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The original Mercurial compression format has a particular weakness in storing and transmitting deltas for branches that are heavily interleaved. In some instances, this can make the size of the manifest data (stored in '''00manifest.d''') balloon by 10x or more. The generaldelta option is an effort to mitigate that, while still maintaining Mercurial's O(1)-bounded performance. |
The original Mercurial compression format has a particular weakness in storing and transmitting deltas for branches that are heavily interleaved. In some instances, this can make the size of the manifest data (stored in '''00manifest.d''') balloon by 10x or more. The generaldelta option is an effort to mitigate that, while still maintaining Mercurial's O(1)-bounded performance. |
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This will actually enable two features: |
This will actually enable two features: |
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| We intend to eventually address this by updating Mercurial's bundle protocol with [[BundleFormat2]], after which the generaldelta feature will be enabled by default on new clones. | We intend to eventually address this by updating Mercurial's bundle protocol with BundleFormat2, after which the generaldelta feature will be enabled by default on new clones. |
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GeneralDelta
Using the generaldelta compression option.
1. Introduction
The original Mercurial compression format has a particular weakness in storing and transmitting deltas for branches that are heavily interleaved. In some instances, this can make the size of the manifest data (stored in 00manifest.d) balloon by 10x or more. The generaldelta option is an effort to mitigate that, while still maintaining Mercurial's O(1)-bounded performance.
The generaldelta feature is available in Mercurial 1.9 and later.
2. Enabling generaldelta
The generaldelta feature can be enabled for new clones with:
[format] generaldelta = true
This will actually enable two features:
- generaldelta
- delta reordering on pulls when this is enabled on the server side
The latter feature will let clients without generaldelta enabled experience some of the disk space and bandwidth benefits.
3. Converting a repo to generaldelta
This is as simple as:
$ hg clone -U --config format.generaldelta=1 --pull project project-generaldelta
The aforementioned reordering can also marginally improve compression for generaldelta clients, which can be tried with a second pass:
$ hg clone -U --config format.generaldelta=1 --pull project-generaldelta project-generaldelta-pass2
Detailed compression statistics for the manifest can be checked with debugrevlog:
$ hg debugrevlog -m
format : 1
flags : generaldelta
revisions : 14932
merges : 1763 (11.81%)
normal : 13169 (88.19%)
revisions : 14932
full : 61 ( 0.41%)
deltas : 14871 (99.59%)
revision size : 3197528
full : 744577 (23.29%)
deltas : 2452951 (76.71%)
avg chain length : 172
compression ratio : 229
uncompressed data size (min/max/avg) : 125 / 80917 / 49156
full revision size (min/max/avg) : 113 / 37284 / 12206
delta size (min/max/avg) : 0 / 27029 / 164
deltas against prev : 13770 (92.60%)
where prev = p1 : 13707 (99.54%)
where prev = p2 : 8 ( 0.06%)
other : 55 ( 0.40%)
deltas against p1 : 1097 ( 7.38%)
deltas against p2 : 4 ( 0.03%)
deltas against other : 0 ( 0.00%)Of particular interest are the number of full revisions and the average delta size.
4. Further work
Mercurial's bundle protocol doesn't yet fully support generaldelta. This creates two barriers to making this Mercurial's default format:
- pulling from a generaldelta repo uses more server CPU as the server has to recalculate some deltas
- more than an optimal amount of bandwidth is still used due to sending old-style deltas
We intend to eventually address this by updating Mercurial's bundle protocol with BundleFormat2, after which the generaldelta feature will be enabled by default on new clones.
5. See also