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diff --git a/Documentation/technical/partial-clone.txt b/Documentation/technical/partial-clone.txt new file mode 100644 index 0000000..cd948b0 --- /dev/null +++ b/Documentation/technical/partial-clone.txt @@ -0,0 +1,367 @@ +Partial Clone Design Notes +========================== + +The "Partial Clone" feature is a performance optimization for Git that +allows Git to function without having a complete copy of the repository. +The goal of this work is to allow Git to better handle extremely large +repositories. + +During clone and fetch operations, Git downloads the complete contents +and history of the repository. This includes all commits, trees, and +blobs for the complete life of the repository. For extremely large +repositories, clones can take hours (or days) and consume 100+GiB of disk +space. + +Often in these repositories there are many blobs and trees that the user +does not need such as: + + 1. files outside of the user's work area in the tree. For example, in + a repository with 500K directories and 3.5M files in every commit, + we can avoid downloading many objects if the user only needs a + narrow "cone" of the source tree. + + 2. large binary assets. For example, in a repository where large build + artifacts are checked into the tree, we can avoid downloading all + previous versions of these non-mergeable binary assets and only + download versions that are actually referenced. + +Partial clone allows us to avoid downloading such unneeded objects *in +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. Initially 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 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 +--------- + +Partial clone is a mechanism to limit the number of blobs and trees downloaded +*within* a given range of commits -- and is therefore independent of and not +intended to conflict with existing DAG-level mechanisms to limit the set of +requested commits (i.e. shallow clone, single branch, or fetch '<refspec>'). + + +Design Overview +--------------- + +Partial clone logically consists of the following parts: + +- A mechanism for the client to describe unneeded or unwanted objects to + the server. + +- A mechanism for the server to omit such unwanted objects from packfiles + sent to the client. + +- A mechanism for the client to gracefully handle missing objects (that + were previously omitted by the server). + +- A mechanism for the client to backfill missing objects as needed. + + +Design Details +-------------- + +- A new pack-protocol capability "filter" is added to the fetch-pack and + upload-pack negotiation. ++ +This uses the existing capability discovery mechanism. +See "filter" in linkgit:gitprotocol-pack[5]. + +- Clients pass a "filter-spec" to clone and fetch which is passed to the + server to request filtering during packfile construction. ++ +There are various filters available to accommodate different situations. +See "--filter=<filter-spec>" in Documentation/rev-list-options.txt. + +- On the server pack-objects applies the requested filter-spec as it + creates "filtered" packfiles for the client. ++ +These filtered packfiles are *incomplete* in the traditional sense because +they may contain objects that reference objects not contained in the +packfile and that the client doesn't already have. For example, the +filtered packfile may contain trees or tags that reference missing blobs +or commits that reference missing trees. + +- On the client these incomplete packfiles are marked as "promisor packfiles" + and treated differently by various commands. + +- On the client a repository extension is added to the local config to + prevent older versions of git from failing mid-operation because of + missing objects that they cannot handle. + See "extensions.partialClone" in Documentation/technical/repository-version.txt" + + +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 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 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 +and handle it appropriately. If not, Git can report a corruption. ++ +This means that there is no need for the client to explicitly maintain an +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 promisor remotes. ++ +When the normal object lookup fails to find an object, Git invokes +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. ++ +Dynamic object fetching tends to be slow as objects are fetched one at +a time. + +- `checkout` (and any other command using `unpack-trees`) has been taught + to bulk pre-fetch all required missing blobs in a single batch. + +- `rev-list` has been taught to print missing objects. ++ +This can be used by other commands to bulk prefetch objects. +For example, a "git log -p A..B" may internally want to first do +something like "git rev-list --objects --quiet --missing=print A..B" +and prefetch those objects in bulk. + +- `fsck` has been updated to be fully aware of promisor objects. + +- `repack` in GC has been updated to not touch promisor packfiles at all, + and to only repack other objects. + +- The global variable "fetch_if_missing" is used to control whether an + object lookup will attempt to dynamically fetch a missing object or + report an error. ++ +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. + + +Fetching Missing Objects +------------------------ + +- Fetching of objects is done by invoking a "git fetch" subprocess. + +- The local repository sends a request with the hashes of all requested + objects, and does not perform any packfile negotiation. + It then receives a packfile. + +- Because we are reusing the existing fetch mechanism, fetching + currently fetches all objects referred to by the requested objects, even + though they are not necessary. + +- Fetching with `--refetch` will request a complete new filtered packfile from + the remote, which can be used to change a filter without needing to + dynamically fetch missing objects. + +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 +------------------- + +- 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. ++ +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. ++ +It is not possible to push at the same time to multiple promisor +remote in a specific order. + +- 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 + distinct partitions and does not mix them. + +- Dynamic object fetching invokes fetch-pack once *for each item* + because most algorithms stumble upon a missing object and need to have + it resolved before continuing their work. This may incur significant + overhead -- and multiple authentication requests -- if many objects are + needed. + +- Dynamic object fetching currently uses the existing pack protocol V0 + which means that each object is requested via fetch-pack. The server + will send a full set of info/refs when the connection is established. + If there are a large number of refs, this may incur significant overhead. + + +Future Work +----------- + +- Improve the way to specify the order in which promisor remotes are + tried. ++ +For example this could allow specifying 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. ++ +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 non-pathname-based filters to make use of packfile bitmaps (when + present). This was just an omission during the initial implementation. + +- Investigate use of a long-running process to dynamically fetch a series + of objects, such as proposed in [5,6] to reduce process startup and + overhead costs. ++ +It would be nice if pack protocol V2 could allow that long-running +process to make a series of requests over a single long-running +connection. + +- Investigate pack protocol V2 to avoid the info/refs broadcast on + each connection with the server to dynamically fetch missing objects. + +- Investigate the need to handle loose promisor objects. ++ +Objects in promisor packfiles are allowed to reference missing objects +that can be dynamically fetched from the server. An assumption was +made that loose objects are only created locally and therefore should +not reference a missing object. We may need to revisit that assumption +if, for example, we dynamically fetch a missing tree and store it as a +loose object rather than a single object packfile. ++ +This does not necessarily mean we need to mark loose objects as promisor; +it may be sufficient to relax the object lookup or is-promisor functions. + + +Non-Tasks +--------- + +- Every time the subject of "demand loading blobs" comes up it seems + that someone suggests that the server be allowed to "guess" and send + additional objects that may be related to the requested objects. ++ +No work has gone into actually doing that; we're just documenting that +it is a common suggestion. We're not sure how it would work and have +no plans to work on it. ++ +It is valid for the server to send more objects than requested (even +for a dynamic object fetch), but we are not building on that. + + +Footnotes +--------- + +[a] expensive-to-modify list of missing objects: Earlier in the design of + partial clone we discussed the need for a single list of missing objects. + This would essentially be a sorted linear list of OIDs that were + omitted by the server during a clone or subsequent fetches. + +This file would need to be loaded into memory on every object lookup. +It would need to be read, updated, and re-written (like the .git/index) +on every explicit "git fetch" command *and* on any dynamic object fetch. + +The cost to read, update, and write this file could add significant +overhead to every command if there are many missing objects. For example, +if there are 100M missing blobs, this file would be at least 2GiB on disk. + +With the "promisor" concept, we *infer* a missing object based upon the +type of packfile that references it. + + +Related Links +------------- +[0] https://crbug.com/git/2 + Bug#2: Partial Clone + +[1] https://lore.kernel.org/git/20170113155253.1644-1-benpeart@microsoft.com/ + + Subject: [RFC] Add support for downloading blobs on demand + + Date: Fri, 13 Jan 2017 10:52:53 -0500 + +[2] https://lore.kernel.org/git/cover.1506714999.git.jonathantanmy@google.com/ + + Subject: [PATCH 00/18] Partial clone (from clone to lazy fetch in 18 patches) + + Date: Fri, 29 Sep 2017 13:11:36 -0700 + +[3] https://lore.kernel.org/git/20170426221346.25337-1-jonathantanmy@google.com/ + + Subject: Proposal for missing blob support in Git repos + + Date: Wed, 26 Apr 2017 15:13:46 -0700 + +[4] https://lore.kernel.org/git/1488999039-37631-1-git-send-email-git@jeffhostetler.com/ + + Subject: [PATCH 00/10] RFC Partial Clone and Fetch + + Date: Wed, 8 Mar 2017 18:50:29 +0000 + +[5] https://lore.kernel.org/git/20170505152802.6724-1-benpeart@microsoft.com/ + + Subject: [PATCH v7 00/10] refactor the filter process code into a reusable module + + Date: Fri, 5 May 2017 11:27:52 -0400 + +[6] https://lore.kernel.org/git/20170714132651.170708-1-benpeart@microsoft.com/ + + Subject: [RFC/PATCH v2 0/1] Add support for downloading blobs on demand + + Date: Fri, 14 Jul 2017 09:26:50 -0400 |