~bzr-pqm/bzr/bzr.dev

2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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Planned changes to the bzr core
2506.1.3 by Alexander Belchenko
merge bzr.dev; fix ReST formatting in planned-performance-changes.txt
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2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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Delivering the best possible performance requires changing the bzr core design
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from that present in 0.16. Some of these changes are incremental and can be
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done with no impact on disk format. Many of them however do require changes to
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the disk format, and these can be broken into two sets of changes, those which
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are sufficiently close to the model bzr uses today to interoperate with the
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0.16 disk formats, and those that are not able to interoperate with the 0.16
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disk formats - specifically some planned changes may result in data which
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cannot be exported to bzr 0.16's disk formats and then imported back to the new
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format without losing critical information. If/when this takes place it will be
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essentially a migration for users to switch from their bzr 0.16 repository to a
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bzr that supports them. We plan to batch all such changes into one large
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'experimental' repository format, which will be complete stable and usable
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before we migrate it to become a supported format. Getting new versions of bzr
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in widespread use at that time will be very important, otherwise the user base
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may be split in two - users that have upgraded and users that have not.
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The following changes are grouped according to their compatability impact:
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library only, disk format but interoperable, disk format interoperability
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unknown, and disk format, not interoperable.
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Library changes
2506.1.3 by Alexander Belchenko
merge bzr.dev; fix ReST formatting in planned-performance-changes.txt
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2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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These changes will change bzrlib's API but will not affect the disk format and
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thus do not pose a significant migration issue.
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 * For our 20 core use cases, we plan to add targeted API's to bzrlib that are
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   repository-representation agnostic. These will instead reflect the shape of
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   data access most optimal for that case.
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 * Deprecate 'versioned files' as a library concept. Instead of asking for
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   information about a file-over-time as a special case, we will move to an API
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   that assumes less coupling between the historical information and the
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   ability to obtain texts/deltas etc. Specifically, we need to remove all
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   API's that act in terms of on disk representation except those within a
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   given repository implementation.
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 * Create a validator for revisions that is more amenable to use by other parts
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   of the code base than just the gpg signing facility. This can be done today
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   without changing disk, possibly with a performance hit until the disk
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   formats match the validatory logic. It will be hard to tell if we have the
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   right routine for that until all the disk changes are complete, so while
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   this is a library only change, its likely one that will be delayed to near
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   the end of the process.
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 * Add an explicit API for managing cached annotations. While annotations are
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   considered a cache this is not exposed in such a way that cache operations
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   like 'drop the cache' can be performed. On current disk formats the cache is
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   mandatory, but an API to manage would allow refreshing of the cache (e.g.
4031.3.3 by Matt Nordhoff
Review tweaks from Ben Finney
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   after ghosts are filled in during baz conversions).
2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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 * Use the _iter_changes API to perform merges. This is a small change that may
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   remove the need to use inventories in merge, making a dramatic difference to
2485.4.10 by Robert Collins
Review feedback.
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   merge performance once the tree shape comparison optimisations are
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   implemented.
2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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 * Create a network-efficient revision graph API. This is the logic at the
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   start of push and pull operations, which currently scales O(graph size).
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   Fixing the scaling can be done, but there are tradeoffs to latency and
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   performance to consider, making it a little tricky to get right.
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 * Working tree disk operation ordering. We plan to change the order in which
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   some operations are done (specifically TreeTransform ones) to improve
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   performance. There is already a 66% performance boost in that area going
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   through review.
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 * Stop requiring full memory copies of files. Currently bzr requires that it
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   can hold 3 copies of any file its versioning in memory. Solving this is
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   tricky, particularly without performance regressions on small files, but
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   without solving it versioning of .iso and other large objects will continue
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   to be extremely painful.
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 * Add an API for per-file graph access that alllows incremental access and is
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   suitable for on-demand generation if desired.
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 * Repository stacking API. Allowing multiple databases to be stacked to give a
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   single 'repository' will allow implementation of some long desired features
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   like history horizons, and bundle usage where the bundle is not added to the
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   local repository just to examine its contents.
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 * Revision data manipulation API. We need a single streaming API for adding
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   data to or getting it from a repository. This will need to allow hints such
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   as 'optimise for size', or 'optimise for fast-addition' to meet the various
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   users planned, but it is a core part of the library today, and its not
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   sufficiently clean to let us simplify/remove a lot of related code today.
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Interoperable disk changes
2506.1.3 by Alexander Belchenko
merge bzr.dev; fix ReST formatting in planned-performance-changes.txt
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--------------------------
2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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 * New container format to allow single-file description of multiple named
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   objects. This will provide the basis for transmission of revisions over the
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   network, the new bundle format, and possibly a new repository format as
4853.1.1 by Patrick Regan
Removed trailing whitespace from files in doc directory
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   well. [Core implemented]
2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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 * Separate the annotation cache from the storage of actual file texts and make
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   the annotation style, and when to do it, configurable. This will reduce data
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   sent over the wire when repositories have had 'needs-annotations' turned
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   off, which very large trees may choose to do - generating just-in-time
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   annotations may be desirable for those trees (even when performing
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   annotation based merges).
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 * Repository disk operation ordering. The order that tasks access data within
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   the repository and the layout of the data should be harmonised. This will
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   require disk format changes but does not inherently alter the model, so its
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   straight forward to export from a repository that has been optimised in this
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   way to a 0.16 based repository.
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 * Inventory representation. An inventory is a logical description of the shape
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   of a version controlled tree. Currently we operate on the whole inventory as
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   a tree broken down per directory, but we store it as a flat file. This scale
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   very poorly as even a minor change between inventories requires us to scan
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   the entire file, and in large trees this is many megabytes of data to
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   consider. We are investigating the exact form, but the intent is to change
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   the serialisation of inventories so that comparing two inventories can be
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   done in some smaller time - e.g. O(log N) scaling. Whatever form this takes,
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   a repository that can export it directly will be able to perform operations
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   between two historical trees much more efficiently than the current
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   repositories.
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 * Greatest distance from origin cache. This is a possible change to introduce,
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   but it may be unnecessary - listed here for completeness till it has been
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   established as [un]needed.
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Possibly non-interoperable disk changes
2506.1.3 by Alexander Belchenko
merge bzr.dev; fix ReST formatting in planned-performance-changes.txt
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2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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 * Removing of derivable data from the core of bzr. Much of the data that bzr
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   stores is derivable from the users source files. For instance the
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   annotations that record who introduced a line. Given the full history for a
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   repository we can recreate that at any time. We want to remove the
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   dependence of the core of bzr on any data that is derivable, because doing
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   this will give us the freedom to:
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   * Improve the derivation algorithm over time.
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   * Deal with bugs in the derivation algorithms without having 'corrupt
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     repositories' or such things.
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   However, some of the data that is technically derived, like the per-file
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   merge graph, is both considered core, and can be generated differently when
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   certain circumstances arive, by bzr 0.16. Any change to the 'core' status of
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   that data will discard data that cannot be recreated and thus lead to the
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   inability to export from a format where that is derived data to bzr 0.16's
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   formats without errors occuring in those circumstances. Some of the data
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   that may be considered for this includes:
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   * Per file merge graphs
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   * Annotations
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Non-interoperable disk changes
2506.1.3 by Alexander Belchenko
merge bzr.dev; fix ReST formatting in planned-performance-changes.txt
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------------------------------
2485.4.9 by Robert Collins
Create the top level changes-list from the London sprint for reference.
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 * Drop the per-file merge graph 'cache' currently held in the FILE-ID.kndx
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   files. A specific case of removing derivable data, this may allow smaller
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   inventory metadata and also make it easier to allow two different trees (in
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   terms of last-change made, e.g. if one is a working tree) to be compared
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   using a hash-tree style approach.
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 * Use hash based names for some objects in the bzr database. Because it would force
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   total-knowledge-of-history on the graph revision objects will not be namable
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   via hash's and neither will revisio signatures. Other than that though we
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   can in principle use hash's e.g. SHA1 for everything else. There are many
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   unanswered questions about hash based naming related to locality of
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   reference impacts, which need to be answered before this becomes a definite
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   item.