Git Rev News: Edition 30 (August 16th, 2017)

Welcome to the 30th edition of Git Rev News, a digest of all things Git. For our goals, the archives, the way we work, and how to contribute or to subscribe, see the Git Rev News page on git.github.io.

This edition covers what happened during the month of July 2017.

Discussions

Reviews

Shawn Pearce sent an email describing a new format to store refs. The email started with:

We’ve been having scaling problems with insane number of references (>866k), so I started thinking a lot about improving ref storage.

I’ve written a simple approach, and implemented it in JGit. Performance is promising:

  • 62M packed-refs compresses to 27M
  • 42.3 usec lookup

This is not the first time that someone proposes a new format to store refs, and Shawn summarized a previous attempt by David Turner:

David Turner proposed using LMDB, as LMDB is lightweight (64k of runtime code) and is licensed under a GPL-compatible license.

A downside of LMDB is its reliance on a single C implementation. This makes embedding inside JGit (a popular reimplementation of Git) difficult, and hosting onto virtual storage (for JGit DFS) virtually impossible.

A common format that can be supported by all major Git implementations (git-core, JGit, libgit2) is strongly preferred.

Shawn also referenced a previous proposal he had worked on:

JGit Ketch proposed RefTree, an encoding of references inside Git tree objects stored as part of the repository’s object database.

He describes the reftable format like this:

A reftable file is a portable binary file format customized for reference storage. References are sorted, enabling linear scans, binary search lookup, and range scans.

Storage in the file is organized into blocks. Prefix compression is used within a single block to reduce disk space. Block size is tunable by the writer.

There are a lot more details in his email and the proposal looked very advanced.

Jeff King, alias Peff, was the first to reply to Shawn. He reviewed the whole email and he summarized what he understood from Shawn’s proposal:

The reftable file is a sequence of blocks, each of which contains a finite set of heavily-compressed refs. You have to read each block sequentially, but since they’re a fixed size, that’s still a constant-time operation (I’m ignoring the “restarts” thing for now). You find the right block by reading the index.

One of his concern was about updating refs:

Updates are where things get dicier. It looks like you just write a new partial reftable file with your updates. And then if there are N reftables present, readers actually have to do a list-merge of the results they get from all of them (where the results from reftable.5 trump ones from reftable.4).

and another one was about reflogs:

One thing I didn’t see is reflogs. They don’t strictly have to be part of a ref-storage solution. But they do still consume at least one inode per ref in the current system.

But overall Peff seemed happy with the performance and the idea.

Stefan Beller replied to Peff. As Stefan works at Google with Shawn, he had already discussed the proposal and gave more details about some points Peff had commented on. He also suggested Shawn give “the whole thing in BNF format from top down” starting with:

initial-block content-blocks* (index-block) footer

Peff replied to Stefan about some points especially regarding gzip compression and agreed that “a high-level overview of the format would have been nice”.

Eric Wong then suggested using “an intrusive critbit tree” to store refs as it could save space and speed up searching for a SHA-1.

Shawn replied to Stefan. He agreed about describing the format using BNF from top down. About gzip compression he showed some numbers and wrote:

The reftable format (for 64k block, 256 restart) is within spitting distance (432 KB) of a default level gzip of packed-refs. We can get fast lookup, and OK compression.

Shawn also replied to Peff’s first email. About reflog support, a short subthread of the discussion started where Dave Borowitz chimed in. The subthread concluded that the reftable format should indeed try to support reflog entries.

About updating refs which was Peff’s other big concern, Peff replied to Shawn suggesting that each reftable points to previous reftables. And then Shawn agreed that it “makes for a very safe atomic reader view”.

Peff eventually suggested to keep the list of reftables in a top-level pointer file, and to rewrite that pointer file on update.

Johannes Sixt agreed with Peff concerns over updates in Shawn’s initial proposal saying:

One of the failure modes is that on Windows a file cannot be deleted while it is open in any process. It can happen that a compacting updater wants to remove a reftable file that is still open in a reader.

Michael Haggerty also replied to Shawn’s initial proposal. Michael first asked if pseudorefs, like HEAD, FETCH_HEAD, should also be stored in reftables, or only the references under refs/.

Micheal then commented on a lot of details. He suggested an algorithm so that reference repacking can be done without blocking other writers.

Shawn replied to Michael. He agreed to use what the suggested algorithm to repack without blocking, but he was not sure about storing pseudo refs in reftables.

Dave Borowitz and Junio Hamano, the Git maintainer, also commented on parts of this discussion.

While the discussion was going on with Michael, Shawn posted a second version of his proposal. It started with:

Biggest changes from the first proposal are:

  • reflog is now integrated into reftable
  • block headers slightly different
  • Peff’s stack management idea is used
  • Michael’s compaction idea is used

The structure of the files was also better documented using the BNF.

Stefan was the first to reply, followed by Junio. Shawn then answered all of their comments. One point that Stefan and Junio both raised was how modification time was stored in the reflog.

Initially Shawn had planned to store them as seconds since the epoch in a 32 bit integer, but that wouldn’t have worked after 2038. So Shawn agreed after also discussing with Michael to have them in an 8 byte field storing microseconds since the epoch which will work “through year 9999”.

Ævar Arnfjörð Bjarmason though chimed in to suggest using 64 bit nanosecond resolution which would “only be good up until the year 2554”, but has other benefits like keeping a one-to-one mapping between file modification time and reflog entries and standardizing on fewer time formats.

Shawn then posted a third version of his proposal starting with the following:

Significant changes from v2:

  • efficient lookup by SHA-1 for allow-tip-sha1-in-want.
  • type 0x4 for FETCH_HEAD, MERGE_HEAD.
  • file size up (27.7 M in v1, 34.4 M in v3)

The file size increase is due to lookup by SHA-1 support. By using unique abbreviations its adding about 7 MB to the file size for 865,258 objects behind 866,456 refs. Average entry for this direction costs 8 bytes, using a 6 byte/12 hex unique abbreviation.

There were more comments from Stefan and Michael, which after short discussions resulted in improvements. So Shawn posted a fourth version starting with:

Significant changes from v3:

  • Incorporated Michael Haggerty’s update_index concept for reflog.
  • Explicitly documented log-only tables.
  • Magic changed to 'REFT'
  • Symlinks now use type 0x3 with "ref: " prefix.
  • Ref-like files (FETCH_HEAD, MERGE_HEAD) also use type 0x3.
  • restart_count simplified, obj block_count simplified

This round led to a lot of discussion. Michael proposed another format altogether. Junio commented on a few things and was worried about the impact of the new format on clients were reading a single ref currently need to open at most 2 files. Dave Borowitz asked about the git stash implementation.

Michael’s proposal started a significant subthread where Peff, Junio and Dave chimed in.

Shawn recently posted a 5th version with the following changes:

Significant changes from v4:

  • Supported Michael Haggerty’s multi-level index.
  • Restart table now appears at start of block, instead of end.
  • The restart_offset is now 3-bytes, instead of 4-bytes.
  • Footer stores obj_id_len abbreviation used by obj blocks.
  • Clarified log-only files can exist.
  • Clarified use of position for byte in file, offset for byte in block.
  • Clarified peeling, and reference name encoding as bag of bytes.
  • Corrected extensions.reftable value to be true.

The comments on this round were mostly about LMDB, as Ævar found a Java binding that could be used. These discussions involved Peff, David Turner and Howard Chu, the Chief Architect of OpenLDAP and LMDB creator.

It turned out that LMDB might not work well on NFS. Also Google (for which Shawn is working) needs something that can be virtualized onto a different filesystem in user space, and it looks like LMDB doesn’t fit this requirement.

So Shawn posted a 6th version:

Changes from v5:

  • extensions.refStorage = reftable is used to select this format.
  • Log records can be explicitly deleted (for refs/stash).
  • Log records may use Michael Haggerty’s chained idea to compress before zlib. This saved ~5.8% on one of my example repositories.

There were a few comments by Stefan, Peff and Junio, but it looks like there is an overall agreement on the design at this point.

Developer Spotlight: Shawn Pearce

  • Who are you and what do you do?

I am an open source developer who really loves working on developer tools. I like making others more happy and productive, making it that much easier for them to change the world.

  • What would you name your most important contribution to Git?

Three things come to mind: git fast-import, git gui, and the offshoot projects I started: libgit2, JGit, EGit, Gerrit Code Review. Sorry, it’s hard to choose just one.

git fast-import format became the lingua franca of version control systems, helping users to move history between CVS, Subversion, Hg, Bazaar, BitKeeper, and of course Git. I originally created the format to help Jon Smirl import the Mozilla repository from CVS into Git. It was so versatile that its adoption has exploded.

git gui has stuck around as a compliment to the awesome gitk, helping users to manage their working directory. I wound up abandoning the project due to lack of time, but several others like Pat Thoyts stepped in and continued fixing bugs. I love open source. :)

I’m most proud of establishing some offshoot projects: JGit, EGit and Gerrit Code Review. These are 3 separate open source projects in their own right, with their own communities of contributors and users. Gerrit Code Review in particular has seen a lot of adoption in the enterprise, and by only supporting Git, has certainly helped to bring Git into many organizations.

  • What are you doing on the Git project these days, and why?

I spend most of my time on JGit and Gerrit Code Review, but this past month I started drafting a proposal for a new file format called reftable, which I hope will replace the existing $GIT_DIR/refs and $GIT_DIR/packed-refs with a more scalable format.

  • If you could get a team of expert developers to work full time on something in Git for a full year, what would it be?

This is a trick question! My $DAY_JOB at Google is to manage a team of developers who work on JGit, Gerrit Code Review, and of course git-core. Google has been investing in the Git ecosystem for many years, and we look forward to continuing our contributions.

This year, part of my team has been working on two focus areas: git submodule improvements (Stefan Beller, Brandon Williams) and lazy clone (Jonathan Tan, Jonathan Nieder). For the latter project we’ve also been working with Ben Peart, and his co-workers at Microsoft.

I’d like to see Git upgrade its hash function. SHA-1 has had a good run for us, but it’s time to start thinking about the next decade. Jonathan Nieder has written an excellent proposal of how to make the transition. I think it’s a matter of implementation effort at this point.

  • If you could remove something from Git without worrying about backwards compatibility, what would it be?

The !*@!*! wire protocol. The server speaks first with an ASCII listing of all of its references is causing scaling bottlenecks for enterprise uses.

There are plans afoot to fix this. Jeff “Peff” King has some ideas, as does Jonathan Tan.

  • What is your favorite Git-related tool/library, outside of Git itself?

Gerrit Code Review, of course. :)

Releases

Other News

Various

Light reading

Git tools and sites

Credits

This edition of Git Rev News was curated by Christian Couder <christian.couder@gmail.com>, Thomas Ferris Nicolaisen <tfnico@gmail.com>, Jakub Narębski <jnareb@gmail.com> and Markus Jansen <mja@jansen-preisler.de> with help from Stefan Beller <sbeller@google.com> and Shawn Pearce <spearce@spearce.org>.