What is changed block tracking (CBT)?
Changed block tracking (CBT) is a feature of hypervisors and some storage platforms that records which blocks of a virtual disk have been written to since a given point in time. Backup software queries that record and reads only the changed blocks, which is what makes incremental backups of large virtual machines fast.
How does changed block tracking work?
The hypervisor keeps a map for each virtual disk. Every write marks the affected block as changed. When a backup runs, the software takes a snapshot, asks for the list of blocks changed since the previous backup, copies only those, and records a new reference point for the next run. On VMware the feature is called CBT. On Hyper-V the equivalent is resilient change tracking.
What happens without changed block tracking?
Without a change map, the backup software has to find the changes itself. It reads the whole disk and compares each block with what it already holds. The resulting restore point is the same size, but the job reads far more data and takes much longer.
| Aspect | With CBT | Without CBT |
|---|---|---|
| Data read from source | Changed blocks only | Entire disk |
| Job duration | Proportional to daily change | Proportional to disk size |
| Load on production storage | Low | High |
| Size of the incremental | Same | Same |
What can go wrong with changed block tracking?
The change map can be reset or become invalid, for example after an unclean host shutdown, a storage migration or certain disk operations. Backup software detects this and falls back to reading the full disk for that run, which makes one night's job unusually long. Rarely, a defect in tracking has caused changed blocks to be missed, which is one reason periodic health checks and restore testing matter even when jobs report success.
How does CBT affect backup storage?
CBT determines how quickly changes are found. The amount written to the backup repository depends on the change rate itself. A system that rewrites many blocks each day produces large increments however efficiently they are detected. For sizing, the measured daily change after compression is the figure to use.
Frequently asked questions
Is changed block tracking enabled by default?
Backup software usually enables it on the virtual machines it protects. It can also be switched on manually in the hypervisor.
Does CBT work for physical servers?
Not the hypervisor feature. Agents for physical machines use their own change-tracking drivers to achieve the same effect.
Does resetting CBT lose any backups?
No. Existing restore points are unaffected. The next job simply reads the full disk to rebuild its reference.