diff options
Diffstat (limited to 'third_party/git/Documentation/technical/partial-clone.txt')
| -rw-r--r-- | third_party/git/Documentation/technical/partial-clone.txt | 368 |
1 files changed, 0 insertions, 368 deletions
diff --git a/third_party/git/Documentation/technical/partial-clone.txt b/third_party/git/Documentation/technical/partial-clone.txt deleted file mode 100644 index 0780d30cac..0000000000 --- a/third_party/git/Documentation/technical/partial-clone.txt +++ /dev/null @@ -1,368 +0,0 @@ -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. - -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 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 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. - - -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. 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 ------------ - -- Improve the way to specify the order in which promisor remotes are - tried. -+ -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. -+ -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://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 |