As the name implies, continuous data protection is a backup format designed to operate on a continuous basis. This stands in stark contrast to legacy backup systems, which write data to tape each night. CDP technology offers numerous benefits over legacy tape backup systems, such as:
Elimination of the backup window. In the past, organizations often struggled to back up growing data sets within a finite backup window. They used various techniques, such as incremental forever backups, to use the backup window as efficiently as possible. Even so, the steadily increasing data set almost always outgrew the backup window.
CDP technology enables an organization to back up data as it is created or modified rather than accumulating it throughout the day and attempting a single, monolithic nightly backup.
Reduced risk of data loss. Legacy tape backups have a much higher recovery point objective (RPO) than CDP backup products.
Let's say an organization performs its tape backups each night at 11. If a user creates a file at 10 a.m., 13 hours will have elapsed from when the file was created to the start of the backup. Depending on how long the backup takes to run, it could conceivably be a few more hours before the file is actually backed up. If a data loss event -- such as an accidental deletion -- should occur between file creation and backup, the file could be lost with no way of restoring it.
With a CDP backup system, a file is backed up soon after it is created. Note that most CDP systems available are not truly continuous -- they simply have a very short RPO. Such systems commonly run backups every few minutes.
Another differentiator between CDP technology and legacy tape backups is the underlying backup method. Backup methodologies vary by data type, but for illustrative purposes, consider how file data is backed up. CDP and legacy backup products both write newly created files to a storage medium. However, the process works differently when it comes to protecting existing files that have recently been modified.
Suppose an organization has a 10 GB file stored on a file server. Assume the file has existed for quite some time and has already been backed up; what happens if 1 MB of the file data is modified?
If the organization is using a legacy tape backup system, the file will be backed up to tape as a part of the next backup cycle. Even if the organization performs incremental backups, the small change to the file's contents will be detected by the backup software and the entire file will be backed up.
Although each vendor has its own way of doing things, most CDP systems perform backups at the storage block level rather than the file level. This means that, like the legacy tape backup system, the CDP backup system detects the change to the file and triggers a backup. Instead of backing up the entire file, however, the CDP system backs up only the 1 MB of data that has been modified. After all, the 10 GB file already exists within the backup media, so there is no need to back it all up again.
Value in block-level backup
There are some major benefits to backing up data at the storage block level.
- Users save disk space on the backup media by not having to back up entire files each time a file modification takes place. Backing up only the blocks that have been modified helps to reduce storage costs and allows backups to complete in a timely manner. This is especially important because disk-based storage arrays have a finite capacity.
- It allows for file versioning. While almost any backup system should allow an administrator to revert a file to a previous state, CDP systems are able to do so without the hassle of trying to locate the tape that contains the correct version of the file and without storing multiple full copies of the file. Some systems are even able to restore a previous version of a file and retain the current version.
Some people are quick to point out that, while CDP technology systems offer many benefits over traditional tape-based backups, their reliance on disk-based storage limits the maximum capacity of the backup. All storage arrays, no matter how large, have finite capacities and will eventually fill up if backups are consistently performed for a long enough period of time. However, many CDP systems use storage deduplication to eliminate redundant storage blocks. While this approach can improve storage efficiency, deduplication alone won't solve the capacity problem.
To deal with limited storage capacity, many CDP systems use retention policies. These policies may, for example, define the primary storage array as short-term storage and a tape-based system as long-term storage. Data retention policies cause the CDP system to move aging data off the storage array and onto tape or some other higher capacity storage medium. This helps to keep the system's primary storage array from filling to capacity.
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