From: Jeff Hostetler <jeffh...@microsoft.com> First draft of design document for partial clone feature.
Signed-off-by: Jeff Hostetler <jeffh...@microsoft.com> --- Documentation/technical/partial-clone.txt | 259 ++++++++++++++++++++++++++++++ 1 file changed, 259 insertions(+) create mode 100644 Documentation/technical/partial-clone.txt diff --git a/Documentation/technical/partial-clone.txt b/Documentation/technical/partial-clone.txt new file mode 100644 index 0000000..731bd8c --- /dev/null +++ b/Documentation/technical/partial-clone.txt @@ -0,0 +1,259 @@ +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. + +During clone and fetch operations, git normally downloads the complete +contents and history of the repository. That is, during clone the client +receives all of the commits, trees, and blobs in the repository into a +local ODB. Subsequent fetches extend the local ODB with any new objects. +For large repositories, this can take significant time to download and +large amounts of diskspace to store. + +The goal of this work is to allow git better handle extremely large +repositories. Often in these repositories there are many files that the +user does not need such as ancient versions of source files, files in +portions of the worktree outside of the user's work area, or large binary +assets. If we can avoid downloading such unneeded objects *in advance* +during clone and fetch operations, we can decrease download times and +reduce ODB disk usage. + + +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 accomodate 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 trees that reference blobs that the client does not have. + +- On the client these incomplete packfiles are marked as "promisor pacfiles" + 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 packfiles obtained from the promisor + remote. + + 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. + + In the future, this ability may be extended to loose objects in case + a promisor packfile is accidentally unpacked. + +- 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, 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. + +- 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. + +- 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. + + Future work may lift this restriction when we figure out how to route + such requests. The current assumption is that partial clone will not be + used for triangular workflows that would need that (at least initially). + +- 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. + + Future work may let repack work to repack promisor packfiles (while + keeping them in a different partition from the others). + +- The current object filtering mechanism does not make use of packfile + bitmaps (when present). + + We should allow this for filters that are not pathname-based. + +- Currently, dynamic object fetching invokes fetch-pack 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. + + We need to investigate use of a long-running process, 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. + +- 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. + + We expect that protocol V2 will allow us to avoid this cost. + + +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. + + +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-benpe...@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.jonathanta...@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-jonathanta...@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-...@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-benpe...@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-benpe...@microsoft.com/ + Subject: [RFC/PATCH v2 0/1] Add support for downloading blobs on demand + Date: Fri, 14 Jul 2017 09:26:50 -0400 -- 2.9.3
