DeltaRPM Support (Presto)
Summary
The presto plugin for yum adds support for downloading deltarpms and using them to generate new packages. If we ship and enable this plugin by default, we can make a substantial dent in the amount of data having to be downloaded by our users for updates.
Owners
- Name: LukeMacken, SethVidal
Current status
- Targeted release: Fedora 11
- Last modified: 2008-09-09
- Percentage of completion: 95%
- bodhi ticket #160 for more details.
Details
- Plugin and tools exist in the repository
- Well over 100 users (closer to 200) have been using the plugin in conjunction with unofficial presto-enabled repositories with no fatal (i.e. yum breaks) errors since August.
- XML format is cleaned up and has a schema for validity checking
- createrepo has code to generate deltarpms and presto xml metadata.
Usage cases/rationale
Justin only has a dial-up connection available for his computer. Currently he has very little chance to actually keep up with the update churn of Fedora due to the amount of data being transferred. Adding presto support will reduce the amount of data he has to transfer and thus make it so that he can keep his Fedora system updated.
Barry pays for his network connection based on usage. Adding presto support will reduce the amount of data he has to transfer and thus reduce the cost for his network connection.
Scope
Requires that the yum-presto plugin be included by default. Also requires some infrastructure work on the buildsystem and compose tools so that we can ensure that deltas are included in the -updates repositories.
How To Test
- Ensure that updates work using deltas rather than downloading full packages.
- Ensure that changing files from a package lead to downloading the full package rather than the delta.
- Ensure that changing a %config file from a package does not lead to downloading the full package.
- Ensure that yum doesn't break when yum-presto runs into problems. (i.e. always fall back to downloading the full package if nothing else works)
Dependencies
The binary diff algorithm used in deltarpm is a version of bsdiff modified for speed.
Details
Contingency Plan
If things aren't working, then we just don't install and enable the plugin by default.
Documentation
See README for the documentation
Release Notes
Normally when you update a package in Fedora, you download an entire replacement package. Most of the time (especially for the larger packages), most of the actual data in the updated package is the same as the original package, but you still end up downloading the full package. Presto allows you to download the difference (called the delta) between the package you have installed and the one you want to update to. This can reduce the download size of updates by 60% - 80%.