Kopia is free, open-source backup software for Windows, macOS and Linux. It saves selected files and folders as encrypted snapshots, preserving earlier states in local, network or cloud storage. These are file backups rather than images of an entire computer.
The desktop interface, KopiaUI, provides repository connections, snapshot creation, scheduling and restoration. A command-line interface uses the same backup engine and adds advanced controls that are not all exposed in the desktop interface. Both are part of the same free software.
Kopia snapshots
A snapshot records a folder’s contents at a particular point in time. Kopia keeps a history of these records, so restoring a file can mean choosing an earlier snapshot rather than retrieving only its latest backed-up state. The command line can also compare two snapshots to identify changes.
Deduplication
Kopia splits file contents into chunks and checks whether those chunks already exist in the repository. Subsequent backups upload new or changed content while reusing content already stored. A change within a large file therefore does not require another complete copy of that file.
Deduplication covers the whole repository. Renaming or moving a file does not make its unchanged contents new, and identical content from several computers can share the same stored chunks.
Compression
New content can be compressed before encryption. Policies let the user choose a compression method and control which file extensions and file sizes are eligible. Kopia includes methods such as s2, pgzip and zstd, allowing different compression choices for different data.
Snapshot policies
Policies determine which files enter a backup, when snapshots run and which historical snapshots remain. Exclusion rules can leave selected files or folders out, while a snapshot frequency turns repeated backups into an automatic task. Without a frequency, snapshots can be started manually.
Retention settings can keep different numbers of hourly, daily, weekly, monthly and yearly snapshots. A global policy supplies defaults, with more specific settings for a computer, user or source path taking precedence. This allows individual folders to have their own schedules and retention without repeating every setting.
Expired snapshots stop retaining content that no remaining snapshot needs. Kopia’s maintenance routines remove that unused content and compact stored data. Space reclamation follows maintenance safety delays, so deleting a snapshot does not necessarily free its storage immediately.
File restoration
KopiaUI lets the user open a snapshot’s directory tree and retrieve an individual file or restore the snapshot’s contents to a chosen local or network destination. Restoring to another location allows the recovered files to be kept separately from the current working folder.
A snapshot can also be mounted as a local filesystem. Its folders can then be browsed and files copied using ordinary file-management tools. The repository password is required to access the encrypted backup contents.
Encrypted repositories
The repository is where Kopia stores snapshots and their shared contents. It can use a local folder, an attached drive, NAS storage, an SFTP or WebDAV server, or services including Amazon S3, compatible S3 storage, Azure Blob Storage, Backblaze B2 and Google Cloud Storage.
The user supplies the storage location and its access credentials. Kopia does not include cloud storage with the software; any storage-provider charges are separate. Backups can also stay entirely on storage the user already controls.
Kopia encrypts file contents and filenames before uploading them. A repository password protects its encryption key, with AES-256 and ChaCha20 available as encryption choices. There is no unencrypted backup mode, and a forgotten repository password cannot be recovered to restore the data.
Repository server
Several computers can connect directly to one repository and share its deduplication. Direct sharing also shares the repository password and access to its data. For managed connections, Kopia can instead run a repository server between the clients and the underlying storage.
Server clients authenticate with their own credentials without receiving the storage provider’s credentials. Access rules control snapshot and policy records, including whose records a user can read or modify. These rules do not provide per-file ownership checks: a user who knows a valid content object ID can retrieve that content.
Backup verification
Kopia can check snapshot directory structures, indexes and the existence of stored data needed for restoration. Ordinary verification runs during full maintenance, but checking that data exists does not read and test every backed-up file.
The command line can additionally download, decrypt and decompress a selected percentage of files for content verification. That percentage can cover the entire backup, and temporary downloaded data is discarded after checking. This content-verification control is available through the command line rather than KopiaUI.
Error correction
New repositories can enable experimental Reed–Solomon error correction to add redundancy against storage errors such as bit flips. The selected overhead determines the extra storage used. This option must be chosen when creating the repository; it cannot be enabled later for an existing one.
