Step 3: Plan capacity and scaling for VMware to Azure replication

Use this article to figure out planning for capacity and scaling, when replicating on-premises VMware VMs and physical servers to Azure with Azure Site Recovery.

Post comments and questions at the bottom of this article, or on the Azure Recovery Services Forum.

How do I start capacity planning?

You gather information about your replication environment, and then plan capacity using this information, together with the considerations highlighted in this article.

Gather information

  1. Download the Deployment Planner tool for VMware replication.
  2. Read this article to understand how to run the tool.
  3. Using the tool you gather information about compatible and incompatible VMs, disks per VM, and data churn per disk. The tool also covers network bandwidth requirements, and the Azure infrastructure needed for successful replication and test failover.

Replication considerations

Note these considerations before you start deployment:

Component Details
Replication Maximum daily change rate: A protected machine can only use one process server, and a single process server can handle a daily change rate up to 2 TB. Thus 2 TB is the maximum daily data change rate that’s supported for a protected machine.

Maximum throughput: A replicated machine can belong to one storage account in Azure. A standard storage account can handle a maximum of 20,000 requests per second, and we recommend that you keep the number of input/output operations per second (IOPS) across a source machine to 20,000. For example, if you have a source machine with 5 disks, and each disk generates 120 IOPS (8K size) on the source machine, then it will be within the Azure per disk IOPS limit of 500. (The number of storage accounts required is equal to the total source machine IOPS, divided by 20,000.)

Configuration server capacity

The configuration server should be able to handle the daily change rate capacity across all workloads running on protected machines, and needs sufficient bandwidth to continuously replicate data to Azure Storage.

As a best practice, locate the configuration server on the same network and LAN segment as the machines you want to protect. It can be located on a different network, but machines you want to protect should have layer 3 network visibility to it.

Sizing recommendations

The table summarizes sizing recommendations based on CPU.

CPU Memory Cache disk size Data change rate Protected machines
8 vCPUs (2 sockets * 4 cores @ 2.5 gigahertz [GHz]) 16 GB 300 GB 500 GB or less Replicate less than 100 machines.
12 vCPUs (2 sockets * 6 cores @ 2.5 GHz) 18 GB 600 GB 500 GB to 1 TB Replicate between 100-150 machines.
16 vCPUs (2 sockets * 8 cores @ 2.5 GHz) 32 GB 1 TB 1 TB to 2 TB Replicate between 150-200 machines.
Deploy another process server > 2 TB Deploy additional process servers if you're replicating more than 200 machines, or if the daily data change rate exceeds 2 TB.


  • Each source machine is configured with 3 disks of 100 GB each.
  • We used benchmarking storage of 8 SAS drives of 10 K RPM, with RAID 10, for cache disk measurements.

Process server capacity

The process server receives replication data from protected machines, and optimizes it with caching, compression, and encryption. Then it sends the data to Azure.

  • The process server machine should have sufficient resources to perform these tasks.
  • The first process server is installed by default on the configuration server. You can deploy additional process servers to scale your environment.
  • The process server uses a disk-based cache. Use a separate cache disk of 600 GB or more to handle data changes stored in the event of a network bottleneck or outage.
  • If you need to protect more than 200 machines, or the daily change rate is greater than 2 TB, you can add process servers to handle the replication load. To scale out, you can:
    • Increase the number of configuration servers. For example, you can protect up to 400 machines with two configuration servers.
    • Add more process servers, and use these to handle traffic instead of (or in addition to) the configuration server.

Example process server scaling

The following table describes a scenario in which:

  • You're not planning to use the configuration server as a process server.
  • You've set up an additional process server.
  • You've configured protected virtual machines to use the additional process server.
  • Each protected source machine is configured with three disks of 100 GB each.
Configuration server Additional process server Cache disk size Data change rate Protected machines
8 vCPUs (2 sockets * 4 cores @ 2.5 GHz), 16 GB memory 4 vCPUs (2 sockets * 2 cores @ 2.5 GHz), 8 GB memory 300 GB 250 GB or less Replicate 85 or fewer machines.
8 vCPUs (2 sockets * 4 cores @ 2.5 GHz), 16 GB memory 8 vCPUs (2 sockets * 4 cores @ 2.5 GHz), 12 GB memory 600 GB 250 GB to 1 TB Replicate between 85-150 machines.
12 vCPUs (2 sockets * 6 cores @ 2.5 GHz), 18 GB memory 12 vCPUs (2 sockets * 6 cores @ 2.5 GHz) 24 GB memory 1 TB 1 TB to 2 TB Replicate between 150-225 machines.

The way in which you scale your servers depends on your preference for a scale-up or scale-out model. You scale up by deploying a few high-end configuration and process servers, or scale out by deploying more servers with fewer resources. For example, if you need to protect 220 machines, you could do either of the following:

  • Set up the configuration server with 12 vCPU, 18 GB of memory, and an additional process server with 12 vCPU, 24 GB of memory. Configure protected machines to use the additional process server only.
  • Set up two configuration servers (2 x 8 vCPU, 16 GB RAM) and two additional process servers (1 x 8 vCPU and 4 vCPU x 1 to handle 135 + 85 [220] machines). Configure protected machines to use the additional process servers only.

Deploy additional process servers

Follow these instructions to set up an additional process server. After setting up the server, you migrate source machines to use it.

  1. In Site Recovery servers, click the configuration server > +Process Server.
  2. In Server type, click Process server (on-premises).

    Process server

  3. Download the Site Recovery Unified Setup file.
  4. Run setup to install the process server, and register it in the vault.
  5. In Before you begin, select Add additional process servers to scale out deployment.
  6. In Configuration Server Details, specify the IP address of the configuration server, and the passphrase. If you don't have the passphrase, get it by running [SiteRecoveryInstallationFolder]\home\sysystems\bin\genpassphrase.exe –n on the configuration server.

    Configuration server

  7. Complete the rest of setup in the same way you did when you set up the configuration server.

Migrate machines to use the process server

  1. In Settings > Site Recovery servers, click the configuration server > Process servers.
  2. Right-click the process server currently in use > Switch.

    Switch process server

  3. In Select target process server, select the process server you want to use, and select the VMs that the server will handle.
  4. Click the information icon. To help you make load decisions, the average space that's needed to replicate each selected VM to the new process server is displayed.
  5. Click the check mark to start replication to the new process server.

Control network bandwidth

After you run the Deployment Planner tool to calculate the bandwidth you need for replication (the initial replication and then delta), you can control the amount of bandwidth used for replication using a couple of options:

  • Throttle bandwidth: VMware traffic that replicates to Azure goes through a specific process server. You can throttle bandwidth on the machines running as process servers.
  • Influence bandwidth: You can influence the bandwidth used for replication by using a couple of registry keys:
    • The HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Azure Backup\UploadThreadsPerVM registry value specifies the number of threads that are used for data transfer (initial or delta replication) of a disk. A higher value increases the network bandwidth used for replication.
    • The HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Azure Backup\DownloadThreadsPerVM specifies the number of threads used for data transfer during failback.

Throttle bandwidth

  1. Open the Azure Backup MMC snap-in on the machine acting as the process server. By default, a shortcut for Backup is available on the desktop, or in the following folder: C:\Program Files\Microsoft Azure Recovery Services Agent\bin\wabadmin.
  2. In the snap-in, click Change Properties.
  3. On the Throttling tab, select Enable internet bandwidth usage throttling for backup operations.
  4. Set the limits for work and non-work hours. Valid ranges are from 512 Kbps to 102 Mbps per second.


You can also use the Set-OBMachineSetting cmdlet to set throttling. Here's a sample:

$mon = [System.DayOfWeek]::Monday
$tue = [System.DayOfWeek]::Tuesday
Set-OBMachineSetting -WorkDay $mon, $tue -StartWorkHour "9:00:00" -EndWorkHour "18:00:00" -WorkHourBandwidth  (512*1024) -NonWorkHourBandwidth (2048*1024)

Set-OBMachineSetting -NoThrottle indicates that no throttling is required.

Influence network bandwidth for a VM

  1. In registry of the VM, go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Azure Backup\Replication.
    • To influence the bandwidth traffic on a replicating disk, modify the value of UploadThreadsPerVM, or create the key if it doesn't exist.
    • To influence the bandwidth for failback traffic from Azure, modify the value of DownloadThreadsPerVM.
  2. The default value is 4. In an overprovisioned network, these registry keys should be modified. The maximum is 32. Monitor traffic to optimize the value.

Next steps

Go to Step 4: Plan networking.