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Reducing FT logging traffic for disk read intensive workloads

Purpose

This article discusses reducing Fault Tolerance logging traffic for disk-read intensive workloads. 

When FT is enabled, all the inputs to the virtual machine are recorded and sent to the secondary host over the NIC designated for Fault Tolerance logging.

The inputs to the virtual machine include incoming network traffic as well as disk reads. If the virtual machine is doing throughput-intensive disk reads (such as sequential I/O), a lot of logging traffic needs to be sent to the secondary.

For example, a virtual machine doing disk reads at a 80MB/sec generates at least 80 x 8 = 640Mb/sec of logging traffic.

Resolution

To reduce the bandwidth consumption on the logging NIC, make the secondary virtual machine issue disk reads directly to the shared disk instead of getting that data over the network.  This can be done as follows:
  1. Power off the virtual machine.
  2. From the ESX command line:

    1. Run this command to find the virtual machine configuration file ( .vmx):

      vmware-cmd -l

      Note: This command outputs the full path of the the .vmx file.
    2. Open the vmx file for editing (using nano or vi, for example).
    3. Add the following line to the file:

      replay.logReadData = checksum
    4. Save your changes and close the file.
  3. Power on the virtual machine.
To undo the changes, power off the virtual machine and remove the "replay.logReadData = checksum" line.
 
Note: Because the default mode offers better performance in most environments, VMware recommends the use of this option only for cases where you are constrained by FT logging network bandwidth.

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Actions
  • KB Article: 1011965
  • Updated: Aug 14, 2009
  • Products:
    VMware ESX
    VMware ESXi
  • Product Versions:
    VMware ESX 4.0.x