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Reducing FT logging traffic for disk read intensive workloads (1011965)

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This article discusses reducing Fault Tolerance (FT) 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 vmkernel port which has been 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 host

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


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 FT virtual machine.
  2. Log in to the Local Tech Support Mode console of the ESX/ESXi host. For more information, see Unable to connect to an ESX host using Secure Shell (SSH) (1003807), Tech Support Mode for Emergency Support (1003677), or Using Tech Support Mode in ESXi 4.1 and ESXi 5.0 (1017910)
  3. Obtain the location of the datastore and directory where the virtual machine configration .vmx file is located (if not already known):

    # vim-cmd vmsvc/getallvms

    The output will show virtual machines which are registered on the ESX/ESXi host and the output will be similar to:

    Vmid   Name         File                        Guest OS                 Version     Annotation
    2848   Win2003_VM   [Datastore1] Win/Win.vmx    winNetEnterpriseGuest    vmx-07      To be used as a template

    In the above example the virtual machine is located datastore Datastore1 in the directory Win . The Vmid  is 2848 

  4. Open the virtual machine configuration .vmx file for editing using an editor such as vi or nano . For more information on using these editors, see Editing files on an ESX host using vi or nano (1020302).
  5. Add the following line to the .vmx file:

    replay.logReadData = checksum

    Save your changes and close the file.

  6. Reload the virtual machine configuration file:

    # vim-cmd vmsvc/reload Vmid

    Note: The Vmid  value for the virtual machine is obtained from the  vim-cmd vmsvc/getallvms command. For more/related information, see Reloading a vmx file without removing the Virtual machine from inventory (1026043).

  7. 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. Unregistering and registering virtual machine removes historic performance data in vCenter Server for Virtual Machine.

See Also

Update History

06/26/2012 - Added ESXi procedure. Updated Product Versions to include ESXi 5.0. 11/02/2012 - Corrected spelling

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