How backup works

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To help protect your data from loss, CrashPlan uses backup to automatically collect and store every version of every file across all your computers. This article explains the concepts that CrashPlan uses to back up your data.

Backup basics

In this example scenario, the CrashPlan app is using its default settings and is backing up your user home directory.

The CrashPlan app constantly watches for new and changed files within your home directory with what we call the real-time file watcher. It adds new and changed files to a to-do list. When you create a document, the real-time file watcher sees that you've created this document and adds it to the to-do list for backup.

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This is what happens when the CrashPlan app starts backing up your document:

  1. Backup begins with a process called data deduplication. The CrashPlan app analyzes a small piece of the file (a block), and checks to see if that block was backed up previously.
  2. If that block was already backed up, the CrashPlan app moves on and analyzes the next block.
  3. If the block has not yet been backed up, the CrashPlan app:
    1. Compresses the block to save storage space
    2. Encrypts the block to secure the data
    3. Sends the block to the backup destination


Data is securely encrypted throughout this process.

The process repeats for the next block within the file until the CrashPlan app has analyzed and backed up the entire file. In this way, only unique information is backed up, which saves bandwidth and storage and makes restoring faster.

Data deduplication
Data deduplication occurs on each computer. If you have the same file on two different computers, the file will be backed up twice—once for each computer.

New files and file changes

When you make changes to the document, the CrashPlan app's real-time file watcher sees that the file has changed and puts the file back into the to-do list. Only the changed blocks are sent to the backup destination, not the entire file. The changes are backed up while you work, creating a new version of your document.

In this example, you've added a paragraph (highlighted in red):


  1. The CrashPlan app scans the file looking for blocks of new data.
  2. The new (red) data blocks are:
    1. Compressed to save space
    2. Encrypted for security
    3. Transmitted to the backup destination for storage

How the CrashPlan app detects changes

The CrashPlan app checks for changes in two ways to make sure the changes are backed up:

  • A real-time file watcher:
    • Uses few resources because it works directly with your computer's operating system.
    • Works in the background without you noticing.
  • File-system scan:
    • Requires more resources.
    • Runs at 3 am (by default) to avoid interfering with you while you're using your computer.

Prioritize files for backup

Of course, you probably have more than one file on your computer that you'd like backed up. The CrashPlan app backs up the newest and most recently changed files first. This ensures that the most recent versions of your files—what you're working on right now—are backed up first, using this priority order:

  1. Newer, smaller files
  2. Newer, larger files
  3. Older, smaller files
  4. Older, larger files

Specify backup priority

If you choose to enable backup sets, you can specify the priority of each set. This allows you to specify which files should be backed up first, if your situation doesn't fit the default prioritization above.

Multiple backup sets with a single destination
When multiple backup sets back up to a single destination, there are some special considerations for nonstandard backup settings.

Back up to multiple destinations

When the CrashPlan app backs up to multiple destinations, it sends files to the destination likely to complete the backup first, then sends files to the next destination. Once all backups are complete, each destination will have the same collection of files.

Frequently asked questions

Why is my data stored size different than the data selected size?

If you compare the number of files on your computer to the number of files the CrashPlan app says it's backing up, you may notice that the size of data stored does not match the size of data selected for back up. There are a few reasons for this size difference:

  • The CrashPlan app detects hidden files, which your computer doesn't display by default, and backs them up alongside visible files.
  • Your backup archive includes multiple versions of your files and it also includes deleted files.
  • Data deduplication and compression during backup may reduce the data stored size.

For more information on this discrepancy, see Why does the CrashPlan app show a different number of files than my computer?

If you are concerned by a large difference between the two sizes, you can verify that all of your files are backed up: 

  1. In the CrashPlan app, go to restore files
  2. Select your device’s name to navigate to your device’s root folder. This selects all of your backed up files.
  3. The CrashPlan app calculates and displays the total size. If this size is close to the selected size, all of your files are backed up.

Is my backup starting over?

Occasionally, the CrashPlan app needs to re-scan your files to see what's already backed up, for data deduplication. When this happens, it may look like the CrashPlan app is backing up all your files from the beginning, but it is actually reviewing each block to see what has been backed up already. If the CrashPlan app is re-scanning your files, you may see one or more of the following:

  • Progress is much, much faster than a full initial backup because information that is already backed up is not re-sent.
  • All your files are available for download during this process.
  • The amount of space used by your backed up files is consistent with the size of your file selection and backup completion percentage. To verify the amount of space used:
    1. Open the CrashPlan app.
    2. From Home, select Settings > Destinations.
    3. From the list of existing destinations, select the destination containing the archive you are verifying.
    4. Verify that the Space used is reasonable for your file selection size and previous backup completion.
The CrashPlan app's cache
The CrashPlan app's cache includes information on deduplicated data. You'll experience the above behavior if the CrashPlan app needs to rebuild its cache for any reason. This is something that happens on occasion under normal use.

Can I backup a backup?

You cannot use the CrashPlan app to back up a CrashPlan backup archive. The CrashPlan app is most efficient at backing up your original files. By encrypting and compressing the files on your computer, the CrashPlan app creates a dense archive for storage. Including the entire archive again in your backup selection has several drawbacks:

  • Increased system resources required to re-compress and re-encrypt the archive
  • Inability to use real-time backups
  • In order to access a single file, you would need to restore the entire archive, which could take a very long time
  • Archive maintenance cannot be performed properly

Due to these restrictions, we automatically exclude CrashPlan archives from your backup selection. If you want additional copies of your archive for redundancy, consider backing up to multiple destinations.

You may be able to back up archives created by other backup applications with the CrashPlan app. However, most backup applications used alongside the CrashPlan app, like Time Machine, should operate independently. There are special considerations if you are backing up really large files. Additionally, if the software uses soft or hard links to create the backup archive, it can create conflicts with the processing of your CrashPlan archive. Consult the manufacturer's documentation to determine whether or not the service is compatible with the CrashPlan app.

You may want to ensure that you have a backup of the data that you have stored on other cloud services. See Backing Up Cloud Services for more information.

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