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

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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. If you're running Yosemite or later, that performance could 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.

Eject the network volume in the Finder

Our first recommendation is to eject your network shares in the Finder, then run your task again. 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 browseable in the Finder.

Disable support for extended attributes

If a performance issue persists despite trying the steps above, you can try dropping the extended attributes from the source. While it is our preference to preserve extended attributes, Apple considers extended attributes to be "disposable" because some filesystems cannot support them.

  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 Troubleshooting Options box.
  4. 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 system files or applications, or many files that are smaller than a few MB, we strongly recommend that you back up to a disk image on your network appliance to improve performance and to maintain important filesystem metadata.