4 ms·

It's true that git is blob based, as opposed to patch based, but it's not the full picture! In practice, git stores a lot more diffs (or rather, deltas) than it

by tux3 5y ago

It's true that git is blob based, as opposed to patch based, but it's not the full picture! In practice, git stores a lot more diffs (or rather, deltas) than it stores loose blobs. (And you probably know this already, but I feel it's still worth making explicit) This is necessary, because when a repo accumulates commits, it becomes a lot more efficient to store most of the objects as deltas instead of separate blobs. If Git didn't do this, it would have a lot of copies, and they would take a lot of space. So the fundamental model of git is truly based on blobs in theory, but in practice many or most git commands will operate on packfiles, and if you look in your .git object store, most likely you will have a few big packfiles containing most objects, and then a much smaller collection of loose blobs. All those diffs are what the "resolving deltas" progress indicator that people see when they do a big clone, fetch, or checkout is about =)

ori_b 5y ago
> In practice, git stores a lot more diffs (or rather, deltas) than it stores loose blobs. The diffs it stores are not the diffs you see in git diff. They're rolling checksum based chunks. The data that the delta is computed against is picked with a heuristic ("sort by name and date, try the top 10, and use the smallest result"). And, in practice, the heuristic diffs the older files against the newer ones, rather than diffing in chronological order, so that getting recent data doesn't involve a lot of delta application. The git deltification is better thought of as a compression method than as diffing.
arghwhat 5y ago
Packfiles and deltas are a storage and transfer optimization for blobs. Any access to them store and yield blobs. It is for all intents and purposes just an internal serialization format, akin to how a filesystem is just a serialization format that makes all your data one large stream. One generally talks about the provided interface (files for a filesystem, blobs for git) rather than where the bits actually go. Compression algorithms are also to some extend diffs as they serialize to a sequence of "repeat previous segment and add this new data" commands, but it is not useful to consider them as such.

by tux3 5y ago

ori_b 5y ago

> In practice, git stores a lot more diffs (or rather, deltas) than it stores loose blobs. The diffs it stores are not the diffs you see in git diff. They're rolling checksum based chunks. The data that the delta is computed against is picked with a heuristic ("sort by name and date, try the top 10, and use the smallest result"). And, in practice, the heuristic diffs the older files against the newer ones, rather than diffing in chronological order, so that getting recent data doesn't involve a lot of delta application. The git deltification is better thought of as a compression method than as diffing.

arghwhat 5y ago

Packfiles and deltas are a storage and transfer optimization for blobs. Any access to them store and yield blobs. It is for all intents and purposes just an internal serialization format, akin to how a filesystem is just a serialization format that makes all your data one large stream. One generally talks about the provided interface (files for a filesystem, blobs for git) rather than where the bits actually go. Compression algorithms are also to some extend diffs as they serialize to a sequence of "repeat previous segment and add this new data" commands, but it is not useful to consider them as such.