threads / discuss / 15726

How Blobs Work ( Blobs Vs. Deltas)

Subject: How Blobs Work ( Blobs Vs. Deltas)

## tl;dr

4 messages between Sep 30, 2008 and Sep 30, 2008.

replies: 3people: 4as markdown or json

Feanil Patel· Sep 30, 2008, 15:14 UTC · lore

Hello,

I was reading about git objects on The Git Book(http://book.git-scm.com/1_the_git_object_model.html) which was posted on the mailing list a while back and I was wondering something about blobs and how files are stored in any particular version. If file A is changed from version one to version two there are two different blobs that exist for the two versions of the file, is that correct? The Book was saying Git does not use delta storage so does this mean that there are two almost identical copies of the file with the difference being the change that was put in from version one to version two?

-Feanil

Bruce Stephens· Sep 30, 2008, 15:28 UTC · re: Feanil Patel · lore

Re: How Blobs Work ( Blobs Vs. Deltas)

"Feanil Patel" <feanil@gmail.com> writes:

> I was reading about git objects on The Git
> Book(http://book.git-scm.com/1_the_git_object_model.html) which was
> posted on the mailing list a while back and I was wondering something
> about blobs and how files are stored in any particular version.

[...]

> The Book was saying Git does not use delta storage so does this mean
> that there are two almost identical copies of the file with the
> difference being the change that was put in from version one to
> version two?

There might be. git may also end up using deltas, see <http://book.git-scm.com/7_the_packfile.html>.

Johannes Sixt· Sep 30, 2008, 15:29 UTC · re: Feanil Patel · lore

Re: How Blobs Work ( Blobs Vs. Deltas)

Feanil Patel schrieb:

Show 10 quoted lines

> I was reading about git objects on The Git
> Book(http://book.git-scm.com/1_the_git_object_model.html) which was
> posted on the mailing list a while back and I was wondering something
> about blobs and how files are stored in any particular version.  If
> file A is changed from version one to version two there are two
> different blobs that exist for the two versions of the file, is that
> correct?  The Book was saying Git does not use delta storage so does
> this mean that there are two almost identical copies of the file with
> the difference being the change that was put in from version one to
> version two?

At the conceptual level, yes. An entire file (== blob) is the smallest unit that you can address. Even git's internals do not work with smaller units.

But there is, of course, a mechanism that stores the database in a more compact format, the so-called pack files, that basically store differences between files as much as possible.

-- Hannes

Jakub Narebski· Sep 30, 2008, 18:54 UTC · re: Feanil Patel · lore

Re: How Blobs Work ( Blobs Vs. Deltas)

"Feanil Patel" <feanil@gmail.com> writes:

Show 12 quoted lines

> Hello,
> 
> I was reading about git objects in "The Git Community Book"
> (http://book.git-scm.com/1_the_git_object_model.html), which was
> posted on the mailing list a while back, and I was wondering something
> about blobs and how files are stored in any particular version.  If
> file A is changed from version one to version two there are two
> different blobs that exist for the two versions of the file, is that
> correct?  The Book was saying Git does not use delta storage so does
> this mean that there are two almost identical copies of the file with
> the difference being the change that was put in from version one to
> version two?

In Git there are two kinds of storage: loose objects and packs. Each object generally starts as a loose object; for those it is like you wrote: if you have two versions of some file, you would have both of those contents of a file stored as separate objects (blobs). Note that those 'blob' objects are compressed, so they usually don't take more time than current version of file and its backup.

But there exists also other type of storage, namely packed. In the past you had to pack (repack) objects by invoking "git repack" and "git prune", and in more modern times by calling "git gc"; nowadays this should be taken care of by git using "git gc --auto" behind. When packing git tries to find objects which are close contents, and store them as base object and binary delta (based on LibXDiff). So you get benefits of delta storage, while on the API and script level you always see single objects.

Note that explicit repacking allow git to not only consider versions of the same file to diff against, tree and not only linear chains of deltas (think branches), and while recency order is preferred it is not enforced; objects and deltas are then compressed individually.

HTH

-- 
Jakub Narebski
Poland
ShadeHawk on #git