Currently Cinder supports snapshot and backup APIs. But these are not enough to implement incremental backups for Cinder volumes. The backup API backups up the entire volume when a backup is performed. Though it performs compression, it still requires to read the entire volume and upload the data to swift. In order to support incremental backups, I would like to propose a new API that returns list of blocks that are changed between two snapshots. Lets assume a Volume v1 has a point in time snapshots s1 and s2 that are taken at t1 and t2 respectively. The changed block api takes s1 and s2 as arguments and returns the list of changed blocks between s1 and s2. These changed blocks are read from S2. This will satisfy the incremental backup scenario. The backup software may perform incremental backups with respect to full backup or with respect to last incremental backup and hence the changed block API need to support changed block list between any two snapshots.
The changed blocks or the delta is represented as QCOW2 file with reference to s1 as the backing file.
