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I have an application that installs itself into the /opt/my_app/ directory. Now I'd like to setup two servers in a cluster (active - passive) and sync the whole directory with DRBD. Now from what I understand DRBD requires a block device. So I would add a new virtual disk (both are ESX VM's) create a partition, next a physical volume, volume group and a logical volume. But the question I have is it technically possible to put /opt/my_app/ on the DRBD device and sync it between two nodes?

EDIT:

[root@server2 otrs]# pcs config
Cluster Name: otrs_cluster
Corosync Nodes:
 server1 server2
Pacemaker Nodes:
 server1 server2

Resources:
 Group: OTRS
  Resource: ClusterIP (class=ocf provider=heartbeat type=IPaddr2)
   Attributes: cidr_netmask=8 ip=10.0.0.60
   Operations: monitor interval=20s (ClusterIP-monitor-interval-20s)
               start interval=0s timeout=20s (ClusterIP-start-interval-0s)
               stop interval=0s timeout=20s (ClusterIP-stop-interval-0s)
  Resource: otrs_file_system (class=ocf provider=heartbeat type=Filesystem)
   Attributes: device=/dev/drbd0 directory=/opt/otrs/ fstype=ext4
   Operations: monitor interval=20 timeout=40 (otrs_file_system-monitor-interval-20)
               start interval=0s timeout=60 (otrs_file_system-start-interval-0s)
               stop interval=0s timeout=60 (otrs_file_system-stop-interval-0s)
 Master: otrs_data_clone
  Meta Attrs: master-node-max=1 clone-max=2 notify=true master-max=1 clone-node-max=1
  Resource: otrs_data (class=ocf provider=linbit type=drbd)
   Attributes: drbd_resource=otrs
   Operations: demote interval=0s timeout=90 (otrs_data-demote-interval-0s)
               monitor interval=30s (otrs_data-monitor-interval-30s)
               promote interval=0s timeout=90 (otrs_data-promote-interval-0s)
               start interval=0s timeout=240 (otrs_data-start-interval-0s)
               stop interval=0s timeout=100 (otrs_data-stop-interval-0s)

Stonith Devices:
Fencing Levels:

Location Constraints:
  Resource: ClusterIP
    Enabled on: server1 (score:INFINITY) (role: Started) (id:cli-prefer-ClusterIP)
Ordering Constraints:
Colocation Constraints:
Ticket Constraints:

Alerts:
 No alerts defined

Resources Defaults:
 No defaults set
Operations Defaults:
 No defaults set

Cluster Properties:
 cluster-infrastructure: corosync
 cluster-name: otrs_cluster
 dc-version: 1.1.16-12.el7_4.8-94ff4df
 have-watchdog: false
 last-lrm-refresh: 1525108871
 stonith-enabled: false

Quorum:
  Options:
[root@server2 otrs]#




[root@server2 otrs]# pcs status
Cluster name: otrs_cluster
Stack: corosync
Current DC: server1 (version 1.1.16-12.el7_4.8-94ff4df) - partition with quorum
Last updated: Mon Apr 30 14:11:54 2018
Last change: Mon Apr 30 13:27:47 2018 by root via crm_resource on server2

2 nodes configured
4 resources configured

Online: [ server1 server2 ]

Full list of resources:

 Resource Group: OTRS
     ClusterIP  (ocf::heartbeat:IPaddr2):       Started server2
     otrs_file_system   (ocf::heartbeat:Filesystem):    Started server2
 Master/Slave Set: otrs_data_clone [otrs_data]
     Masters: [ server2 ]
     Slaves: [ server1 ]

Failed Actions:
* otrs_file_system_start_0 on server1 'unknown error' (1): call=78, status=complete, exitreason='Couldn't mount filesystem /dev/drbd0 on /opt/otrs',
    last-rc-change='Mon Apr 30 13:21:13 2018', queued=0ms, exec=151ms


Daemon Status:
  corosync: active/enabled
  pacemaker: active/enabled
  pcsd: active/enabled
[root@server2 otrs]#
1

It's certainly possible.

After you've added the block device and created the LVM to back the DRBD device, you would configure and initialize the DRBD device (drbdadm create-md <res> and drbdadm up <res>.

Promote one node to Primary (NOTE: you only need to force primary the first time you're promoting a device since you have Inconsistent/Inconsistent disk states): drbdadm primary <res> --force

Then you can put a filesystem on the device and mount it anywhere on the system, including /opt/my_app, just like you would with an ordinary block device.

If there is existing data in /opt/my_app/ that you need to move to the DRBD device, you could mount the device somewhere else, move/copy the data from /opt/my_app/ to the mount point, then remount the DRBD device on /opt/myapp, or you could use symlinks to point /opt/my_app at the DRBD device's mount point.

Updated answer after EDIT:

You need to add colocation and ordering constraints to your cluster configuration to tell the OTRS resource group to only run on the DRBD Master and to only start after the DRBD Master has been promoted.

These commands should add those constraints:

# pcs constraint colocation add OTRS with otrs_data_clone INFINITY with-rsc-role=Master
# pcs constraint order promote otrs_data_clone then start OTRS
| improve this answer | |
  • ok, I think I figured it out but after integrating drbd with pacemaker I'm facing issues, it appears that pacemaker tries to mount the file system on both nodes (please see edit in my question). Thank you in advance. – postFix May 1 '18 at 17:53
  • I edited my answer. Hope that helps – Matt Kereczman May 2 '18 at 16:36
  • Thank you Matt. Wouldn't I achieve the same configuring the OTRS group ? Isn't it the idea of the group that the resources run on the same node ? And the order in which pacemaker starts the resource isn't controlled by the resource order ? This is at least what I deduct. – postFix May 6 '18 at 19:11
  • You need the constraints to tell the OTRS group to start on the DRBD master. – Matt Kereczman May 7 '18 at 3:10
  • Hello Matt. Ok it looks like that it works now although I think that # pcs constraint colocation add otrs_data_clone INFINITY with-rsc-role=Master was missing one more resource, is it possible? It worked with the following pcs constraint colocation add otrs_file_system otrs_data_clone INFINITY with-rsc-role=Master – postFix May 9 '18 at 13:56

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