Troubleshooting slow performance when copying files to or from a network volume


Network performance is usually the bottleneck of a backup task that copies files to or from a network volume, but there are several other factors that can affect performance as well. Here are some suggestions for improving the performance of your NAS-based backups.

Use ethernet instead of WiFi

Backing up data over a wireless connection will be considerably slower than backing up over an ethernet connection. 802.11n networks support approximately 300 Mb/s of rated (theoretical) bandwidth under the best conditions, but they usually operate at much lower speeds (130 Mbps and below, which is comparable to 16 MB/s). Bandwidth drops considerably as you get further from the base station (a wooden door between your Mac and the router will cut the signal in half), and the file sharing protocol overhead will reduce your achievable bandwidth yet more. So practically speaking, you're lucky to get 8 MB/s over a wireless connection while sitting right next to the base station. That performance can be cut in half due to Apple Wireless Direct Link (AWDL), which causes the Airport card's interface bandwidth to be shared between your ordinary WiFi network and an ad hoc network hosted by your Mac.

We performed a simple bandwidth test to a fourth generation Airport Extreme Base Station (802.11n) to demonstrate the performance decline. We copied a 100MB file to an external hard drive attached to the base station via USB in three scenarios: 1. An ethernet connection to the base station, 2. Sitting a few feet from the base station, and 3. Sitting across the house from the base station (~35 feet, no line of sight to the base station). The results were 6.5s (15.5 MB/s), 18.7s (5.3 MB/s), and 256s (0.39 MB/s) for the three scenarios, respectively. So, before you try to back up over a wireless network, consider running a simple test in the Finder to see just how fast your connection is. If it takes more than a minute to copy a 100MB file, your connection is too slow to be practical for backup purposes.

Use Quick Update after establishing a backup of a local source

Once you have established the initial, complete backup to a destination network volume, you can use CCC's Quick Update feature to greatly reduce the length of subsequent backup tasks. When Quick Update is enabled, CCC queries the FSEvents service for a list of folders that were modified on the source since the last backup event. In many cases, this folder list is just a small fraction of the total number of folders. By limiting the scope of the task to just the modified folders, CCC will have far fewer folders to enumerate on the destination.

Related Documentation

Eject the network volume in the Finder

We have run several tests and positively identified an issue in which the Finder will make repeated and ceaseless access attempts to the items of a folder on your network share if you simply open the network volume in the Finder. This persists even after closing the window. If you eject the network volume(s), then run your CCC backup tasks, CCC will mount the network volume privately such that it is not browsable in the Finder.

Disable support for extended attributes

Most NAS volumes are very slow at working with extended attributes, so we recommend disabling this setting if you do not specifically require them to be backed up. Apple considers extended attributes to be "disposable" because some filesystems cannot support them.

CCC automatically disables this setting when backing up to or from a network volume

  1. Open CCC and select your backup task.
  2. Click the Advanced Settings button.
  3. Check the box next to Don't preserve extended attributes in the File Copying Settings tab.
  4. Save and run the task.

Try using AFP instead of SMB to connect to the NAS

Apple deprecated AFP many years ago, but it still remains faster and more reliable than SMB in many cases. We last tested this assertion on macOS Big Sur, where AFP was 30% faster than SMB. To try AFP instead of SMB:

  1. Eject the NAS volume if it's currently mounted
  2. Open CCC and select the applicable backup task
  3. Click on the Source or Destination selector (whichever references the NAS volume)
  4. Hold down the Option key and choose "Switch to AFP" (provide the credentials for the NAS volume again if prompted)
  5. Save and run the task

Avoid running tasks simultaneously if they read from or write to the same NAS device

Especially with locally-attached source volumes, CCC won't have any trouble saturating your network connection with a single backup task. If you run more than one task at the same time, especially to the same NAS device, the network connection or the NAS device may not be able to handle the load. Leverage CCC's task chaining functionality, or place your tasks into a task group so that they will be run sequentially instead.

Consider backing up to a disk image on the NAS device rather than directly to it

Network file sharing is a surprisingly CPU-intensive task. While network appliances are well suited to the task of serving media to multiple workstations, the overhead of individual filesystem transactions makes them less suited to the task of backing up millions of files. Media files, in comparison, are generally large and the required data rate for streaming media is relatively low. Consider a 1-hour, 1GB HD movie file. Streaming 1GB over the course of an hour requires only 0.27MB/s. That's an easy task, even over a weak wireless network. But if you want to back up 100GB of data in an hour, and that 100GB is made up of a million smaller files, then a network appliance may not be up to that task.

The actual bandwidth that you achieve in your backup task will be based on the number of files you're copying, the file size distribution, and the number and size of extended attributes in the source data set. Copying large files (e.g. media files) to a network volume will achieve the maximum potential bandwidth, while copying lots of small files will take quite a bit longer due to network filesystem overhead. If the data that you're backing up consists primarily of large files, e.g. music, photos, video — backing up directly to a network appliance will be fine. If you're backing up hundreds of thousands of files that are smaller than 1 MB, we recommend that you back up to a disk image on your network appliance to improve performance.

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