Zerto – vSphere WAN Sizing Best Practices

Are you planning to deploy a Zerto Virtual Replication solution, and you are worried about bandwidth consumption? Keep in mind these considerations to estimate and optimize WAN traffic:

Estimating the Bandwidth Requirements Between Sites

Collect Data Characteristics for Protected VMs.

Enabling vCenter Server Data Collection
  1. Connect to the vCenter Server using the vSphere Web Client.
  2. Select your vCenter and go to Manage -> Settings -> General -> Edit.
  3. Select Statistics and make sure that the Statistics Level value for all interval duration is at least Level 3.
    • When you choose Level 3, you will be able to collect past week statistics.
    • If you choose 2, you will only be able to collect past day statistics.
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  4. Click OK and wait for at least 1 week before using the aggregate usage data.
Collecting Data Characteristics for VMs

You can collect data characteristics for the VMs in one of the following ways:

  1. Via vSphere Web Client console performance statistics.
  2. By running a script to collect the data characteristics.
  3. By using operating system performance monitors, such as the Microsoft Performance Monitor utility for Windows operating systems or the iostat command for Linux operating systems.

***Collect data for a minimum of one week. The collection period should be long enough to gather a true representation of usage.***

To collect data characteristics for the VMs via the vSphere Web Client console performance statistics:
  1. In the vSphere Web Client console select the VM and go to Monitor -> Performance -> Advanced -> Chart Options
  2. Select Disk in Chart Metrics and Last Week in Timespan.
  3. In Counters, click None to clear all the selections and then select Write Rate.
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  4. Click OK to generate the chart.
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  5. Collect the Average Write Rate value.
  6. Repeat the procedure for each VM you are planning to replicate.
To collect data characteristics for the VMs via a script:

PowerCLI script should be similar to the following:

$report = @()
Get-VM | %{
   $stats = Get-Stat -Entity $ -Stat disk.write.average -Start (Get-Date).adddays(-7) -
ErrorAction SilentlyContinue
      $statsGrouped = $stats | Group-Object -Property MetricId
      $row = "" | Select Name, WriteAvgKBps, WriteAvgMBps
      $row.Name = $_.Name
      $row.WriteAvgKBps = ($statsGrouped |
         where {$_.Name -eq "disk.write.average"} |
         %{$_.Group | Measure-Object -Property Value -Average}).Average
      $row.WriteAvgMBps = $row.WriteAvgKBps/1024
      $row.WriteAvgKBps = "{0:N2}" -f $row.WriteAvgKbps
      $row.WriteAvgMBps = "{0:N2}" -f $row.WriteAvgMBps
      $report += $row
$report | Export-Csv "C:\ZertoOutput.csv"

***After this procedures, make sure to re-configure Statistics Level value for all interval duration on Level 1 or 2. Keeping Statistics Level on Level 3 may quickly fill out the vCenter Server database disk.***

Calculate the Estimated Bandwidth Requirements.

To estimate sizing using the Zerto WAN Sizing Estimator:

Use the “Zerto WAN Sizing Estimator v2.xlsx” to estimate the minimum bandwidth required. You can find it in My Zerto Portal: Zerto Virtual Replication WAN Sizing.zip

Once you have downloaded the tool, you must complete the excel cells for each VM.


  • VM Name: … don’t need to be explained…
  • Write KB/s: use the Average Write Rate value.
  • Compression?: you must decide whether compression will be enabled for the VM, based on the data characteristics. For example, a File Server that only store PDF files won’t be a good candidate for compression. Check “3. WAN Optimization Considerations” section for more details.
  • SQL Server?: if the VM is a SQL Server, compression will be better.

In WAN Mbps column, you can find the minimum required bandwidth for each VM, and in the top table you can find Total Required BW with and without Zerto savings.

Remember to take in count at least 20% extra BW for bursts of change.

To estimate sizing without using the Zerto WAN Sizing Estimator:
  1. For each VM multiply the KB/sec, based on the statistics gathered, by 8 and divide the result by 1024 to provide an answer in Mb/sec. Divide this result by 2 if compression is enabled for the VM and the data is compressible.
  2. Sum the results of step 1.
  3. WAN Mb/sec = SUM(KB/sec * (8/1024/(1 or 2 if compressible data that will be compressed)))

If the result is less than 5 Mb/sec, you must use a minimum dedicated bandwidth of at least 5 Mb/sec.

WAN Optimization Considerations

Zerto employs sophisticated data compression algorithms to reduce the bandwidth required between the protected and recovery sites. Although this compression can be very effective in reducing bandwidth requirements, its efficiency is highly dependent on the specific characteristics of the data being replicated.

Zerto uses the maximum bandwidth configured as well as optimization and internal compression to maintain low RPOs. This removes the need to use third-party WAN acceleration devices. However, you can also work with third-party devices, but as a best practice, you should choose just 1 WAN compression mechanism (Zerto embedded or third-party).

Bandwidth Throttling to Temporarily Free Bandwidth

Zerto can be configured to throttle the bandwidth it uses during a specific time period. During this period, replication does not use more than the bandwidth selected. At the end of the period, bandwidth throttling ends.

To configure time-based throttling:
  1. In the Zerto user interface, select Site Settings -> Performance and Throttling.
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  2. To specify time-based throttling, check the Time-based Throttling checkbox.
  3. Specify the maximum bandwidth for the period.
    • If the Unlimited checkbox is checked, the bandwidth is always unlimited.
    • In the From fields, select the start time for throttling.
    • In the To fields, select the end time for throttling.
  4. Click APPLY or SAVE.

Using Swap Disks to Lessen the Bandwidth Load

A swap disk is a virtual disk that Zerto replicates and then isolates from further changes. If temporary files on a protected VM are moved to a swap disk, although the disk itself is replicated to the recovery site, changes to it are not replicated. This reduces replication traffic, journal size, and storage use.

Preseeding to Initially Lessen the Bandwidth Load

In preseeding, the entire VM is block-copied and moved to the recovery site. Only changes made after the preseeding are replicated.

Setting the Priority for Using the WAN by Multiple VPGs (Virtual Protection Groups)

Zerto enables prioritizing replication for each VPG. The priorities are:

  • High
  • Medium
  • Low

Prioritization is used when there is bandwidth contention between sites or when the load on the VRA is more than can be accommodated. It enables maintaining a low RPO on the most critical VPGs. As bandwidth becomes available, lower priority VPGs are synchronized and returned to a continuous protection state.



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