Storage Policies, Deduplication, and Auxiliary Copy
Domain 2 - Storage Policies, Deduplication, and Auxiliary Copy
This domain tests copy design: how retention, deduplication placement and auxiliary copy interact, and why a copy chain that looks correct on paper stalls in production.
Core rules
- A storage policy has exactly one primary copy and any number of secondary copies. Each copy is bound to a storage pool (library plus MediaAgents) and carries its own retention.
- Secondary copy retention must be at least as long as the primary copy retention. If it is shorter, the auxiliary copy job is blocked - you cannot hold a copy longer than its source.
- Auxiliary copy reads completed jobs from the source copy and writes them to the destination. When source and destination MediaAgents differ, data crosses the network, so place MediaAgents to match the real data flow.
- Inline copy writes the secondary copy at backup time instead of waiting for an aux copy window. It only works well when the destination media is available and fast enough to keep up with the backup stream.
- Deduplication is block-based and tracked in the deduplication database of the writing MediaAgent. Identical blocks are stored once across all jobs that share that database; jobs written through different dedupe databases cannot dedupe against each other.
- Rehydration: copying deduplicated data to a non-dedupe copy or to tape expands it to full size, so plan destination capacity at the inflated figure.
- Synthetic full backups build a new full from an earlier full plus incrementals, avoiding another complete read of the source.
Worked example
A policy holds 30 days of deduplicated data on disk as the primary copy and needs one year on tape. Set primary retention to 30 days and secondary retention to one year, enable inline copy or schedule auxiliary copy outside the backup window, and confirm the aux copy job finishes before the primary copy begins pruning those jobs. Calculate tape capacity at full (rehydrated) size, not at the dedupe ratio, and verify the source MediaAgent can stream to the destination MediaAgent without crossing a constrained link.
Traps that cost marks
- Setting secondary retention equal to or shorter than primary and expecting aux copy to run.
- Deleting the primary copy while assuming the secondary copy stays usable and browsable.
- Sizing tape or a non-dedupe copy from the deduplicated on-disk figure.
- Assuming a dedupe database can be hosted on the CommServe or moved between MediaAgents without a rebuild.