GIT Performance question

Hello,

Our company is evaluating SCM solutions, one of our most important requirements is performance as we develop over 3 differents sites across the world. I read that GIT doesn't use deltas, it uses snapshots. My question is: how could GIT have high performance (most of the users say that) if for synchronization (pull/push command) with e.g. a shared repository GIT transfers all modified files (and references) instead of the respective deltas?

Thanks in advance,

Santos

On Apr 17, 2010, at 05:55, santos2010 wrote:

Git does use deltas between snapshots.

  -Geert

On Sat, Apr 17, 2010 at 01:55:49AM -0800, santos2010 wrote:

Short answer: Git does store and transfer deltas. It generally beats any other system in terms of repo size.

Longer answer:

Git separates the concept of the history graph and the actual storage mechanism. So conceptually the history is a directed graph of snapshots, each representing the whole tree. But there are two things that save space:

  1. Git addresses content by its sha1. So each snapshot may refer to a
     file by the sha1 of its content, meaning we only have to store that
     content once.

  2. Git packs "objects" (where each file's content is in a single
     object) into "packfiles", in which it aggressively deltas objects
     against each other, including objects which do not come from the
     same path in your tree.

Git will store "loose" objects when performing most operations, but will occasionally pack when the number of objects get too high. You can also initiate a full pack by running "git gc".

For transferring between repositories, git will figure out which parts of the history each side has, and will only send the objects that the other side needs. In addition, it will send them as a packfile using delta compression, including deltas against objects that are not being sent but that it knows the other side has.

-Peff

On Sat, Apr 17, 2010 at 01:55:49AM -0800, santos2010 wrote:

Well, Git _does_ use deltas for storage and synchronization, but this deltas are unrelated to history of changes stored in the repository. So, conceptually, Git just stores snapshots, but files in those snapshots are deltified against some old files based on some heuristic of finding similar files, which allows Git to create deltas not only to previous version of the same file (which most VCSes do), but potentially to any file stored in the repository if it similar enough. So, typically, Git has the most compact storage comparing to other VCSes, in particular, in case of complex history with a lot of branches and merges.

Dmitry

Thanks a lot for the quick answer. Are there some references (on-line books or web pages) where i could find details about this approach? I need this to justify my evaluation :)

On Sat, Apr 17, 2010 at 03:21:13AM -0800, santos2010 wrote:

Try the "Git Internals" chapter of Scott Chacon's Pro Git book, which is available online here:

  http://progit.org/book/ch9-0.html

He has some pretty pictures, too.

-Peff