S3QL is a file system that stores all its data online using storage services like Google Storage, Amazon S3 or OpenStack. S3QL effectively provides a hard disk of dynamic, infinite capacity that can be accessed from any computer with Internet access. S3QL is a standard conforming, full featured UNIX file system that is conceptually indistinguishable from any local file system. Furthermore, S3QL has additional features like compression, encryption, data de-duplication, immutable trees and snapshotting which make it especially suitable for on-line backup and archival. S3QL is designed to favor simplicity and elegance over performance and feature- creep. Care has been taken to make the source code as readable and serviceable as possible. Solid error detection and error handling have been included from the very first line, and S3QL comes with extensive automated test cases for all its components. == Features == * Transparency. Conceptually, S3QL is indistinguishable from a local file system. For example, it supports hardlinks, symlinks, standard unix permissions, extended attributes and file sizes up to 2 TB. * Dynamic Size. The size of an S3QL file system grows and shrinks dynamically as required. * Compression. Before storage, all data may compressed with the LZMA, bzip2 or deflate algorithm. * Encryption. After compression , all data can AES encrypted with a 256 bit key. An additional SHA256 HMAC checksum is used to protect the data against manipulation. * Data De-duplication. If several files have identical contents, the redundant data will be stored only once. This works across all files stored in the file system, and also if only some parts of the files are identical while other parts differ. * Immutable Trees. Directory trees can be made immutable, so that their contents can no longer be changed in any way whatsoever. This can be used to ensure that backups can not be modified after they have been made. * Copy-on-Write/Snapshotting. S3QL can replicate entire directory trees without using any additional storage space. Only if one of the copies is modified, the part of the data that has been modified will take up additional storage space. This can be used to create intelligent snapshots that preserve the state of a directory at different points in time using a minimum amount of space. * High Performance independent of network latency. All operations that do not write or read file contents are very fast because they are carried out without any network transactions. S3QL achieves this by saving the entire file and directory structure in a database. This database is locally cached and the remote copy updated asynchronously. * Support for low bandwidth connections. S3QL splits file contents into smaller blocks and caches blocks locally. This minimizes both the number of network transactions required for reading and writing data, and the amount of data that has to be transferred when only parts of a file are read or written.