New Default Branch Format in Bzr
One of the new features in the soon to be released bzr 0.8 is the new “knit” storage format.
When comparing the size of the repository data for jhbuild with “knit” and “metadir” formats (metadir is just the old storage format with repository, branch and checkout bookkeeping separated), I see the following:
| metadir | knit | |
|---|---|---|
| Size | 9.9MB | 5.5MB |
| Number of files | 1267 | 307 |
The reason for the smaller number of files is that information about all revisions in the repository is now stored together rather than in separate files. So the file count comes out at a constant plus 2 times the number of tracked files (a knit index file plus the knit data file). For comparison, the CVS repository I imported this from was 4.4MB, and comprised 143 files.
As well as reducing storage requirements, the new knit repository format is designed to reduce network traffic. With the current weave repository format, the weave file for each file touched by a commit gets rewritten to include the contents of the new revision. In contrast to this, the information about the new revision can simply be appended to the knit data file and the knit index file updated to match. This means publishing a branch to a server via sftp mainly involves append operations, resulting in a nice speed up.
Similarly when pulling new changes from a published branch, bzr only needs to download a knit index to find out which sections of the knit data are missing locally. It can then ask for just the changed sections (by an HTTP range request or a partial read with sftp), rather than downloading the entire contents of the changed weaves.
Overall, this should make bzr 0.8 a lot more usable than 0.7 for various network operations.