9. Convert Storage to Active/Active

The primary requirement for an active/active cluster is that the data required for your services is available, simultaneously, on both machines. Pacemaker makes no requirement on how this is achieved; you could use a Storage Area Network (SAN) if you had one available, but since DRBD supports multiple Primaries, we can continue to use it here.

9.1. Install Cluster Filesystem Software

The only hitch is that we need to use a cluster-aware filesystem. The one we used earlier with DRBD, xfs, is not one of those. Both OCFS2 and GFS2 are supported; here, we will use GFS2.

On both nodes, install Distributed Lock Manager (DLM) and the GFS2 command- line utilities required by cluster filesystems:

# dnf config-manager --set-enabled resilientstorage
# dnf install -y dlm gfs2-utils

9.2. Configure the Cluster for the DLM

The DLM control daemon needs to run on both nodes, so we’ll start by creating a resource for it (using the ocf:pacemaker:controld resource agent), and clone it:

[root@pcmk-1 ~]# pcs cluster cib dlm_cfg
[root@pcmk-1 ~]# pcs -f dlm_cfg resource create dlm \
    ocf:pacemaker:controld op monitor interval=60s
[root@pcmk-1 ~]# pcs -f dlm_cfg resource clone dlm clone-max=2 clone-node-max=1
[root@pcmk-1 ~]# pcs resource status
  * ClusterIP       (ocf:heartbeat:IPaddr2):         Started pcmk-1
  * WebSite (ocf:heartbeat:apache):  Started pcmk-1
  * Clone Set: WebData-clone [WebData] (promotable):
    * Promoted: [ pcmk-1 ]
    * Unpromoted: [ pcmk-2 ]
  * WebFS   (ocf:heartbeat:Filesystem):      Started pcmk-1

Activate our new configuration, and see how the cluster responds:

[root@pcmk-1 ~]# pcs cluster cib-push dlm_cfg --config
CIB updated
[root@pcmk-1 ~]# pcs resource status
  * ClusterIP       (ocf:heartbeat:IPaddr2):         Started pcmk-1
  * WebSite (ocf:heartbeat:apache):  Started pcmk-1
  * Clone Set: WebData-clone [WebData] (promotable):
    * Promoted: [ pcmk-1 ]
    * Unpromoted: [ pcmk-2 ]
  * WebFS   (ocf:heartbeat:Filesystem):      Started pcmk-1
  * Clone Set: dlm-clone [dlm]:
    * Started: [ pcmk-1 pcmk-2 ]
[root@pcmk-1 ~]# pcs resource config
 Resource: ClusterIP (class=ocf provider=heartbeat type=IPaddr2)
  Attributes: cidr_netmask=24 ip=192.168.122.120
  Operations: monitor interval=30s (ClusterIP-monitor-interval-30s)
              start interval=0s timeout=20s (ClusterIP-start-interval-0s)
              stop interval=0s timeout=20s (ClusterIP-stop-interval-0s)
 Resource: WebSite (class=ocf provider=heartbeat type=apache)
  Attributes: configfile=/etc/httpd/conf/httpd.conf statusurl=http://localhost/server-status
  Operations: monitor interval=1min (WebSite-monitor-interval-1min)
              start interval=0s timeout=40s (WebSite-start-interval-0s)
              stop interval=0s timeout=60s (WebSite-stop-interval-0s)
 Clone: WebData-clone
  Meta Attrs: clone-max=2 clone-node-max=1 notify=true promotable=true promoted-max=1 promoted-node-max=1
  Resource: WebData (class=ocf provider=linbit type=drbd)
   Attributes: drbd_resource=wwwdata
   Operations: demote interval=0s timeout=90 (WebData-demote-interval-0s)
               monitor interval=29s role=Promoted (WebData-monitor-interval-29s)
               monitor interval=31s role=Unpromoted (WebData-monitor-interval-31s)
               notify interval=0s timeout=90 (WebData-notify-interval-0s)
               promote interval=0s timeout=90 (WebData-promote-interval-0s)
               reload interval=0s timeout=30 (WebData-reload-interval-0s)
               start interval=0s timeout=240 (WebData-start-interval-0s)
               stop interval=0s timeout=100 (WebData-stop-interval-0s)
 Resource: WebFS (class=ocf provider=heartbeat type=Filesystem)
  Attributes: device=/dev/drbd1 directory=/var/www/html fstype=xfs
  Operations: monitor interval=20s timeout=40s (WebFS-monitor-interval-20s)
              start interval=0s timeout=60s (WebFS-start-interval-0s)
              stop interval=0s timeout=60s (WebFS-stop-interval-0s)
 Clone: dlm-clone
  Meta Attrs: interleave=true ordered=true
  Resource: dlm (class=ocf provider=pacemaker type=controld)
   Operations: monitor interval=60s (dlm-monitor-interval-60s)
               start interval=0s timeout=90s (dlm-start-interval-0s)
               stop interval=0s timeout=100s (dlm-stop-interval-0s)

9.3. Create and Populate GFS2 Filesystem

Before we do anything to the existing partition, we need to make sure it is unmounted. We do this by telling the cluster to stop the WebFS resource. This will ensure that other resources (in our case, WebSite) using WebFS are not only stopped, but stopped in the correct order.

[root@pcmk-1 ~]# pcs resource disable WebFS
[root@pcmk-1 ~]# pcs resource
  * ClusterIP       (ocf:heartbeat:IPaddr2):         Started pcmk-1
  * WebSite (ocf:heartbeat:apache):  Stopped
  * Clone Set: WebData-clone [WebData] (promotable):
    * Promoted: [ pcmk-1 ]
    * Unpromoted: [ pcmk-2 ]
  * WebFS   (ocf:heartbeat:Filesystem):      Stopped (disabled)
  * Clone Set: dlm-clone [dlm]:
    * Started: [ pcmk-1 pcmk-2 ]

You can see that both WebSite and WebFS have been stopped, and that pcmk-1 is currently running the promoted instance for the DRBD device.

Now we can create a new GFS2 filesystem on the DRBD device.

Warning

This will erase all previous content stored on the DRBD device. Ensure you have a copy of any important data.

Important

Run the next command on whichever node has the DRBD Primary role. Otherwise, you will receive the message:

/dev/drbd1: Read-only file system

[root@pcmk-1 ~]# mkfs.gfs2 -p lock_dlm -j 2 -t mycluster:web /dev/drbd1
It appears to contain an existing filesystem (xfs)
This will destroy any data on /dev/drbd1
Are you sure you want to proceed? [y/n] y
Discarding device contents (may take a while on large devices): Done
Adding journals: Done
Building resource groups: Done
Creating quota file: Done
Writing superblock and syncing: Done
Device:                    /dev/drbd1
Block size:                4096
Device size:               0.50 GB (131059 blocks)
Filesystem size:           0.50 GB (131055 blocks)
Journals:                  2
Journal size:              8MB
Resource groups:           4
Locking protocol:          "lock_dlm"
Lock table:                "mycluster:web"
UUID:                      19712677-7206-4660-a079-5d17341dd720

The mkfs.gfs2 command required a number of additional parameters:

• -p lock_dlm specifies that we want to use DLM-based locking.
• -j 2 indicates that the filesystem should reserve enough space for two journals (one for each node that will access the filesystem).
• -t mycluster:web specifies the lock table name. The format for this field is <CLUSTERNAME>:<FSNAME>. For CLUSTERNAME, we need to use the same value we specified originally with pcs cluster setup --name (which is also the value of cluster_name in /etc/corosync/corosync.conf). If you are unsure what your cluster name is, you can look in /etc/corosync/corosync.conf or execute the command pcs cluster corosync | grep cluster_name.

Now we can (re-)populate the new filesystem with data (web pages). We’ll create yet another variation on our home page.

[root@pcmk-1 ~]# mount /dev/drbd1 /mnt
[root@pcmk-1 ~]# cat <<-END >/mnt/index.html
<html>
<body>My Test Site - GFS2</body>
</html>
END
[root@pcmk-1 ~]# chcon -R --reference=/var/www/html /mnt
[root@pcmk-1 ~]# umount /dev/drbd1
[root@pcmk-1 ~]# drbdadm verify wwwdata

9.4. Reconfigure the Cluster for GFS2

With the WebFS resource stopped, let’s update the configuration.

[root@pcmk-1 ~]# pcs resource config WebFS
 Resource: WebFS (class=ocf provider=heartbeat type=Filesystem)
  Attributes: device=/dev/drbd1 directory=/var/www/html fstype=xfs
  Meta Attrs: target-role=Stopped
  Operations: monitor interval=20s timeout=40s (WebFS-monitor-interval-20s)
              start interval=0s timeout=60s (WebFS-start-interval-0s)
              stop interval=0s timeout=60s (WebFS-stop-interval-0s)

The fstype option needs to be updated to gfs2 instead of xfs.

[root@pcmk-1 ~]# pcs resource update WebFS fstype=gfs2
[root@pcmk-1 ~]# pcs resource config WebFS
 Resource: WebFS (class=ocf provider=heartbeat type=Filesystem)
  Attributes: device=/dev/drbd1 directory=/var/www/html fstype=gfs2
  Meta Attrs: target-role=Stopped
  Operations: monitor interval=20s timeout=40s (WebFS-monitor-interval-20s)
              start interval=0s timeout=60s (WebFS-start-interval-0s)
              stop interval=0s timeout=60s (WebFS-stop-interval-0s)

GFS2 requires that DLM be running, so we also need to set up new colocation and ordering constraints for it:

[root@pcmk-1 ~]# pcs constraint colocation add WebFS with dlm-clone
[root@pcmk-1 ~]# pcs constraint order dlm-clone then WebFS
Adding dlm-clone WebFS (kind: Mandatory) (Options: first-action=start then-action=start)
[root@pcmk-1 ~]# pcs constraint
Location Constraints:
  Resource: WebSite
    Enabled on:
      Node: pcmk-2 (score:50)
Ordering Constraints:
  start ClusterIP then start WebSite (kind:Mandatory)
  promote WebData-clone then start WebFS (kind:Mandatory)
  start WebFS then start WebSite (kind:Mandatory)
  start dlm-clone then start WebFS (kind:Mandatory)
Colocation Constraints:
  WebSite with ClusterIP (score:INFINITY)
  WebFS with WebData-clone (score:INFINITY) (rsc-role:Started) (with-rsc-role:Promoted)
  WebSite with WebFS (score:INFINITY)
  WebFS with dlm-clone (score:INFINITY)
Ticket Constraints:

We also need to update the no-quorum-policy property to freeze. By default, the value of no-quorum-policy is set to stop indicating that once quorum is lost, all the resources on the remaining partition will immediately be stopped. Typically this default is the safest and most optimal option, but unlike most resources, GFS2 requires quorum to function. When quorum is lost both the applications using the GFS2 mounts and the GFS2 mount itself cannot be correctly stopped. Any attempts to stop these resources without quorum will fail, which will ultimately result in the entire cluster being fenced every time quorum is lost.

To address this situation, set no-quorum-policy to freeze when GFS2 is in use. This means that when quorum is lost, the remaining partition will do nothing until quorum is regained.

[root@pcmk-1 ~]# pcs property set no-quorum-policy=freeze

9.5. Clone the Filesystem Resource

Now that we have a cluster filesystem ready to go, we can configure the cluster so both nodes mount the filesystem.

Clone the Filesystem resource in a new configuration. Notice how pcs automatically updates the relevant constraints again.

[root@pcmk-1 ~]# pcs cluster cib active_cfg
[root@pcmk-1 ~]# pcs -f active_cfg resource clone WebFS
[root@pcmk-1 ~]# pcs -f active_cfg constraint
Location Constraints:
  Resource: WebSite
    Enabled on:
      Node: pcmk-2 (score:50)
Ordering Constraints:
  start ClusterIP then start WebSite (kind:Mandatory)
  promote WebData-clone then start WebFS-clone (kind:Mandatory)
  start WebFS-clone then start WebSite (kind:Mandatory)
  start dlm-clone then start WebFS-clone (kind:Mandatory)
Colocation Constraints:
  WebSite with ClusterIP (score:INFINITY)
  WebFS-clone with WebData-clone (score:INFINITY) (rsc-role:Started) (with-rsc-role:Promoted)
  WebSite with WebFS-clone (score:INFINITY)
  WebFS-clone with dlm-clone (score:INFINITY)
Ticket Constraints:

Tell the cluster that it is now allowed to promote both instances to be DRBD Primary.

[root@pcmk-1 ~]# pcs -f active_cfg resource update WebData-clone promoted-max=2

Finally, load our configuration to the cluster, and re-enable the WebFS resource (which we disabled earlier).

[root@pcmk-1 ~]# pcs cluster cib-push active_cfg --config
CIB updated
[root@pcmk-1 ~]# pcs resource enable WebFS

After all the processes are started, the status should look similar to this.

[root@pcmk-1 ~]# pcs resource
  * ClusterIP       (ocf:heartbeat:IPaddr2):         Started pcmk-1
  * WebSite (ocf:heartbeat:apache):  Started pcmk-1
  * Clone Set: WebData-clone [WebData] (promotable):
    * Promoted: [ pcmk-1 pcmk-2 ]
  * Clone Set: dlm-clone [dlm]:
    * Started: [ pcmk-1 pcmk-2 ]
  * Clone Set: WebFS-clone [WebFS]:
    * Started: [ pcmk-1 pcmk-2 ]

9.6. Test Failover

Testing failover is left as an exercise for the reader.

With this configuration, the data is now active/active. The website administrator could change HTML files on either node, and the live website will show the changes even if it is running on the opposite node.

If the web server is configured to listen on all IP addresses, it is possible to remove the constraints between the WebSite and ClusterIP resources, and clone the WebSite resource. The web server would always be ready to serve web pages, and only the IP address would need to be moved in a failover.