The CrashPlan app stores the files backed up from your computer in a proprietary backup archive at each of your backup destinations. This article explains what an archive is, where it's located, what it contains, and how it's maintained.
How archives work
An archive is a storage folder that contains all the elements of your backup, including files, file properties, and settings. The CrashPlan app creates a unique archive for each computer you back up, and the archive is stored in its entirety at each of your backup destinations. During the backup process, files are split into blocks of data that are then de-duplicated, compressed, encrypted, and stored in a proprietary archive format at each backup destination.
The CrashPlan app creates an index of transactions (also called a manifest) in the archive to determine what needs to be backed up and what has already been backed up, as well as track backup and restore status. The CrashPlan app indexes this information about your files, settings, and versions to maintain the health of your backup.
When restoring a file, the CrashPlan app uses the indexes in the archive to find the relevant blocks of data, re-assemble them, send the file to your device, and then decompress and decrypt the file back to its original state.
There are many reasons for storing your files in an archive, the most important being security. If your files were stored in their normal state, as they may be with a cloud storage service, anyone with access to the destination could view your files. Splitting your files into blocks and encrypting the data at the source allows us to transmit and store it securely in an archive at the destination. In this state, the files are completely unreadable without first reassembling and decrypting the files, which requires you to sign in to the CrashPlan app.
Our proprietary archive format also speeds up the process of your backup by reducing the size of your files and de-duplicating the data before it is sent to the destination. This also serves to reduce storage space and conserve bandwidth.
Your computer identifies its archive at the destination by a folder marked with its computer identity. When backing up to a folder or an external drive, you can select any location you like. When you back up to a CrashPlan server, your archive is stored in our secure data centers.
The CrashPlan app divides the backup into several files at the destination and builds archives containing the following information:
|GUID file||A file named for the 18 digit GUID for your system.|
Archive properties status file
|Lists the current properties of the archive, including: size, last completed, next backup, and last archive maintenance.|
|Data containment folder||Contains the encrypted data from your backup and a manifest of all blocks of data backed up in your archive.|
File to-do queue
|Index of files remaining to be backed up.|
The file scan status
|Information on the last time the file selection was scanned. If the CrashPlan app is currently scanning, current progress is stored.|
|Index of file information for all selected files.|
|Index of block information for all selected files.|
File version history data
|File and block information for all previous versions of the selected files.|
|Backup retention policy||Information regarding frequency and versions settings for your backup.|
|A log of operations that happened on the archive.|
Archive maintenance is a regularly-scheduled task that runs on each backup destination. The purpose of archive maintenance is to maintain archive integrity and to optimize the size of the archives.
During archive maintenance, backup and restore functions are disabled. Once maintenance has completed, these functions resume normally.