KnitPack repository format

Contents

Using KnitPack repositories

Motivation

KnitPack is a new repository format for Bazaar, which is expected to be faster both locally and over the network, is usually more compact, and will work with more FTP servers.

Our benchmarking results to date have been very promising. We fully expect to make a pack-based format the default in the near future. We would therefore like as many people as possible using KnitPack repositories, benchmarking the results and telling us where improvements are still needed.

Preparation

A small percentage of existing repositories may have some inconsistent data within them. It's is a good idea to check the integrity of your repositories before migrating them to knitpack format. To do this, run:

bzr check

If that reports a problem, run this command:

bzr reconcile

Note that this can take many hours for repositories with deep history so be sure to set aside some time for this if it is required.

Creating a new knitpack branch

If you're starting a project from scratch, it's easy to make it a knitpack one. Here's how:

cd my-stuff
bzr init --knitpack-experimental
bzr add
bzr commit -m "initial import"

In other words, use the normal sequence of commands but add the --knitpack-experimental option to the init command.

Creating a new knitpack repository

If you're starting a project from scratch and wish to use a shared repository for branches, you can make it a knitpack repository like this:

cd my-repo
bzr init-repo --knitpack-experimental .
cd my-stuff
bzr init
bzr add
bzr commit -m "initial import"

In other words, use the normal sequence of commands but add the --knitpack-experimental option to the init-repo command.

Upgrading an existing branch or repository to knitpack format

If you have an existing branch and wish to migrate it to a knitpack format, use the upgrade command like this:

bzr upgrade --knitpack-experimental path-to-my-branch

If you are using a shared repository, run:

bzr upgrade --knitpack-experimental ROOT_OF_REPOSITORY

to upgrade the history database. Note that this will not alter the branch format of each branch, so you will need to also upgrade each branch individually if you are upgrading from an old (e.g. < 0.17) bzr. More modern bzr's will already have the branch format at our latest branch format which adds support for tags.

Starting a new knitpack branch from one in an older format

This can be done in one of several ways:

  1. Create a new branch and pull into it
  2. Create a standalone branch and upgrade its format
  3. Create a knitpack shared repository and branch into it

Here are the commands for using the pull approach:

bzr init --knitpack-experimental my-new-branch
cd my-new-branch
bzr pull my-source-branch

Here are the commands for using the upgrade approach:

bzr branch my-source-branch my-new-branch
cd my-new-branch
bzr upgrade --knitpack-experimental .

Here are the commands for the shared repository approach:

cd my-repo
bzr init-repo --knitpack-experimental .
bzr branch my-source-branch my-new-branch
cd my-new-branch

As a reminder, any of the above approaches can fail if the source branch has inconsistent data within it and hasn't been reconciled yet. Please be sure to check that before reporting problems.

Testing packs for bzr-svn users

If you are using bzr-svn or are testing the prototype subtree support, you can still use and assist in testing KnitPacks. The commands to use are identical to the ones given above except that the name of the format to use is knitpack-subtree-experimental.

WARNING: Note that the subtree formats, distate-subtree and knitpack-subtree-experimental, are not production strength yet and may cause unexpected problems. They are required for the bzr-svn plug-in but should otherwise ony be used by people happy to live on the bleeding edge. If you are using bzr-svn, you're on the bleeding edge anyway. :-)

Reporting problems

If you need any help or encounter any problems, please contact the developers via the usual ways, i.e. chat to us on IRC or send a message to our mailing list. See http://bazaar-vcs.org/BzrSupport for contact details.

Technical notes

Bazaar 0.92 adds a new format (experimental at first) implemented in bzrlib.repofmt.pack_repo.py.

This format provides a knit-like interface which is quite compatible with knit format repositories: you can get a VersionedFile for a particular file-id, or for revisions, or for the inventory, even though these do not correspond to single files on disk.

The on-disk format is that the repository directory contains these files and subdirectories:

packs/ completed readonly packs
indices/ indices for completed packs
upload/ temporary files for packs currently being written
obsolete_packs/ packs that have been repacked and are no longer normally needed
pack-names index of all live packs
lock/ lockdir

Note that for consistency we always write "indices" not "indexes".

This is implemented on top of pack files, which are written once from start to end, then left alone. A pack consists of a body file, plus several index files. There are four index files for each pack, which have the same basename and an extension indicating the purpose of the index:

extn Purpose Key References
.tix File texts file_id, revision_id per-file parents, compression basis per-file parents
.six Signatures revision_id,
.rix Revisions revision_id, revision parents
.iix Inventory revision_id, revision parents, compression base

Indices are accessed through the bzrlib.index.GraphIndex class. Indices are stored as sorted files on disk. Each line is one record, and contains:

  • key fields
  • a value string - for all these indices, this is an ascii decimal pair of "offset length" giving the position of the refenced data within the pack body file
  • a list of zero or more reference lists

The reference lists let a graph be stored within the index. Each reference list entry points to another entry in the same index. The references are represented as a byte offset for the target within the index file.

When a compression base is given, it indicates that the body of the text or inventory is a forward delta from the referenced revision. The compression base list must have length 0 or 1.

Like packs, indexes are written only once and then unmodified. A GraphIndex builder is a mutable in-memory graph that can be sorted, cross-referenced and written out when the write group completes.

There can also be index entries with a value of 'a' for absent. These records exist just to be pointed to in a graph. This is used, for example, to give the revision-parent pointer when the parent revision is in a previous pack.

The data content for each record is a knit data chunk. The knits are always unannotated - the annotations must be generated when needed. (We'd like to cache/memoize the annotations.) The data hunks can be moved between packs without needing to recompress them.

It is not possible to regenerate an index from the body file, because it contains information stored in the knit index that's not in the body. (In particular, the per-file graph is only stored in the index.) We would like to change this in a future format.

The lock is a regular LockDir lock. The lock is only held for a much reduced scope, while updating the pack-names file. The bulk of the insertion can be done without the repository locked. This is an implementation detail; the repository user should still call repository.lock_write at the regular time but be aware this does not correspond to a physical mutex.

Read locks control caching but do not affect writers.

The newly-added repository write group concept is very important to KnitPack repositories. When start_write_group is called, a new temporary pack is created and all modifications to the repository will go into it until either commit_write_group or abort_write_group is called, at which time it is either finished and moved into place or discarded respectively. Write groups cannot be nested, only one can be underway at a time on a Repository instance and they must occur within a write lock.

Normally the data for each revision will be entirely within a single pack but this is not required.

When a pack is finished, it gets a final name based on the md5 of all the data written into the pack body file.

The pack-names file gives the list of all finished non-obsolete packs. (This should always be the same as the list of files in the packs/ directory, but the file is needed for readonly http clients that can't easily list directories, and it includes other information.) The constraint on the pack-names list is that every file mentioned must exist in the packs/ directory.

In rare cases, when a writer is interrupted, about-to-be-removed packs may still be present in the directory but removed from the list.

As well as the list of names, the pack-names file also contains the size, in bytes, of each of the four indices. This is used to bootstrap bisection search within the indices.

In normal use, one pack will be created for each commit to a repository. This would build up to an inefficient number of files over time, so a repack operation is available to recombine them, by producing larger files containing data on multiple revisions. This can be done manually by running bzr pack, and it also may happen automatically when a write group is committed.

The repacking strategy used at the moment tries to balance not doing too much work during commit with not having too many small files left in the repository. The algorithm is roughly this: the total number of revisions in the repository is expressed as a decimal number, e.g. "532". Then we'll repack until we have five packs containing a hundred revisions each, three packs containing ten revisions each, and two packs with single revisions. This means that each revision will normally initially be created in a single-revision pack, then moved to a ten-revision pack, then to a 100-pack, and so on.

As with other repositories, in normal use data is only inserted. However, in some circumstances we may want to garbage-collect or prune existing data, or reconcile indexes.

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