Why rsync is bad for backups

While rsync is an excellent tool for transferring files, it has some limitations when it comes to creating consistent backups. You want at least crash consistent backups. There must be some kind of snapshotting of the filesystem.

Why Rsync Can’t Do Consistent Backups

Here are the main reasons why rsync can’t do consistent backups:

1. File system snapshots: To create a consistent backup, you need to take a snapshot of the file system at a specific point in time. However, rsync relies on the file system’s metadata to determine which files have changed, and it doesn’t capture any information about the overall consistency of the file system.
2. Transaction logs: Modern databases use transaction logs to maintain consistency. These logs track all changes made to the database since the last checkpoint or backup. rsync can’t understand these logs or replicate them, which means it can’t ensure consistency.
3. Locking and concurrency: In a multi-user environment, multiple users might be modifying files simultaneously. rsync has no way of knowing whether a file was modified before or after the point at which you want to create a consistent backup.
4. Partial writes: When writing data to disk, many applications don’t write the entire buffer in one go; instead, they break it up into smaller chunks and perform multiple partial writes. rsync can’t detect these partial writes or ensure that all parts of a file are written correctly.

What Rsync Does Instead

While rsync can’t create consistent backups like some other tools (e.g., snapshotting software), it excels at:

1. Incremental backups: By keeping track of which files have changed, rsync allows you to perform incremental backups, significantly reducing the time and space needed for backup purposes.
2. File-level consistency: rsync ensures that each file is consistent within itself; it just doesn’t guarantee overall system consistency.

Alternatives for Consistent Backups

If you need consistent backups, consider using other tools specifically designed for this purpose:

1. Snapshots: Take regular snapshots of your file systems or volumes using software like LVM (Logical Volume Manager) or ZFS.
2. Database backup solutions: Use specialized database backup tools, such as PostgreSQL’s pg_dump or MySQL’s mysqldump, to capture the entire database state at a given point in time.
3. Backup software with consistency features: Utilize backup software that includes consistency features, like Veeam backup and replication, which can create consistent backups by taking snapshots of file systems and capturing transaction logs.