author | mpm@selenic.com |
Wed, 04 May 2005 22:45:52 -0800 | |
changeset 24 | 57a1eef79415 |
parent 0 | 9117c6561b0b |
child 47 | 45cc818f2445 |
permissions | -rw-r--r-- |
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Some notes about Mercurial's design |
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Revlogs: |
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The fundamental storage type in Mercurial is a "revlog". A revlog is |
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the set of all revisions to a file. Each revision is either stored |
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compressed in its entirety or as a compressed binary delta against the |
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previous version. The decision of when to store a full version is made |
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based on how much data would be needed to reconstruct the file. This |
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lets us ensure that we never need to read huge amounts of data to |
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reconstruct a file, regardless of how many revisions of it we store. |
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In fact, we should always be able to do it with a single read, |
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provided we know when and where to read. This is where the index comes |
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in. Each revlog has an index containing a special hash (nodeid) of the |
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text, hashes for its parents, and where and how much of the revlog |
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data we need to read to reconstruct it. Thus, with one read of the |
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index and one read of the data, we can reconstruct any version in time |
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proportional to the file size. |
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Similarly, revlogs and their indices are append-only. This means that |
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adding a new version is also O(1) seeks. |
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Generally revlogs are used to represent revisions of files, but they |
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also are used to represent manifests and changesets. |
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Manifests: |
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A manifest is simply a list of all files in a given revision of a |
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project along with the nodeids of the corresponding file revisions. So |
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grabbing a given version of the project means simply looking up its |
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manifest and reconstruction all the file revisions pointed to by it. |
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Changesets: |
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A changeset is a list of all files changed in a check-in along with a |
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change description and some metadata like user and date. It also |
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contains a nodeid to the relevent revision of the manifest. Changesets |
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and manifests are one-to-one, but contain different data for |
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convenience. |
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Nodeids: |
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Nodeids are unique ids that are used to represent the contents of a |
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file AND its position in the project history. That is, if you change a |
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file and then change it back, the result will have a different nodeid |
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because it has different history. This is accomplished by including |
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the parents of a given revision's nodeids with the revision's text |
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when calculating the hash. |
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Graph merging: |
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Nodeids are implemented as they are to simplify merging. Merging a |
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pair of directed acyclic graphs (aka "the family tree" of the file |
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history) requires some method of determining if nodes in different |
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graphs correspond. Simply comparing the contents of the node (by |
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comparing text of given revisions or their hashes) can get confused by |
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identical revisions in the tree. |
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The nodeid approach makes it trivial - the hash uniquely describes a |
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revision's contents and its graph position relative to the root, so |
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merge is simply checking whether each nodeid in graph A is in the hash |
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table of graph B. If not, we pull them in, adding them sequentially to |
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the revlog. |
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Graph resolving: |
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Mercurial does branching by copying (or COWing) a repository and thus |
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keeps everything nice and linear within a repository. However, when a |
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merge of repositories (a "pull") is done, we may often have two head |
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revisions in a given graph. To keep things simple, Mercurial forces |
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the head revisions to be merged. |
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It first finds the closest common ancestor of the two heads. If one is |
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a child of the other, it becomes the new head. Otherwise, we call out |
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to a user-specified 3-way merge tool. |
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Merging files, manifests, and changesets: |
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We begin by comparing changeset DAGs, pulling all nodes we don't have |
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in our DAG from the other repository. As we do so, we collect a list |
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of changed files to merge. |
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Then for each file, we perform a graph merge and resolve as above. |
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It's important to merge files using per-file DAGs rather than just |
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changeset level DAGs as this diagram illustrates: |
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M M1 M2 |
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AB |
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|`-------v M2 clones M |
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aB AB file A is change in mainline |
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|`---v AB' file B is changed in M2 |
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| aB / | M1 clones M |
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| ab/ | M1 changes B |
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| ab' | M1 merges from M2, changes to B conflict |
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| | A'B' M2 changes A |
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`---+--.| |
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| a'B' M2 merges from mainline, changes to A conflict |
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`--.| |
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??? depending on which ancestor we choose, we will have |
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to redo A hand-merge, B hand-merge, or both |
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but if we look at the files independently, everything |
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is fine |
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After we've merged files, we merge the manifest log DAG and resolve |
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additions and deletions. Then we are ready to resolve the changeset |
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DAG - if our merge required any changes (the new head is not a |
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decendent of our tip), we must create a new changeset describing all |
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of the changes needed to merge it into the tip. |
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Merge performance: |
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The I/O operations for performing a merge are O(changed files), not |
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O(total changes) and in many cases, we needn't even unpack the deltas |
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to add them to our repository (though this optimization isn't |
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necessary). |
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Rollback: |
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Rollback is not yet implemented, but will be easy to add. When |
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performing a commit or a merge, we order things so that the changeset |
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entry gets added last. We keep a transaction log of the name of each |
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file and its length prior to the transaction. On abort, we simply |
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truncate each file to its prior length. This is one of the nice |
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properties of the append-only structure of the revlogs. |
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Remote access: |
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Mercurial currently supports pulling from "serverless" repositories. |
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Simply making the repo directory accessibly via the web and pointing |
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hg at it can accomplish a pull. This is relatively bandwidth efficient |
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but no effort has been spent on pipelining, so it won't work |
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especially well over LAN yet. |
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|
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It's also quite amenable to rsync, if you don't mind keeping an intact |
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copy of the master around locally. |
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Also note the append-only and ordering properties of the commit |
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guarantee that readers will always see a repository in a consistent |
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state and no special locking is necessary. As there is generally only |
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one writer to an hg repository, there is in fact no exclusion |
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implemented yet. |
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Some comparisons to git: |
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Most notably, Mercurial uses delta compression and repositories |
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created with it will grow much more slowly over time. This also allows |
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it to be much more bandwidth efficient. I expect repos sizes and sync |
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speeds to be similar to or better than BK, given the use of binary diffs. |
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Mercurial is roughly the same performance as git and is faster in |
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others as it keeps around more metadata. One example is listing and |
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retrieving past versions of a file, which it can do without reading |
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all the changesets. This metadata will also allow it to perform better |
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merges as described above. |
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159 |