The Short Answer
Changed block tracking (CBT) is a hypervisor-level mechanism that records which disk blocks have changed since the last backup, so an incremental backup reads only those blocks instead of scanning the entire disk.
The difference it makes is not subtle. Consider a 2 TB virtual disk with 1% daily churn: without CBT, every incremental still has to read all 2 TB to discover the 20 GB that changed; with CBT, the platform hands the backup tool a map and it reads 20 GB. CBT is the reason incremental-forever strategies are possible at all — without it, "incremental" only describes what gets written, not what gets read.
CBT on VMware vs CloudStack/KVM
On VMware vSphere, CBT is a native feature: the hypervisor maintains the change map per virtual disk, and backup tools query it through the storage APIs. On KVM — and therefore on Apache CloudStack — there is no platform-wide equivalent, so the backup vendor has to build change tracking itself, per storage path, and keep it consistent across every VM lifecycle event. That is why most backup products on CloudStack quietly fall back to full-image copies.
Sendense protects QCOW2/NFS, ZFS and Linstor/DRBD through qualified snagent host-assisted changed-block paths where available. Ceph RBD, raw block, XCP-ng storage repositories and other supported paths use CloudStack-aware full-source reads and EBA repository deduplication. For eligible RAW Block protection patterns on version-aware EBA repositories, Sendense CBT can store suitable later recovery points as synthetic incrementals while still performing a full source scan and read.
The operational caveat: trust, but verify the map
One caveat applies everywhere: change tracking can be invalidated — by storage migrations, certain snapshot operations, or crashes mid-cycle. A trustworthy backup platform detects invalid tracking data and fails safely rather than trusting a stale map. With Sendense CBT, early ineligibility stays on the genuine-full path, while a late safety failure can fail the job and require the next backup to establish a genuine full baseline.
FAQ
What is changed block tracking (CBT)?
Changed block tracking is a hypervisor-level mechanism that records which disk blocks changed since the last backup, so incremental backups read only those blocks instead of scanning the whole disk. It is what makes incremental-forever backup strategies practical.
Does CloudStack have changed block tracking?
Not as a platform-wide API. Sendense uses qualified snagent host-assisted changed-block paths for QCOW2/NFS, ZFS, and LINSTOR DRBD. Other supported paths scan and read the full source and use EBA repository deduplication. For eligible RAW Block protection patterns on version-aware EBA repositories, Sendense CBT can represent suitable later recovery points as synthetic incrementals. The source is still fully scanned and read, and the first and explicitly forced fulls remain genuine fulls.
What happens if CBT data is lost or invalid?
The backup platform must detect invalid tracking and fail safely. With Sendense CBT, early ineligibility stays on the ordinary genuine-full path; a late coverage, proof, persistence, or finalization failure can fail the job and require the next backup to establish a genuine full baseline.