~bzr-pqm/bzr/bzr.dev

==========================
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 --pack-0.92
  bzr add
  bzr commit -m "initial import"

In other words, use the normal sequence of commands but add the
``--pack-0.92`` option to the ``init`` command.

**Note:** In bzr 0.92, this format was called ``knitpack-experimental``.

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 --pack-0.92 .
  cd my-stuff
  bzr init
  bzr add
  bzr commit -m "initial import"

In other words, use the normal sequence of commands but add the
``--pack-0.92`` 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 --pack-0.92 path-to-my-branch

If you are using a shared repository, run::

  bzr upgrade --pack-0.92 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 --pack-0.92 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 --pack-0.92 .

Here are the commands for the shared repository approach::

  cd my-repo
  bzr init-repo --pack-0.92 .
  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, ``dirstate-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 only 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://wiki.bazaar.canonical.com/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 referenced 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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