Disaster Recovery Tiers
Once recovery time and recovery point objectives have been established, the next step is to select a recovery solution that can match those objectives. Unfortunately, disaster recovery facilities do come at a cost. A D/R plan should not be so expensive that it outweighs any benefit gained by doing it at all. It is, after all, like an insurance policy and should be cost-justified. In a perfect world, all data and applications would be treated equally in disaster recovery planning. Unfortunately, this is financially impractical.
To assist with this selection, analysts generally aim to categorize business functions into recovery classes or tiers, with different recovery point and recovery time objectives for each tier.
The RPOs and RTOs are used to guide the disaster recovery strategy of each tier. There is no industry standard classification. However, for purposes of comparison, we will use a tier classification first presented at an IBM SHARE user conference in 1992 that’s since been extended and is still in common use in IBM circles today.
At one extreme, the highest tier, the business function and data are deemed to be so critical that the recovery point objective is to the "last committed transaction" and Recovery Time Objective is "immediate recovery".
At the other extreme is tier zero (no recovery strategy). This tier provides no preparation in saving information, establishing a backup hardware platform, or developing a contingency plan. The length of time for recovery is unpredictable. Your data can be regarded as unprotected and if you are in this tier, you really do not care about your data.
The image later in this topic of tier classification for disaster recovery solutions based on IBM SHARE definitions illustrates the various tiers of recovery on a chart of cost versus recovery time. In general, there are three main bands of application criticality; within each band, there are tiers. The tiers can be summarized as follows.
| Tier | Description |
|---|---|
| Tier 0 | No recovery capability |
| Tier 1 | Offsite backup |
| Tier 2 | Offsite backup, tape intensive recovery, business recovery service |
| Tier 3 | Electronic vaulting |
| Tier 4 | Warm standby site, fuzzy disk mirroring, point-in-time copies |
| Tier 5 | Hot standby site, software database-specific recovery |
| Tier 6 | Near zero or zero transaction loss |
| Automated takeover | |
| Tier 7 | Near zero or zero transaction loss |
| Highly automated takeover |
