diff options
Diffstat (limited to 'Documentation/technical')
| -rw-r--r-- | Documentation/technical/partial-clone.txt | 117 |
1 files changed, 84 insertions, 33 deletions
diff --git a/Documentation/technical/partial-clone.txt b/Documentation/technical/partial-clone.txt index 896c7b3878..210373e258 100644 --- a/Documentation/technical/partial-clone.txt +++ b/Documentation/technical/partial-clone.txt @@ -30,12 +30,20 @@ advance* during clone and fetch operations and thereby reduce download times and disk usage. Missing objects can later be "demand fetched" if/when needed. +A remote that can later provide the missing objects is called a +promisor remote, as it promises to send the objects when +requested. Initialy Git supported only one promisor remote, the origin +remote from which the user cloned and that was configured in the +"extensions.partialClone" config option. Later support for more than +one promisor remote has been implemented. + Use of partial clone requires that the user be online and the origin -remote be available for on-demand fetching of missing objects. This may -or may not be problematic for the user. For example, if the user can -stay within the pre-selected subset of the source tree, they may not -encounter any missing objects. Alternatively, the user could try to -pre-fetch various objects if they know that they are going offline. +remote or other promisor remotes be available for on-demand fetching +of missing objects. This may or may not be problematic for the user. +For example, if the user can stay within the pre-selected subset of +the source tree, they may not encounter any missing objects. +Alternatively, the user could try to pre-fetch various objects if they +know that they are going offline. Non-Goals @@ -100,18 +108,18 @@ or commits that reference missing trees. Handling Missing Objects ------------------------ -- An object may be missing due to a partial clone or fetch, or missing due - to repository corruption. To differentiate these cases, the local - repository specially indicates such filtered packfiles obtained from the - promisor remote as "promisor packfiles". +- An object may be missing due to a partial clone or fetch, or missing + due to repository corruption. To differentiate these cases, the + local repository specially indicates such filtered packfiles + obtained from promisor remotes as "promisor packfiles". + These promisor packfiles consist of a "<name>.promisor" file with arbitrary contents (like the "<name>.keep" files), in addition to their "<name>.pack" and "<name>.idx" files. - The local repository considers a "promisor object" to be an object that - it knows (to the best of its ability) that the promisor remote has promised - that it has, either because the local repository has that object in one of + it knows (to the best of its ability) that promisor remotes have promised + that they have, either because the local repository has that object in one of its promisor packfiles, or because another promisor object refers to it. + When Git encounters a missing object, Git can see if it is a promisor object @@ -123,12 +131,12 @@ expensive-to-modify list of missing objects.[a] - Since almost all Git code currently expects any referenced object to be present locally and because we do not want to force every command to do a dry-run first, a fallback mechanism is added to allow Git to attempt - to dynamically fetch missing objects from the promisor remote. + to dynamically fetch missing objects from promisor remotes. + When the normal object lookup fails to find an object, Git invokes -fetch-object to try to get the object from the server and then retry -the object lookup. This allows objects to be "faulted in" without -complicated prediction algorithms. +promisor_remote_get_direct() to try to get the object from a promisor +remote and then retry the object lookup. This allows objects to be +"faulted in" without complicated prediction algorithms. + For efficiency reasons, no check as to whether the missing object is actually a promisor object is performed. @@ -157,8 +165,7 @@ and prefetch those objects in bulk. + We are not happy with this global variable and would like to remove it, but that requires significant refactoring of the object code to pass an -additional flag. We hope that concurrent efforts to add an ODB API can -encompass this. +additional flag. Fetching Missing Objects @@ -182,21 +189,63 @@ has been updated to not use any object flags when the corresponding argument though they are not necessary. +Using many promisor remotes +--------------------------- + +Many promisor remotes can be configured and used. + +This allows for example a user to have multiple geographically-close +cache servers for fetching missing blobs while continuing to do +filtered `git-fetch` commands from the central server. + +When fetching objects, promisor remotes are tried one after the other +until all the objects have been fetched. + +Remotes that are considered "promisor" remotes are those specified by +the following configuration variables: + +- `extensions.partialClone = <name>` + +- `remote.<name>.promisor = true` + +- `remote.<name>.partialCloneFilter = ...` + +Only one promisor remote can be configured using the +`extensions.partialClone` config variable. This promisor remote will +be the last one tried when fetching objects. + +We decided to make it the last one we try, because it is likely that +someone using many promisor remotes is doing so because the other +promisor remotes are better for some reason (maybe they are closer or +faster for some kind of objects) than the origin, and the origin is +likely to be the remote specified by extensions.partialClone. + +This justification is not very strong, but one choice had to be made, +and anyway the long term plan should be to make the order somehow +fully configurable. + +For now though the other promisor remotes will be tried in the order +they appear in the config file. + Current Limitations ------------------- -- The remote used for a partial clone (or the first partial fetch - following a regular clone) is marked as the "promisor remote". +- It is not possible to specify the order in which the promisor + remotes are tried in other ways than the order in which they appear + in the config file. + -We are currently limited to a single promisor remote and only that -remote may be used for subsequent partial fetches. +It is also not possible to specify an order to be used when fetching +from one remote and a different order when fetching from another +remote. + +- It is not possible to push only specific objects to a promisor + remote. + -We accept this limitation because we believe initial users of this -feature will be using it on repositories with a strong single central -server. +It is not possible to push at the same time to multiple promisor +remote in a specific order. -- Dynamic object fetching will only ask the promisor remote for missing - objects. We assume that the promisor remote has a complete view of the +- Dynamic object fetching will only ask promisor remotes for missing + objects. We assume that promisor remotes have a complete view of the repository and can satisfy all such requests. - Repack essentially treats promisor and non-promisor packfiles as 2 @@ -218,15 +267,17 @@ server. Future Work ----------- -- Allow more than one promisor remote and define a strategy for fetching - missing objects from specific promisor remotes or of iterating over the - set of promisor remotes until a missing object is found. +- Improve the way to specify the order in which promisor remotes are + tried. + -A user might want to have multiple geographically-close cache servers -for fetching missing blobs while continuing to do filtered `git-fetch` -commands from the central server, for example. +For example this could allow to specify explicitly something like: +"When fetching from this remote, I want to use these promisor remotes +in this order, though, when pushing or fetching to that remote, I want +to use those promisor remotes in that order." + +- Allow pushing to promisor remotes. + -Or the user might want to work in a triangular work flow with multiple +The user might want to work in a triangular work flow with multiple promisor remotes that each have an incomplete view of the repository. - Allow repack to work on promisor packfiles (while keeping them distinct |