How is that for clickbait?

With the now released git-cinnabar 0.5.2, the cinnabarclone feature is enabled by default, which means it doesn’t need to be enabled manually anymore.

Cinnabarclone is to git-cinnabar what clonebundles is to Mercurial (to some extent). Clonebundles allow Mercurial to download a pre-generated bundle of a repository, which reduces work on the server side. Similarly, Cinnabarclone allows git-cinnabar to download a pre-generated bundle of the git form of a Mercurial repository.

Thanks to Connor Sheehan, who deployed the necessary extension and configuration on the server side, cinnabarclone is now enabled for mozilla-central and mozilla-unified, making git-cinnabar clone faster than ever for these repositories. In fact, under some conditions (mostly depending on network bandwidth), cloning with git-cinnabar is now faster than cloning with Mercurial:

$ time git clone hg::https://hg.mozilla.org/mozilla-unified mozilla-unified_git
Cloning into 'mozilla-unified_git'...
Fetching cinnabar metadata from https://index.taskcluster.net/v1/task/github.glandium.git-cinnabar.bundle.mozilla-unified/artifacts/public/bundle.git
Receiving objects: 100% (12153616/12153616), 2.67 GiB | 41.41 MiB/s, done.
Resolving deltas: 100% (8393939/8393939), done.
Reading 172 changesets
Reading and importing 170 manifests
Reading and importing 758 revisions of 570 files
Importing 172 changesets
It is recommended that you set "remote.origin.prune" or "fetch.prune" to "true".
git config remote.origin.prune true
or
git config fetch.prune true

Run the following command to update tags:
git fetch --tags hg::tags: tag "*"
Checking out files: 100% (279688/279688), done.

real    4m57.837s
user    9m57.373s
sys     0m41.106s

$ time hg clone https://hg.mozilla.org/mozilla-unified
destination directory: mozilla-unified
applying clone bundle from https://hg.cdn.mozilla.net/mozilla-unified/5ebb4441aa24eb6cbe8dad58d232004a3ea11b28.zstd-max.hg
adding changesets
adding manifests
adding file changes
added 537259 changesets with 3275908 changes to 523698 files (+13 heads)
finished applying clone bundle
searching for changes
adding changesets
adding manifests
adding file changes
added 172 changesets with 758 changes to 570 files (-1 heads)
new changesets 8b3c35badb46:468e240bf668
537259 local changesets published
updating to branch default
(warning: large working directory being used without fsmonitor enabled; enable fsmonitor to improve performance; see "hg help -e fsmonitor")
279688 files updated, 0 files merged, 0 files removed, 0 files unresolved

real    21m9.662s
user    21m30.851s
sys     1m31.153s

To be fair, the Mozilla Mercurial repos also have a faster “streaming” clonebundle that they only prioritize automatically if the client is on AWS currently, because they are much larger, and could take longer to download. But you can opt-in with the --stream command line argument:

$ time hg clone --stream https://hg.mozilla.org/mozilla-unified mozilla-unified_hg
destination directory: mozilla-unified_hg
applying clone bundle from https://hg.cdn.mozilla.net/mozilla-unified/5ebb4441aa24eb6cbe8dad58d232004a3ea11b28.packed1.hg
525514 files to transfer, 2.95 GB of data
transferred 2.95 GB in 51.5 seconds (58.7 MB/sec)
finished applying clone bundle
searching for changes
adding changesets
adding manifests
adding file changes
added 172 changesets with 758 changes to 570 files (-1 heads)
new changesets 8b3c35badb46:468e240bf668
updating to branch default
(warning: large working directory being used without fsmonitor enabled; enable fsmonitor to improve performance; see "hg help -e fsmonitor")
279688 files updated, 0 files merged, 0 files removed, 0 files unresolved

real    1m49.388s
user    2m52.943s
sys     0m43.779s

If you’re using Mercurial and can download 3GB in less than 20 minutes (in other words, if you can download faster than 2.5MB/s), you’re probably better off with the streaming clone.

Bonus fact: the Git clone is smaller than the Mercurial clone

The Mercurial streaming clone bundle contains data in a form close to what Mercurial puts on disk in the .hg directory, meaning the size of .hg is close to that of the clone bundle. The Cinnabarclone bundle contains a git pack, meaning the size of .git is close to that of the bundle, plus some more for the pack index file that unbundling creates.

The amazing fact is that, to my own surprise, the git pack, containing the repository contents along with all git-cinnabar needs to recreate Mercurial changesets, manifests and files from the contents, takes less space than the Mercurial streaming clone bundle.

And that translates in local repository size:

$ du -h -s --apparent-size mozilla-unified_hg/.hg
3.3G    mozilla-unified_hg/.hg
$ du -h -s --apparent-size mozilla-unified_git/.git
3.1G    mozilla-unified_git/.git

And because Mercurial creates so many files (essentially, two per file that ever was in the repository), there is a larger difference in block size used on disk:

$ du -h -s mozilla-unified_hg/.hg
4.7G    mozilla-unified_hg/.hg
$ du -h -s mozilla-unified_git/.git
3.1G    mozilla-unified_git/.git

It’s even more mind blowing when you consider that Mercurial happily creates delta chains of several thousand revisions, when the git pack’s longest delta chain is 250 (set arbitrarily at pack creation, by which I mean I didn’t pick a larger value because it didn’t make a significant difference). For the casual readers, Git and Mercurial try to store object revisions as a diff/delta from a previous object revision because that takes less space. You get a delta chain when that previous object revision itself is stored as a diff/delta from another object revision itself stored as a diff/delta … etc.

My guess is that the difference is mainly caused by the use of line-based deltas in Mercurial, but some Mercurial developer should probably take a deeper look. The fact that Mercurial cannot delta across file renames is another candidate.

https://glandium.org/blog/?p=3913