Disk Chaining and Redo Logs

In VMDK terminology, all the following are synonyms: child disk, redo log, and delta link. From the original parent disk, each child constitutes a redo log pointing back from the present state of the virtual disk, one step at a time, to the original. This pseudo equation represents the relative complexity of backups and snapshots:

backup image < child disk = redo log = delta link < snapshot

A backup image is less than a child disk because a backup image is merely a data stream. A snapshot is more than a child disk because it contains virtual machine state, with pointers to associated file states on VMDK.

There exist other types of redo logs, such as those that perform progressive protection in vSphere Replication (VR). For disk chaining, the “redo” terminology is especially appropriate for the snapshot revert operation, when changed blocks in the redo log are applied to the base disk, before deleting the redo log. Afterwards the base disk contains a “redo” of all changes that the virtual machine made while the snapshot was active.