Related to issue #7499 and #7718.
The cache policy decides the root file system disk usage in the pod where data movement is running, and on the other hand, it also impact the restore performance significantly.
Therefore, besides storing the cache in the root file system, we should allow users to add a dedicate volume to hold the cache.

How does this issue affects the scalability:

• In many distributions, the system disk size is predefined, non configurable and limit, e.g., the system disk size may be 20G or less
• The default cache limit is 5G
• This means each node allows no more than 4 concurrent data mover restores, otherwise, they will run out of the system disk and cause significant problem that pods running in the node are evicted