Branch Office DD3300 Sizing and WAN Replication Planning
A retail company plans a new backup appliance for a branch office. The branch has 60 TB of full backup data, a 10% daily change rate, 14-day retention, and an estimated 6:1 deduplication ratio. The disaster recovery requirement is an RPO of 6 hours to a central DD9400. The WAN circuit is 200 Mbps with 30 ms latency and is shared with voice traffic; the carrier guarantees 60% usable bandwidth during business hours. The branch rack has 6U free, one 20A 120V circuit, and no redundant power. The proposal lists a DD3300 with 8 TB usable capacity, a single power supply, and replication over the existing WAN. The branch has 120 virtual machines and 40 physical SQL servers. The customer wants to avoid adding a second WAN circuit and asks whether the DD3300 will meet capacity and RPO, and what site-prep issues must be fixed before installation.
1. Calculate whether the proposed DD3300 capacity is sufficient. State your assumptions and the recommended action.
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Use a working-set estimate rather than the 60 TB full alone. With 14-day retention and 10% daily change, the logical data is roughly the full plus 13 days of change: 60 TB + (13 x 6 TB) = 138 TB. At 6:1 deduplication that is about 23 TB of deduplicated data. Add 20% headroom for metadata, snapshots, cleaning, and growth, giving roughly 27.6 TB usable. The proposed 8 TB DD3300 is therefore far too small, even if some daily changes are absorbed by deduplication. The DD3300 also has limited expansion and stream capacity compared with larger models.
The correct action is to resize to a platform that can present at least 28-30 TB usable with growth, such as a DD6400 or DD6900 with the appropriate capacity license, or to use a smaller active tier plus DD Cloud Tier if retention and recall requirements allow it. Do not approve the 8 TB unit on the assumption that 6:1 deduplication will hold for all data; deduplication varies by workload, and SQL backups may deduplicate differently from VM backups.
2. Calculate the replication bandwidth required to meet the 6-hour RPO and determine whether the existing 200 Mbps circuit is adequate.
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Daily change is 10% of 60 TB = 6 TB. At 6:1 deduplication, the replicated change is about 1 TB per day. One TB is 8,000 Gb, so at a dedicated 200 Mbps link the transfer time is 8,000 / 200 = 40,000 seconds, or about 11.1 hours. The carrier guarantees only 60% usable bandwidth during business hours, so effective throughput is about 120 Mbps, giving 8,000 / 120 = 66,667 seconds, or about 18.5 hours. Both exceed the 6-hour RPO.
To meet 6 hours, the required rate is 8,000 Gb / 21,600 s = about 370 Mbps dedicated, or roughly 617 Mbps if only 60% is usable. That is well beyond the existing circuit. Options are: add a dedicated WAN circuit or increase bandwidth to at least 400-600 Mbps; reduce the daily change or increase deduplication; replicate more frequently so the per-window change is smaller; use DD Replicator low-bandwidth optimization and QoS; or accept a longer RPO. The existing 200 Mbps shared circuit cannot meet the stated RPO.
3. Identify the site-preparation issues in the branch and explain what must be corrected before installation.
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The rack has only one 20A 120V circuit and no redundant power. A production DD appliance should have dual power feeds, ideally from separate PDUs or UPS sources. The proposal's single power supply is not acceptable for the stated DR posture; specify dual power supplies and confirm the branch electrical capacity supports the appliance plus the existing load. Verify heat load and airflow; the rack must have enough free U space for the appliance and any future expansion shelf, and the floor must support the weight.
Network preparation also needs attention: confirm 10 GbE data ports, a separate management network, VLAN assignment, and switchports configured for the planned bonding and MTU. The WAN link must be tested for latency, loss, and throughput, and QoS should reserve replication bandwidth or schedule replication outside voice peaks. Management access, DNS, NTP, and email alerts must be available before handoff. Finally, validate that the chosen model can be licensed for the required capacity and that the replication target has enough capacity and stream limits for this branch.