70df0a..112041f407 100644 --- a/Documentation/config.txt +++ b/Documentation/config.txt @@ -2684,6 +2684,21 @@ Note that changing the compression level will not automatically recompress all existing objects. You can force recompression by passing the -F option to linkgit:git-repack[1]. +pack.island:: + An extended regular expression configuring a set of delta + islands. See "DELTA ISLANDS" in linkgit:git-pack-objects[1] + for details. + +pack.islandCore:: + Specify an island name which gets to have its objects be + packed first. This creates a kind of pseudo-pack at the front + of one pack, so that the objects from the specified island are + hopefully faster to copy into any pack that should be served + to a user requesting these objects. In practice this means + that the island specified should likely correspond to what is + the most commonly cloned in the repo. See also "DELTA ISLANDS" + in linkgit:git-pack-objects[1]. + pack.deltaCacheSize:: The maximum memory in bytes used for caching deltas in linkgit:git-pack-objects[1] before writing them out to a pack. @@ -3218,6 +3233,10 @@ repack.packKeptObjects:: index is being written (either via `--write-bitmap-index` or `repack.writeBitmaps`). +repack.useDeltaIslands:: + If set to true, makes `git repack` act as if `--delta-islands` + was passed. Defaults to `false`. + repack.writeBitmaps:: When true, git will write a bitmap index when packing all objects to disk (e.g., when `git repack -a` is run). This diff --git a/Documentation/git-pack-objects.txt b/Documentation/git-pack-objects.txt index d95b472d16..40c825c381 100644 --- a/Documentation/git-pack-objects.txt +++ b/Documentation/git-pack-objects.txt @@ -289,6 +289,103 @@ Unexpected missing object will raise an error. --unpack-unreachable:: Keep unreachable objects in loose form. This implies `--revs`. +--delta-islands:: + Restrict delta matches based on "islands". See DELTA ISLANDS + below. + + +DELTA ISLANDS +------------- + +When possible, `pack-objects` tries to reuse existing on-disk deltas to +avoid having to search for new ones on the fly. This is an important +optimization for serving fetches, because it means the server can avoid +inflating most objects at all and just send the bytes directly from +disk. This optimization can't work when an object is stored as a delta +against a base which the receiver does not have (and which we are not +already sending). In that case the server "breaks" the delta and has to +find a new one, which has a high CPU cost. Therefore it's important for +performance that the set of objects in on-disk delta relationships match +what a client would fetch. + +In a normal repository, this tends to work automatically. The objects +are mostly reachable from the branches and tags, and that's what clients +fetch. Any deltas we find on the server are likely to be between objects +the client has or will have. + +But in some repository setups, you may have several related but separate +groups of ref tips, with clients tending to fetch those groups +independently. For example, imagine that you are hosting several "forks" +of a repository in a single shared object store, and letting clients +view them as separate repositories through `GIT_NAMESPACE` or separate +repos using the alternates mechanism. A naive repack may find that the +optimal delta for an object is against a base that is only found in +another fork. But when a client fetches, they will not have the base +object, and we'll have to find a new delta on the fly. + +A similar situation may exist if you have many refs outside of +`refs/heads/` and `refs/tags/` that point to related objects (e.g., +`refs/pull` or `refs/changes` used by some hosting providers). By +default, clients fetch only heads and tags, and deltas against objects +found only in those other groups cannot be sent as-is. + +Delta islands solve this problem by allowing you to group your refs into +distinct "islands". Pack-objects computes which objects are reachable +from which islands, and refuses to make a delta from an object `A` +against a base which is not present in all of `A`'s islands. This +results in slightly larger packs (because we miss some delta +opportunities), but guarantees that a fetch of one island will not have +to recompute deltas on the fly due to crossing island boundaries. + +When repacking with delta islands the delta window tends to get +clogged with candidates that are forbidden by the config. Repacking +with a big --window helps (and doesn't take as long as it otherwise +might because we can reject some object pairs based on islands before +doing any computation on the content). + +Islands are configured via the `pack.island` option, which can be +specified multiple times. Each value is a left-anchored regular +expressions matching refnames. For example: + +------------------------------------------- +[pack] +island = refs/heads/ +island = refs/tags/ +------------------------------------------- + +puts heads and tags into an island (whose name is the empty string; see +below for more on naming). Any refs which do not match those regular +expressions (e.g., `refs/pull/123`) is not in any island. Any object +which is reachable only from `refs/pull/` (but not heads or tags) is +therefore not a candidate to be used as a base for `refs/heads/`. + +Refs are grouped into islands based on their "names", and two regexes +that produce the same name are considered to be in the same +island. The names are computed from the regexes by concatenating any +capture groups from the regex, with a '-' dash in between. (And if +there are no capture groups, then the name is the empty string, as in +the above example.) This allows you to create arbitrary numbers of +islands. Only up to 14 such capture groups are supported though. + +For example, imagine you store the refs for each fork in +`refs/virtual/ID`, where `ID` is a numeric identifier. You might then +configure: + +------------------------------------------- +[pack] +island = refs/virtual/([0-9]+)/heads/ +island = refs/virtual/([0-9]+)/tags/ +island = refs/virtual/([0-9]+)/(pull)/ +------------------------------------------- + +That puts the heads and tags for each fork in their own island (named +"1234" or similar), and the pull refs for each go into their own +"1234-pull". + +Note that we pick a single island for each regex to go into, using "last +one wins" ordering (which allows repo-specific config to take precedence +over user-wide config, and so forth). + SEE ALSO -------- linkgit:git-rev-list[1] diff --git a/Documentation/git-repack.txt b/Documentation/git-repack.txt index d056250968..aa0cc8bd44 100644 --- a/Documentation/git-repack.txt +++ b/Documentation/git-repack.txt @@ -160,6 +160,11 @@ depth is 4095. being removed. In addition, any unreachable loose objects will be packed (and their loose counterparts removed). +-i:: +--delta-islands:: + Pass the `--delta-islands` option to `git-pack-objects`, see + linkgit:git-pack-objects[1]. + Configuration ------------- |