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How SmartScan Works in ChronoSync


SmartScan is a significant new feature that has been added to ChronoSync v4.6. It is an Advanced, Heuristic Algorithm that allows ChronoSync to detect file system changes very quickly. The amount of time spent scanning the file system can be very time consuming, especially when there are many files and folders in the sync tree. While it won't speed up file copying, SmartScan takes what can be a very lengthy part of the process and reduces it to a negligible portion of the sync as a whole.

SmartScan is enabled in the “Special File/Folder Handling” section of the Options panel:

SmartScan in Special File/Folder Handling

The option states to use SmartScan "when available". This is because not all storage devices are SmartScan compatible. Local hard drives that are HFS+ formatted qualify but removable storage devices and file servers do not. HFS+ formatted hard drives accessed over a ChronoAgent connection DO qualify, however. This is yet another advantage to using ChronoAgent versus built-in file sharing for Mac-to-Mac based syncs and backups.

Note: Once SmartScan is enabled, it will take one full run of your synchronizer document to determine if SmartScan can be used and two full runs of your synchronizer document before the the full benefits will be realized. While older sync documents (ones created before ChronoSync v4.4) can be upgraded to support SmartScan, it is generally better to create new sync documents from scratch.


The performance improvements SmartScan offers can be drastic — such as a sync that used to take hours completing in mere minutes. Or the improvements can be negligible — it all depends on the structure of the file system being scanned and, more important, how those files are used in between syncs. There are also situations where SmartScan should not be used, so understanding all these factors is important.

When is SmartScan good: When there are a LOT of files to scan, your devices are relatively slow or the connection to those devices (bus or network) is relatively slow — or any combination thereof. ChronoAgent-based syncs almost always benefit from SmartScan and, the slower the network connection, the greater the performance boost!

When SmartScan is not-so-good: If you’re running small syncs (< 100,000 files) and/or have exceptionally fast hardware, you will see minimal performance gains. Also, if your changes are spread out evenly across your sync hierarchy, ChronoSync will still have to access a large portion of your file system. SmartScan works by telling ChronoSync what parts of your file system it can avoid, so if it can’t avoid any of it, there's no performance gain.

When SmartScan should not be used: If your target volume is transported between machines, you probably should avoid SmartScan. If the volume is used on other operating systems, including versions of macOS prior to 10.8, SmartScan should definitely be left disabled. Important: One not-so-obvious example of moving a volume between systems is if a single computer reboots into different partitions that run different operating systems. This includes using Boot Camp to run Windows.

Full details on how SmartScan works and when it should and should not be used can be found in the Understanding SmartScan tech-note.

SmartScan on Target tab


As mentioned above, after enabling SmartScan, it will take one full run of your synchronizer document to determine if SmartScan can be used with a target volume. Once this has occurred, you can tell if SmartScan is being used by looking at the target details section of the Setup Panel.

If SmartScan is being used, “SmartScan Active” will be displayed.

SmartScan Target Options


From the target info pane of the Setup panel, you can click the “Options” button to configure target-specific options.

Here you will see two SmartScan specific options:

The "Allow SmartScan" option allows you turn off SmartScan on a per target basis. You may want to do this, for instance, if one of your targets is a volume that is transported between different computers but the other target isn’t — such as would be the case if you were synchronizing your internal hard drive with a portable hard disk drive. Leaving SmartScan enabled on your internal HD but disabled on your portable HD allows you to safely realize half the benefit of SmartScan.

The "Aggressive Mode" setting is an option that causes SmartScan to operate even faster at the expense of disabling some safeguards for recognizing certain kinds of (rare) file system events that may cause SmartScan to miss some changes. Details can be found in the Understanding SmartScan tech-note.