14.1.1.3. Setting Up a DRBD Primary Node

To set up a DRBD primary node you need to configure the DRBD service, create the first DRBD block device and then create a file system on the device so that you can store files and data.

The DRBD configuration file /etc/drbd.conf defines a number of parameters for your DRBD configuration, including the frequency of updates and block sizes, security information and the definition of the DRBD devices that you want to create.

The key elements to configure are the on sections which specify the configuration of each node.

To follow the configuration, the sequence below shows only the changes from the default drbd.conf file. Configurations within the file can be both global or tied to specific resource.

  1. Set the synchronization rate between the two nodes. This is the rate at which devices are synchronized in the background after a disk failure, device replacement or during the initial setup. Keep this in check compared to the speed of your network connection. Gigabit Ethernet can support up to 125 MB/second, 100Mbps Ethernet slightly less than a tenth of that (12MBps). If you are using a shared network connection, rather than a dedicated, then gauge accordingly.

    To set the synchronization rate, edit the rate setting within the syncer block:

    syncer {
        rate 10M;
    }

    You may additionally want to set the al-extents parameter. The default for this parameter is 257.

    For more detailed information on synchronization, the effects of the synchronization rate and the effects on network performance, see Section 14.1.3.2, “Optimizing the Synchronization Rate”.

  2. Set up some basic authentication. DRBD supports a simple password hash exchange mechanism. This helps to ensure that only those hosts with the same shared secret are able to join the DRBD node group.

    cram-hmac-alg “sha1”;
    shared-secret "shared-string";
  3. Now you must configure the host information. Remember that you must have the node information for the primary and secondary nodes in the drbd.conf file on each host. You need to configure the following information for each node:

    • device: The path of the logical block device that is created by DRBD.

    • disk: The block device that stores the data.

    • address: The IP address and port number of the host that holds this DRBD device.

    • meta-disk: The location where the metadata about the DRBD device is stored. If you set this to internal, DRBD uses the physical block device to store the information, by recording the metadata within the last sections of the disk. The exact size depends on the size of the logical block device you have created, but it may involve up to 128MB.

    A sample configuration for our primary server might look like this:

    on drbd-one {
    device /dev/drbd0;
    disk /dev/hdd1;
    address 192.168.0.240:8888;
    meta-disk internal;
    }

    The on configuration block should be repeated for the secondary node (and any further) nodes:

    on drbd-two {
    device /dev/drbd0;
    disk /dev/hdd1;
    address 192.168.0.241:8888;
    meta-disk internal;
    }

    The IP address of each on block must match the IP address of the corresponding host. Do not set this value to the IP address of the corresponding primary or secondary in each case.

  4. Before starting the primary node, create the metadata for the devices:

    root-shell> drbdadm create-md all
  5. You are now ready to start DRBD:

    root-shell> /etc/init.d/drbd start

    DRBD should now start and initialize, creating the DRBD devices that you have configured.

  6. DRBD creates a standard block device - to make it usable, you must create a file system on the block device just as you would with any standard disk partition. Before you can create the file system, you must mark the new device as the primary device (that is, where the data is written and stored), and initialize the device. Because this is a destructive operation, you must specify the command line option to overwrite the raw data:

    root-shell> drbdadm -- --overwrite-data-of-peer primary all

    If you are using a version of DRBD 0.7.x or earlier, then you need to use a different command-line option:

    root-shell> drbdadm -- --do-what-I-say primary all

    Now create a file system using your chosen file system type:

    root-shell> mkfs.ext3 /dev/drbd0
  7. You can now mount the file system and if necessary copy files to the mount point:

    root-shell> mkdir /mnt/drbd
    root-shell> mount /dev/drbd0 /mnt/drbd
    root-shell> echo "DRBD Device" >/mnt/drbd/samplefile

Your primary node is now ready to use. Next, configure your secondary node or nodes.

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