Aspera Sync FAQ

What does Aspera Sync actually do?

Aspera Sync synchronizes new and modified files and directories between remote endpoints. It moves, deletes, renames, and transfers new file contents as needed. For example:

• Moving a file out of the synchronized directory results in deletion at the remote peer.
• Moving a file into the synchronized directory results in a copy at the remote peer.
• Renaming a file in a previously synchronized directory renames the file at the remote peer; moving a file in a previously synchronized directory results in the same move operation at the peer.

For additional details on Aspera Sync's core features, see the Introduction.

How does Aspera Sync differ from rsync?

Aspera Sync is a high-speed replacement for rsync in uni-directional mode, and is designed to be a drop-in replacement with similar command-line options (async Command-Line Options). Aspera Sync also supports bi-directional and multi-directional synchronization. The following key capabilities distinguish it from rsync:

• Uses Aspera's high-speed FASP transport technology, while rsync transfers over traditional TCP.
• Operates in push, pull and bi-directional modes.
• Circumvents the typically slower comparison of the local system to the remote system over the WAN, and instead, it efficiently compares the current file system state to a snapshot of the last sync.
• Detects and implements file or directory moves and renames to avoid unnecessary transfers over the network.
• Waits for the systems to become stable (that is, it detects whether files are still being modified) before performing synchronization.

To view an example of async options versus rsync options, see rsync vs. async Uni-directional Example.

How is one-time mode different from continuous mode?

As described in Synchronization and Direction Modes, Aspera Sync offers two modes of operation: one-time ("on-demand") synchronization and continuous synchronization. When running in one-time mode, it synchronizes once and exits. In continuous mode, on the other hand, it offers constant synchronization between file systems.

Continuous mode can only be used where file system change notification (that is, inotify, which monitors file system events) is available on the systems that are running async. NFS-mounted file systems do not support inotify change notification for updates made by remote NFS clients, so in these scenarios, async should be run in one-time mode (which can be scheduled through cron). The Sync scan mode is designed for maximum speed and is fully pipelined with transfer, so as to allow for maximum performance even in one-time mode.

In what directions does Aspera Sync work?

Sync works in multiple directions: push, pull, and bi-directional. For more information about Sync direction modes, see Synchronization and Direction Modes.

• Aspera Sync supports pushing files/directories from the local system to a remote system (as does rsync), and pulling files/directories from a remote system to the local system.
• Bi-directional synchronization occurs between two endpoints, which means that file system changes occurring at either end (local or remote) are replicated on both sides.
• Multi-directional synchronization syncs between multiple endpoints in a hub-and-spoke model or a collaborative model.

How are conflicts handled in bi-directional mode?

A conflict situation can arise in bi-directional mode when a file or directory changes content, an entity is renamed before synchronization has completed, or the change occurs on both endpoints concurrently such that the "newer" version cannot be reliably determined. Aspera Sync reports such conflicts and does not modify either file system, leaving the file systems in conflict. To resolve this type of conflict, delete the file on one side and run async again.

How much space is required for an Aspera Sync snapshot?

Snapshots require up to 1 GB of disk space for every 1 million files, and an additional 1 GB for cleanup purposes. For optimum performance, Aspera recommends that the file system have at least 2 GB free per 1 million files, and 3 GB free per 1 million files on Windows (due to the poor performance of Windows NTFS when more than half of the available disk space is occupied).