324 lines
13 KiB
Text
324 lines
13 KiB
Text
Partial Clone Design Notes
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==========================
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The "Partial Clone" feature is a performance optimization for Git that
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allows Git to function without having a complete copy of the repository.
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The goal of this work is to allow Git better handle extremely large
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repositories.
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During clone and fetch operations, Git downloads the complete contents
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and history of the repository. This includes all commits, trees, and
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blobs for the complete life of the repository. For extremely large
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repositories, clones can take hours (or days) and consume 100+GiB of disk
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space.
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Often in these repositories there are many blobs and trees that the user
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does not need such as:
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1. files outside of the user's work area in the tree. For example, in
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a repository with 500K directories and 3.5M files in every commit,
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we can avoid downloading many objects if the user only needs a
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narrow "cone" of the source tree.
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2. large binary assets. For example, in a repository where large build
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artifacts are checked into the tree, we can avoid downloading all
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previous versions of these non-mergeable binary assets and only
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download versions that are actually referenced.
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Partial clone allows us to avoid downloading such unneeded objects *in
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advance* during clone and fetch operations and thereby reduce download
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times and disk usage. Missing objects can later be "demand fetched"
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if/when needed.
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Use of partial clone requires that the user be online and the origin
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remote be available for on-demand fetching of missing objects. This may
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or may not be problematic for the user. For example, if the user can
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stay within the pre-selected subset of the source tree, they may not
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encounter any missing objects. Alternatively, the user could try to
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pre-fetch various objects if they know that they are going offline.
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Non-Goals
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---------
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Partial clone is a mechanism to limit the number of blobs and trees downloaded
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*within* a given range of commits -- and is therefore independent of and not
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intended to conflict with existing DAG-level mechanisms to limit the set of
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requested commits (i.e. shallow clone, single branch, or fetch '<refspec>').
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Design Overview
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---------------
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Partial clone logically consists of the following parts:
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- A mechanism for the client to describe unneeded or unwanted objects to
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the server.
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- A mechanism for the server to omit such unwanted objects from packfiles
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sent to the client.
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- A mechanism for the client to gracefully handle missing objects (that
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were previously omitted by the server).
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- A mechanism for the client to backfill missing objects as needed.
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Design Details
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--------------
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- A new pack-protocol capability "filter" is added to the fetch-pack and
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upload-pack negotiation.
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+
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This uses the existing capability discovery mechanism.
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See "filter" in Documentation/technical/pack-protocol.txt.
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- Clients pass a "filter-spec" to clone and fetch which is passed to the
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server to request filtering during packfile construction.
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+
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There are various filters available to accommodate different situations.
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See "--filter=<filter-spec>" in Documentation/rev-list-options.txt.
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- On the server pack-objects applies the requested filter-spec as it
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creates "filtered" packfiles for the client.
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+
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These filtered packfiles are *incomplete* in the traditional sense because
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they may contain objects that reference objects not contained in the
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packfile and that the client doesn't already have. For example, the
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filtered packfile may contain trees or tags that reference missing blobs
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or commits that reference missing trees.
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- On the client these incomplete packfiles are marked as "promisor packfiles"
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and treated differently by various commands.
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- On the client a repository extension is added to the local config to
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prevent older versions of git from failing mid-operation because of
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missing objects that they cannot handle.
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See "extensions.partialClone" in Documentation/technical/repository-version.txt"
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Handling Missing Objects
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------------------------
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- An object may be missing due to a partial clone or fetch, or missing due
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to repository corruption. To differentiate these cases, the local
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repository specially indicates such filtered packfiles obtained from the
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promisor remote as "promisor packfiles".
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+
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These promisor packfiles consist of a "<name>.promisor" file with
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arbitrary contents (like the "<name>.keep" files), in addition to
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their "<name>.pack" and "<name>.idx" files.
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- The local repository considers a "promisor object" to be an object that
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it knows (to the best of its ability) that the promisor remote has promised
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that it has, either because the local repository has that object in one of
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its promisor packfiles, or because another promisor object refers to it.
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+
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When Git encounters a missing object, Git can see if it is a promisor object
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and handle it appropriately. If not, Git can report a corruption.
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This means that there is no need for the client to explicitly maintain an
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expensive-to-modify list of missing objects.[a]
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- Since almost all Git code currently expects any referenced object to be
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present locally and because we do not want to force every command to do
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a dry-run first, a fallback mechanism is added to allow Git to attempt
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to dynamically fetch missing objects from the promisor remote.
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+
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When the normal object lookup fails to find an object, Git invokes
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fetch-object to try to get the object from the server and then retry
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the object lookup. This allows objects to be "faulted in" without
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complicated prediction algorithms.
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For efficiency reasons, no check as to whether the missing object is
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actually a promisor object is performed.
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Dynamic object fetching tends to be slow as objects are fetched one at
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a time.
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- `checkout` (and any other command using `unpack-trees`) has been taught
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to bulk pre-fetch all required missing blobs in a single batch.
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- `rev-list` has been taught to print missing objects.
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+
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This can be used by other commands to bulk prefetch objects.
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For example, a "git log -p A..B" may internally want to first do
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something like "git rev-list --objects --quiet --missing=print A..B"
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and prefetch those objects in bulk.
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- `fsck` has been updated to be fully aware of promisor objects.
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- `repack` in GC has been updated to not touch promisor packfiles at all,
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and to only repack other objects.
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- The global variable "fetch_if_missing" is used to control whether an
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object lookup will attempt to dynamically fetch a missing object or
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report an error.
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+
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We are not happy with this global variable and would like to remove it,
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but that requires significant refactoring of the object code to pass an
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additional flag. We hope that concurrent efforts to add an ODB API can
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encompass this.
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Fetching Missing Objects
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------------------------
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- Fetching of objects is done using the existing transport mechanism using
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transport_fetch_refs(), setting a new transport option
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TRANS_OPT_NO_DEPENDENTS to indicate that only the objects themselves are
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desired, not any object that they refer to.
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Because some transports invoke fetch_pack() in the same process, fetch_pack()
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has been updated to not use any object flags when the corresponding argument
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(no_dependents) is set.
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- The local repository sends a request with the hashes of all requested
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objects as "want" lines, and does not perform any packfile negotiation.
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It then receives a packfile.
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- Because we are reusing the existing fetch-pack mechanism, fetching
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currently fetches all objects referred to by the requested objects, even
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though they are not necessary.
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Current Limitations
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-------------------
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- The remote used for a partial clone (or the first partial fetch
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following a regular clone) is marked as the "promisor remote".
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We are currently limited to a single promisor remote and only that
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remote may be used for subsequent partial fetches.
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We accept this limitation because we believe initial users of this
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feature will be using it on repositories with a strong single central
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server.
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- Dynamic object fetching will only ask the promisor remote for missing
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objects. We assume that the promisor remote has a complete view of the
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repository and can satisfy all such requests.
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- Repack essentially treats promisor and non-promisor packfiles as 2
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distinct partitions and does not mix them. Repack currently only works
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on non-promisor packfiles and loose objects.
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- Dynamic object fetching invokes fetch-pack once *for each item*
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because most algorithms stumble upon a missing object and need to have
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it resolved before continuing their work. This may incur significant
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overhead -- and multiple authentication requests -- if many objects are
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needed.
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- Dynamic object fetching currently uses the existing pack protocol V0
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which means that each object is requested via fetch-pack. The server
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will send a full set of info/refs when the connection is established.
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If there are large number of refs, this may incur significant overhead.
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Future Work
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-----------
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- Allow more than one promisor remote and define a strategy for fetching
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missing objects from specific promisor remotes or of iterating over the
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set of promisor remotes until a missing object is found.
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+
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A user might want to have multiple geographically-close cache servers
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for fetching missing blobs while continuing to do filtered `git-fetch`
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commands from the central server, for example.
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Or the user might want to work in a triangular work flow with multiple
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promisor remotes that each have an incomplete view of the repository.
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- Allow repack to work on promisor packfiles (while keeping them distinct
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from non-promisor packfiles).
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- Allow non-pathname-based filters to make use of packfile bitmaps (when
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present). This was just an omission during the initial implementation.
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- Investigate use of a long-running process to dynamically fetch a series
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of objects, such as proposed in [5,6] to reduce process startup and
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overhead costs.
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It would be nice if pack protocol V2 could allow that long-running
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process to make a series of requests over a single long-running
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connection.
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- Investigate pack protocol V2 to avoid the info/refs broadcast on
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each connection with the server to dynamically fetch missing objects.
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- Investigate the need to handle loose promisor objects.
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Objects in promisor packfiles are allowed to reference missing objects
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that can be dynamically fetched from the server. An assumption was
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made that loose objects are only created locally and therefore should
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not reference a missing object. We may need to revisit that assumption
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if, for example, we dynamically fetch a missing tree and store it as a
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loose object rather than a single object packfile.
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This does not necessarily mean we need to mark loose objects as promisor;
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it may be sufficient to relax the object lookup or is-promisor functions.
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Non-Tasks
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---------
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- Every time the subject of "demand loading blobs" comes up it seems
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that someone suggests that the server be allowed to "guess" and send
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additional objects that may be related to the requested objects.
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No work has gone into actually doing that; we're just documenting that
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it is a common suggestion. We're not sure how it would work and have
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no plans to work on it.
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It is valid for the server to send more objects than requested (even
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for a dynamic object fetch), but we are not building on that.
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Footnotes
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---------
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[a] expensive-to-modify list of missing objects: Earlier in the design of
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partial clone we discussed the need for a single list of missing objects.
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This would essentially be a sorted linear list of OIDs that the were
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omitted by the server during a clone or subsequent fetches.
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This file would need to be loaded into memory on every object lookup.
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It would need to be read, updated, and re-written (like the .git/index)
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on every explicit "git fetch" command *and* on any dynamic object fetch.
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The cost to read, update, and write this file could add significant
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overhead to every command if there are many missing objects. For example,
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if there are 100M missing blobs, this file would be at least 2GiB on disk.
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With the "promisor" concept, we *infer* a missing object based upon the
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type of packfile that references it.
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Related Links
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-------------
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[0] https://crbug.com/git/2
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Bug#2: Partial Clone
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[1] https://public-inbox.org/git/20170113155253.1644-1-benpeart@microsoft.com/ +
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Subject: [RFC] Add support for downloading blobs on demand +
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Date: Fri, 13 Jan 2017 10:52:53 -0500
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[2] https://public-inbox.org/git/cover.1506714999.git.jonathantanmy@google.com/ +
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Subject: [PATCH 00/18] Partial clone (from clone to lazy fetch in 18 patches) +
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Date: Fri, 29 Sep 2017 13:11:36 -0700
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[3] https://public-inbox.org/git/20170426221346.25337-1-jonathantanmy@google.com/ +
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Subject: Proposal for missing blob support in Git repos +
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Date: Wed, 26 Apr 2017 15:13:46 -0700
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[4] https://public-inbox.org/git/1488999039-37631-1-git-send-email-git@jeffhostetler.com/ +
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Subject: [PATCH 00/10] RFC Partial Clone and Fetch +
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Date: Wed, 8 Mar 2017 18:50:29 +0000
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[5] https://public-inbox.org/git/20170505152802.6724-1-benpeart@microsoft.com/ +
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Subject: [PATCH v7 00/10] refactor the filter process code into a reusable module +
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Date: Fri, 5 May 2017 11:27:52 -0400
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[6] https://public-inbox.org/git/20170714132651.170708-1-benpeart@microsoft.com/ +
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Subject: [RFC/PATCH v2 0/1] Add support for downloading blobs on demand +
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Date: Fri, 14 Jul 2017 09:26:50 -0400
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