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Before you configure coordinator disks

I/O fencing requires coordinator disks to be configured in a disk group that each cluster system can access. The use of coordinator disks enables the vxfen driver to resolve potential split-brain conditions and prevent data corruption. A coordinator disk is not used for data storage, so it can be configured as the smallest LUN on a disk array to avoid wasting space.

Coordinator disks must meet the following requirements:

  There must be at least three coordinator disks and the total number of coordinator disks must be an odd number. This ensures a majority of disks can be achieved.

  Each of the coordinator disks must use a physically separate disk or LUN.

  Each of the coordinator disks should be on a different disk array, if possible.

  Coordinator disks in a disk array should use hardware-based mirroring.

  The coordinator disks must support SCSI-3 PR. Note that the use of the vxfentsthdw utility to test for SCSI-3 PR support requires that disks be 1MB or greater. Smaller disks can be tested manually. Contact Veritas support (http://support.veritas.com) for the procedure.

Setting up the disk group for coordinator disks

If you have already added and initialized disks you intend to use as coordinator disks, you can begin the following procedure at step 5.

 To set up the disk group for coordinator disks

  1. Physically add the three disks you intend to use for coordinator disks. All cluster nodes should physically share them. Veritas recommends that you use the smallest size disks or LUNs, so that space for data is not wasted.
  2. If necessary, use the vxdisk scandisks command to scan the disk drives and their attributes. This command updates the VxVM device list and reconfigures DMP with the new devices. For example:

    # vxdisk scandisks

  3. Use the command vxdisksetup command to initialize a disk as a VxVM disk. The example command that follows specifies the CDS format:

    # vxdisksetup -i vxvm_device_name format=cdsdisk

    For example:

  4. Test the disks for SCSI-3 persistent reservations support.
  5. From one node, create a disk group named vxfencoorddg. This group must contain an odd number of disks or LUNs and a minimum of three disks. Symantec recommends that you use only three coordinator disks, and that you use the smallest size disks or LUNs to conserve disk space.

    For example, assume the disks have the device names /dev/sdz /dev/sdaa, and sdab.

  6. On the master node, create the disk group by specifying the device name of one of the disks.

    # vxdg -o coordinator=on init vxfencoorddg sdz

  7. Add the other two disks to the disk group.

    # vxdg -g vxfencoorddg adddisk sdaa

    # vxdg -g vxfencoorddg adddisk sdab

See the Veritas Volume Manager Administrator's Guide.

Requirements for testing the coordinator disk group

Running the vxfentsthdw utility

Review these guidelines on testing support for SCSI-3:

Testing the coordinator disk group

After setting up, test the coordinator disk group.

 To test the coordinator disk group

  1. From one node, start the utility:

    # /opt/VRTSvcs/vxfen/bin/vxfentsthdw

    Make sure system-to-system communication is functioning properly before performing this step.

    See the vxfentsthdw(1M) man page.

  2. After reviewing the overview and warning about overwriting data on the disks, confirm to continue the process and enter the node names.
  3. Enter the name of the disk you are checking.

    For example, /dev/sdz.

Creating the vxfendg file

After setting up and testing the coordinator disk group, configure it for use.

 To create the vxfendg file

  1. Deport the disk group:

    # vxdg deport vxfencoorddg

  2. Import the disk group with the -t option to avoid automatically importing it when the nodes restart:

    # vxdg -t import vxfencoorddg

  3. Deport the disk group. This operation prevents the coordinator disks from serving other purposes:

    # vxdg deport vxfencoorddg

  4. On all nodes, type:

    # echo "vxfencoorddg" > /etc/vxfendg

    Do no use spaces between the quotes in the "vxfencoorddg" text.

    This command creates the /etc/vxfendg file, which includes the name of the coordinator disk group. Based on the contents of the /etc/vxfendg file, the rc script creates the /etc/vxfentab file for use by the vxfen driver when the system starts. The rc script also invokes the vxfenconfig command, which configures the vxfen driver to start and use the coordinator disks listed in /etc/vxfentab. /etc/vxfentab is a generated file; do not modify this file.