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diff --git a/Documentation/technical/partial-clone.txt b/Documentation/technical/partial-clone.txt new file mode 100644 index 0000000000..0bed2472c8 --- /dev/null +++ b/Documentation/technical/partial-clone.txt @@ -0,0 +1,324 @@ +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 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. + +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. + + +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 Documentation/technical/pack-protocol.txt. + +- 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 the + promisor remote 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 + its promisor packfiles, or because another promisor object refers to it. + + When Git encounters a missing object, Git can see if it 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 the promisor remote. + + 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. + + 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. We hope that concurrent efforts to add an ODB API can + encompass this. + + +Fetching Missing Objects +------------------------ + +- Fetching of objects is done using the existing transport mechanism using + transport_fetch_refs(), setting a new transport option + TRANS_OPT_NO_DEPENDENTS to indicate that only the objects themselves are + desired, not any object that they refer to. + + Because some transports invoke fetch_pack() in the same process, fetch_pack() + has been updated to not use any object flags when the corresponding argument + (no_dependents) is set. + +- The local repository sends a request with the hashes of all requested + objects as "want" lines, and does not perform any packfile negotiation. + It then receives a packfile. + +- Because we are reusing the existing fetch-pack mechanism, fetching + currently fetches all objects referred to by the requested objects, even + though they are not necessary. + + +Current Limitations +------------------- + +- The remote used for a partial clone (or the first partial fetch + following a regular clone) is marked as the "promisor remote". + + We are currently limited to a single promisor remote and only that + remote may be used for subsequent partial fetches. + + We accept this limitation because we believe initial users of this + feature will be using it on repositories with a strong single central + server. + +- Dynamic object fetching will only ask the promisor remote for missing + objects. We assume that the promisor remote has 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. Repack currently only works + on non-promisor packfiles and loose objects. + +- 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 large number of refs, this may incur significant overhead. + + +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. + + 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. + + Or 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 + from non-promisor packfiles). + +- 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 the 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://bugs.chromium.org/p/git/issues/detail?id=2 + Chromium work item for: Partial Clone + +[1] https://public-inbox.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://public-inbox.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://public-inbox.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://public-inbox.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://public-inbox.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://public-inbox.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 |