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How Backup Works

Applies to:

    Overview

    This article explains the concepts that CrashPlan uses to back up your data.

    Backup basics

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

    Home Directory Selected For Backup

    CrashPlan 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.

    One File On Backup To-Do List

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

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

    Backup Process: New File

    Data is securely encrypted throughout this process.

    The process repeats for the next block within the file until CrashPlan 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 de-duplication
    Data de-duplication 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, CrashPlan's real-time file watcher sees that the file has changed, and CrashPlan puts the file back into the to-do list. Only the changes are actually sent to the destination, however, 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):

    Backup Process: File Changes

    1. CrashPlan's data de-duplication scans the file looking for new blocks of 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 CrashPlan detects changes

    CrashPlan checks for changes in two ways to make sure your 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. CrashPlan 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 CrashPlan backs up to multiple destinations, it sends files to each destination at the same time. This ensures that each destination has the same collection of files.

    Does my backup start over?

    Occasionally, CrashPlan's data de-duplication needs to re-scan your files to see what's already backed up. When this happens, it may look like CrashPlan is backing up all your files from the beginning, but it is actually reviewing each block to see what's backed up already. If CrashPlan 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. Go to device preferences:
        • Windows: Select View > Options
        • OS X: Select CrashPlan > Device Preferences
      3. Select Destinations.
      4. From the list of existing destinations, select the destination containing the archive you are verifying.
      5. Verify that the Space used is reasonable for your file selection size and previous backup completion.
    CrashPlan's cache
    CrashPlan's cache includes information on de-duplicated data. You'll experience the above behavior if CrashPlan needs to rebuild its cache for any reason. This is something that happens on occasion under normal use.