Full volume copy
Full Volume Copy overwrites an existing volume, and at the time of its creation it is logically equivalent to the source volume.
After the copy is made, both volumes are independent of each other. Hosts can write to either one of them without affecting the other. This is somewhat similar to creating a writable (unlocked) snapshot, with the following differences and similarities:
- Creation time and availability
- Both Full Volume Copy and creating a snapshot happen almost instantly. Both the new snapshot and volume are immediately available to the host. This is because at the time of creation, both the source and the destination of the copy operation contain the exact same data and share the same physical storage.
- Singularity of the copy operation
- Full Volume Copy is implemented as a single copy operation into an existing volume, overriding its content and potentially its size. The existing target of a volume copy can be mapped to a host. From the host perspective, the content of the volume is changed within a single transaction. In contrast, creating a new writable snapshot creates a new object that has to be mapped to the host.
- Space allocation
- With Full Volume Copy, all the required space for the target volume is reserved at the time of the copy. If the storage pool that contains the target volume cannot allocate the required capacity, the operation fails and has no effect. This is unlike writable snapshots, which are different in nature.
- Taking snapshots and mirroring the copied volume
- The target of the Full Volume Copy is a Primary volume. This Primary volume can later be used as a source for taking a snapshot or creating a mirror. However, at the time of the copy, neither snapshots nor remote mirrors of the target volume are allowed.
- Redirect-on-write implementation
- With both Full Volume Copy and writable snapshots, while one volume is being changed, a redirect-on-write operation will ensure a split so that the other volume maintains the original data.
- Unlike writable snapshots, with Full Volume Copy, the copying process is performed in the background even if no I/O operations are performed. Within a certain amount of time, the two volumes will use different copies of the data, even though they contain the same logical content. This means that the redirect-on-write overhead of writes occur only before the initial copy is complete. After this initial copy, there is no additional overhead.
- Full Volume Copy can be performed with source and target volumes in different storage pools.