Changed block tracking (CBT) offers a way of speeding up backups by scanning only those data blocks in the backup selection that have changed since the previous backup for changes, instead of whole files. CBT relies on Microsoft's Volume Shadow Copy Service (VSS), which makes it possible to identify the differences in data blocks between backups. You can enable CBT from your ESE data management agent.
- CBT is not compatible with local copy. For local copy, we read and process entire files instead of patch chains. Since the purpose of CBT is to negate the need to process entire files, the feature is not compatible with local copy. Enabling local copy will automatically disable CBT, and this will be noted in the logs.
- It is recommended that CBT be used only on systems where large files are routinely modified. CBT is not ideal for servers where files are added or removed but not modified.
How much space does CBT use?
ESE needs space for at least two shadow copies (snapshots) at all times: one of the current backup, and one of the previous backup. Because of how CBT works, a backup cannot use CBT if the shadow copy that preceded it was not persisted.
Once a backup has completed, the cleanup service removes shadow copies older than the immediately preceding copy in order to save space. Note that when Windows runs low on disk space, it aggressively cleans out shadow copies even if they occupy less than the allocated space. In such cases, ESE will fall back to regular patching until it has two shadow copies to work with again. For more information on storage options for shadow copies, see Microsoft's documentation.
How do I enable CBT?
Open the Redstor ESE Agent and go to Options and Settings > Advanced. Under VSS, tick Use changed block tracking (CBT) for faster backups.
You can also specify what should happen in cases where CBT fails (abort the backup completely or continue with a standard backup without CBT).