An organization is designing a disaster recovery (DR) solution from the Azure North Europe… — Microsoft Azure Solutions Architect (AZ-305)

An organization is designing a disaster recovery (DR) solution from the Azure North Europe region to the Azure West Europe region for a critical three-tier application.

The application workloads and performance metrics are:
- Web and Application Tiers: 4 VMs in total, each running on Standard SSDs. The write churn per VM is $3\text{ MB/s}$ . The required recovery point objective (RPO) is $1\text{ hour}$ , and the recovery time objective (RTO) is $4\text{ hours}$ .
- Database Tier: 2 VMs running SQL Server Enterprise Edition on Windows Server. Each VM has:
- 1 Premium SSD OS disk ( $4\text{ MB/s}$ write churn)
- 1 Premium SSD TempDB disk ( $30\text{ MB/s}$ write churn)
- 1 Premium SSD Data disk ( $22\text{ MB/s}$ write churn)
- 1 Premium SSD Transaction Log disk ( $35\text{ MB/s}$ write churn)
- The required RPO for the database tier is $10\text{ seconds}$ , and the RTO is $15\text{ minutes}$ .

Which two of the following strategies should you include in the disaster recovery design? (Select TWO).

Replicate the Web and Application tier VMs to the target region using Azure Site Recovery.Answer
Deploy SQL Server Always On Availability Groups with asynchronous commit replication to SQL Server VMs in the target region.Answer
C
Replicate the database tier VMs to the target Azure region using Azure Site Recovery standard replication.
D
Configure Azure Traffic Manager to handle the real-time data replication of the database logs between the primary and target regions.

Answer

Replicate the Web and Application tier VMs to the target region using Azure Site Recovery, and deploy SQL Server Always On Availability Groups with asynchronous commit replication to SQL Server VMs in the target region.

The correct strategy involves combining Azure Site Recovery for the Web and Application tiers with SQL Server Always On Availability Groups for the Database tier. Azure Site Recovery is optimal for standard VM-level replication because the Web and Application tiers have low write churn and flexible RPO/RTO objectives. For the database tier, the 10-second RPO and the high write churn (specifically the log disk at 35 MB/s, which exceeds the 20 MB/s standard ASR per-disk limit, and the total VM churn of 91 MB/s) require a native application-level replication solution. SQL Server Always On Availability Groups with asynchronous commit provides low-latency replication that meets the 10-second RPO without running into VM-level storage replication thresholds.

Step-by-Step Solution

Evaluate the disaster recovery requirements for the Web and Application tiers.

The Web/App tiers require a 1-hour RPO and 4-hour RTO, and have low write churn (3 MB/s per VM). Azure Site Recovery (ASR) is fully supported and meets these constraints.

ASR provides continuous replication for Azure VMs with a default RPO alert threshold of 15 minutes, which easily satisfies the 1-hour requirement.

Evaluate the database tier requirements against Azure Site Recovery limits.

The database VMs have a total write churn of 91 MB/s (OS: 4, TempDB: 30, Data: 22, Log: 35), and the log disk has 35 MB/s churn. Under standard ASR, write churn limits are 20 MB/s per Premium SSD and 40 MB/s per VM. Standard ASR cannot support this churn workload. Furthermore, ASR cannot consistently achieve a 10-second RPO.

Checking ASR constraints helps determine whether a VM-level replication or a native database replication strategy is required.

Determine the optimal replication strategy for the database tier.

SQL Server Always On Availability Groups with asynchronous commit replication should be configured between the primary region and SQL Server VMs in the target region.

Native SQL Server replication tools handle high transaction rates natively, bypass VM-level storage replication limits, and can achieve near-zero RPO, satisfying the 10-second RPO limit.

Key Concept

Disaster recovery design involves aligning workload metrics (such as write churn and RPO/RTO) with Azure service capabilities. While Azure Site Recovery is ideal for standard virtual machine workloads, native database replication techniques (like SQL Server Always On Availability Groups) must be used for databases with high write churn or extremely low RPO targets.

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