This article describes how data deduplication and data compression increase the efficiency of the CrashPlan app's backup process and decrease the amount of storage space needed while still preserving your ability to restore the files to their original state.
Data deduplication is a form of single-instance storage, which means that we do not store the same information twice, even if you have the exact same file (or part of a file) duplicated in two or more places on your computer.
For instance, your email may contain many instances of the signature file that is attached to your emails. Without deduplication, each instance of your signature would be saved independently when backing up your email, taking up additional space with multiple copies. With data deduplication, only one instance of the signature is actually stored in your backup, and additional instances point to the stored copy.
How does deduplication work?
The CrashPlan app uses block-level deduplication when backing up your files, which splits the files into smaller blocks of data before sending them to your backup destination. During the initial backup of your files, all of the unique blocks of data are transferred to the destination.
If there are duplicate versions of the same file on your computer, the CrashPlan app detects the duplicate blocks of data and does not send them again. If the file changes, only the changed blocks are transferred. In the example below, only the shaded blocks of data would be sent to the destination.
This makes block deduplication very efficient. The CrashPlan app uses block-level data deduplication in conjunction with compression to optimize storage space at each destination and reduce the bandwidth required for your backup.
When does deduplication occur?
The CrashPlan app deduplicates your data any time a backup occurs. In addition, there are times when deduplication is triggered as part of a file verification scan.
By default, the scan is set to automatically run every day, but this schedule can be changed by updating the Preferred time for verification scan setting in device backup settings. The CrashPlan app periodically runs additional verification scans to detect data corruption, purge files that are no longer selected for backup, and prune file versions and deleted files according to your frequency and version settings.
In addition, there are times when a file verification scan is automatically triggered:
- Changing the file selection: If you update your backup file selection, either to add or remove files, the scan runs to look for new, changed, or deleted files.
- Replacing your device: After replacing a device (for example, if you recently changed devices), the scan runs to compare the files on your device to the files in your existing backup archive.
- Clearing your cache: The cache includes information about your destinations and the data on your device. When the cache is cleared, a file verification scan initiates to help rebuild this information.
- Attaching an external drive: When you attach an external drive, the scan runs to compare the files on the drive to the files in your existing backup archive.
- At device reconnection: If the device is powered off or asleep at the scheduled scan time, the scan runs 15 minutes after the device reconnects.
- Manually: The file verification scan can be triggered at any time from the Backup Set Settings Menu.
Does my backup start 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:
- Open the CrashPlan app.
- Select Settings > Destinations to navigate to the list of destinations.
- From the list of existing destinations, select the destination containing the archive you are verifying.
- Verify that the Space used is reasonable for your file selection size and previous backup completion.
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.
The CrashPlan app analyzes, compresses, and encrypts your data before sending it to your backup destinations. Data compression is the process of reducing file size by encoding the data in a more efficient way. There are many algorithms that can be used to compress and decompress data, which fall in to two categories: lossless, and lossy (used to make MP3s and other media smaller). The CrashPlan app losslessly compresses your data before sending it for backup.
How does compression help?
When the CrashPlan app identifies new or changed data in a file, it breaks the data into blocks and compresses each block. The smaller file size increases your effective transfer rate, which makes both backing up and restoring the files faster.
By reducing the file size, compression also reduces the amount of storage space needed at the destination. Between compression and deduplication, CrashPlan app can save a significant amount of disk space. However, data savings can vary greatly based on the type of files being backed up. For example, text documents compress extremely well, but movies do not. Typically, we observe 10-30% savings in disk space as a result of compression and deduplication.
Does compression harm my data?
The CrashPlan app uses lossless compression when backing up your data, so your data can be restored to the state that it was in prior to being backed up. This differs from lossy compression:
- Lossless compression reduces file size by identifying and eliminating any redundant data within the file and minimizing wasted space. No data is lost in lossless compression.
- Lossy compression uses partial data discarding to represent the content being encoded. Discarding the data reduces the quality of images, videos, and music. Using this compression type reduces the size of the file by discarding some details of the file that were in the original, such as reducing the number of colors displayed in an image.