# Data Model Seafile internally uses a data model similar to GIT's. It consists of `Repo`, `Branch`, `Commit`, `FS`, and `Block`. ### Repo A repo is also called a library. Every repo has an unique id (UUID), and attributes like description, creator, password. ### Branch Unlike git, only two predefined branches is used, i.e., `local` and `master`. In PC client, modifications will first be committed to the `local` branch.Then the `master` branch is downloaded from server, and merged into `local` branch.After that the `local` branch will be uploaded to server. Then the server will fast-forwardits `master` branch to the head commit of the just uploaded branch. When users update a repo on the web, modifications will first be committed to temporary branchon the server, then merged into the `master` branch. ### Commit Like in GIT. ### FS There are two types of FS objects, `SeafDir Object` and `Seafile Object`.`SeafDir Object` represents a directory, and `Seafile Object` represents a file. ### Block A file is further divided into blocks with variable lengths. We use Content Defined Chunking algorithm todivide file into blocks. A clear overview of this algorithm can be found at [http://pdos.csail.mit.edu/papers/lbfs:sosp01/lbfs.pdf](http://pdos.csail.mit.edu/papers/lbfs:sosp01/lbfs.pdf).On average, a block's size is around 1MB. This mechanism makes it possible to deduplicate data between different versions of frequently updated files,improving storage efficiency. It also enables transferring data to/from multiple servers in parallel.