Even if you’re serving up static websites, having to manually synchronize the contents of that website to all the machines in the cluster is not ideal. For dynamic websites, such as a wiki, it’s not even an option. Not everyone can afford network-attached storage, but somehow the data needs to be kept in sync.
Enter DRBD, which can be thought of as network-based RAID-1 [1].
DRBD itself is included in the upstream kernel [2], but we do need some utilities to use it effectively.
CentOS does not ship these utilities, so we need to enable a third-party repository to get them. Supported packages for many OSes are available from DRBD’s maker LINBIT [http://www.linbit.com/], but here we’ll use the free ELRepo [http://elrepo.org/] repository.
On both nodes, import the ELRepo package signing key, and enable the repository:
# rpm --import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
# rpm -Uvh https://www.elrepo.org/elrepo-release-8.el8.elrepo.noarch.rpm
Retrieving https://www.elrepo.org/elrepo-release-8.el8.elrepo.noarch.rpm
Verifying... ################################# [100%]
Preparing... ################################# [100%]
Updating / installing...
1:elrepo-release-8.2-1.el8.elrepo ################################# [100%]
Now, we can install the DRBD kernel module and utilities:
# yum install -y kmod-drbd90 drbd90-utils
DRBD will not be able to run under the default SELinux security policies. If you are familiar with SELinux, you can modify the policies in a more fine-grained manner, but here we will simply exempt DRBD processes from SELinux control:
# yum install -y policycoreutils-python-utils
# semanage permissive -a drbd_t
We will configure DRBD to use port 7789, so allow that port from each host to the other:
[root@pcmk-1 ~]# firewall-cmd --permanent --add-rich-rule='rule family="ipv4" \
source address="192.168.122.102" port port="7789" protocol="tcp" accept'
success
[root@pcmk-1 ~]# firewall-cmd --reload
success
[root@pcmk-2 ~]# firewall-cmd --permanent --add-rich-rule='rule family="ipv4" \
source address="192.168.122.101" port port="7789" protocol="tcp" accept'
success
[root@pcmk-2 ~]# firewall-cmd --reload
success
Note
In this example, we have only two nodes, and all network traffic is on the same LAN. In production, it is recommended to use a dedicated, isolated network for cluster-related traffic, so the firewall configuration would likely be different; one approach would be to add the dedicated network interfaces to the trusted zone.
DRBD will need its own block device on each node. This can be a physical disk partition or logical volume, of whatever size you need for your data. For this document, we will use a 512MiB logical volume, which is more than sufficient for a single HTML file and (later) GFS2 metadata.
[root@pcmk-1 ~]# vgdisplay | grep -e Name -e Free
VG Name cs_pcmk-1
Free PE / Size 3583 / <14.00 GiB
[root@pcmk-1 ~]# lvcreate --name drbd-demo --size 512M cs_pcmk-1
Logical volume "drbd-demo" created.
[root@pcmk-1 ~]# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
drbd-demo centos_pcmk-1 -wi-a----- 512.00m
root centos_pcmk-1 -wi-ao---- 3.00g
swap centos_pcmk-1 -wi-ao---- 1.00g
Repeat for the second node, making sure to use the same size:
[root@pcmk-1 ~]# ssh pcmk-2 -- lvcreate --name drbd-demo --size 512M cs_pcmk-2
Logical volume "drbd-demo" created.
There is no series of commands for building a DRBD configuration, so simply run this on both nodes to use this sample configuration:
# cat <<END >/etc/drbd.d/wwwdata.res
resource wwwdata {
protocol C;
meta-disk internal;
device /dev/drbd1;
syncer {
verify-alg sha1;
}
net {
allow-two-primaries;
}
on pcmk-1 {
disk /dev/cs_pcmk-1/drbd-demo;
address 192.168.122.101:7789;
}
on pcmk-2 {
disk /dev/cs_pcmk-2/drbd-demo;
address 192.168.122.102:7789;
}
}
END
Important
Edit the file to use the hostnames, IP addresses and logical volume paths of your nodes if they differ from the ones used in this guide.
Note
Detailed information on the directives used in this configuration (and other alternatives) is available in the DRBD User’s Guide [https://docs.linbit.com/docs/users-guide-8.4/#ch-configure]. The allow-two-primaries option would not normally be used in an active/passive cluster. We are adding it here for the convenience of changing to an active/active cluster later.
With the configuration in place, we can now get DRBD running.
These commands create the local metadata for the DRBD resource, ensure the DRBD kernel module is loaded, and bring up the DRBD resource. Run them on one node:
[root@pcmk-1 ~]# drbdadm create-md wwwdata
initializing activity log
initializing bitmap (16 KB) to all zero
Writing meta data...
New drbd meta data block successfully created.
[root@pcmk-1 ~]# modprobe drbd
[root@pcmk-1 ~]# drbdadm up wwwdata
--== Thank you for participating in the global usage survey ==--
The server's response is:
you are the 801th user to install this version
We can confirm DRBD's status on this node:
[root@pcmk-1 ~]# drbdadm status
wwwdata role:Secondary
disk:Inconsistent
pcmk-2 connection:Connecting
Because we have not yet initialized the data, this node’s data is marked as Inconsistent. Because we have not yet initialized the second node, the pcmk-2 connection is Connecting (waiting for connection).
Now, repeat the above commands on the second node, starting with creating wwwdata.res. After giving it time to connect, when we check the status of the first node, it shows:
[root@pcmk-1 ~]# drbdadm status
wwwdata role:Secondary
disk:Inconsistent
pcmk-2 role:Secondary
peer-disk:Inconsistent
You can see that pcmk-2 connection:Connecting longer appears in the output, meaning the two DRBD nodes are communicating properly, and both nodes are in Secondary role with Inconsistent data.
To make the data consistent, we need to tell DRBD which node should be considered to have the correct data. In this case, since we are creating a new resource, both have garbage, so we’ll just pick pcmk-1 and run this command on it:
[root@pcmk-1 ~]# drbdadm primary --force wwwdata
Note
If you are using a different version of DRBD, the required syntax may be different. See the documentation for your version for how to perform these commands.
If we check the status of both nodes immediately, we’ll see something like this:
[root@pcmk-1 ~]# drbdadm status
wwwdata role:Primary
disk:UpToDate
pcmk-1 role:Secondary
peer-disk:UpToDate
[root@pcmk-2 ~]# drbdadm status
wwwdata role:Secondary
disk:UpToDate
pcmk-1 role:Primary
peer-disk:Inconsistent
We can see that the first node has the Primary role, its partner node has the Secondary role, the first node’s data is now considered UpToDate, the partner node’s data is still Inconsistent.
After a while, the sync should finish, and you’ll see something like:
[root@pcmk-1 ~]# drbdadm status
wwwdata role:Primary
disk:UpToDate
pcmk-1 role:Secondary
peer-disk:UpToDate
[root@pcmk-2 ~]# drbdadm status
wwwdata role:Secondary
disk:UpToDate
pcmk-1 role:Primary
peer-disk:UpToDate
Both sets of data are now UpToDate, and we can proceed to creating and populating a filesystem for our WebSite resource’s documents.
On the node with the primary role (pcmk-1 in this example), create a filesystem on the DRBD device:
[root@pcmk-1 ~]# mkfs.xfs /dev/drbd1
meta-data=/dev/drbd1 isize=512 agcount=4, agsize=32765 blks
= sectsz=512 attr=2, projid32bit=1
= crc=1 finobt=1, sparse=1, rmapbt=0
= reflink=1
data = bsize=4096 blocks=131059, imaxpct=25
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0, ftype=1
log =internal log bsize=4096 blocks=1368, version=2
= sectsz=512 sunit=0 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
Discarding blocks...Done.
Note
In this example, we create an xfs filesystem with no special options. In a production environment, you should choose a filesystem type and options that are suitable for your application.
Mount the newly created filesystem, populate it with our web document, give it the same SELinux policy as the web document root, then unmount it (the cluster will handle mounting and unmounting it later):
[root@pcmk-1 ~]# mount /dev/drbd1 /mnt
[root@pcmk-1 ~]# cat <<-END >/mnt/index.html
<html>
<body>My Test Site - DRBD</body>
</html>
END
[root@pcmk-1 ~]# chcon -R --reference=/var/www/html /mnt
[root@pcmk-1 ~]# umount /dev/drbd1
One handy feature pcs has is the ability to queue up several changes into a file and commit those changes all at once. To do this, start by populating the file with the current raw XML config from the CIB.
[root@pcmk-1 ~]# pcs cluster cib drbd_cfg
Using pcs’s -f option, make changes to the configuration saved in the drbd_cfg file. These changes will not be seen by the cluster until the drbd_cfg file is pushed into the live cluster’s CIB later.
Here, we create a cluster resource for the DRBD device, and an additional clone resource to allow the resource to run on both nodes at the same time.
[root@pcmk-1 ~]# pcs -f drbd_cfg resource create WebData ocf:linbit:drbd \
drbd_resource=wwwdata op monitor interval=60s
[root@pcmk-1 ~]# pcs -f drbd_cfg resource promotable WebData \
promoted-max=1 promoted-node-max=1 clone-max=2 clone-node-max=1 \
notify=true
[root@pcmk-1 ~]# pcs resource status
* ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-1
* WebSite (ocf::heartbeat:apache): Started pcmk-1
[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)
After you are satisfied with all the changes, you can commit them all at once by pushing the drbd_cfg file into the live CIB.
[root@pcmk-1 ~]# pcs cluster cib-push drbd_cfg --config
CIB updated
Let’s see what the cluster did with the new configuration:
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-1 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Wed Feb 3 09:04:23 2021
* Last change: Wed Feb 3 09:04:18 2021 by root via cibadmin on pcmk-1
* 2 nodes configured
* 4 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-1
* WebSite (ocf::heartbeat:apache): Started pcmk-1
* Clone Set: WebData-clone [WebData] (promotable):
* Masters: [ pcmk-1 ]
* Slaves: [ pcmk-2 ]
Daemon Status:
corosync: active/disabled
pacemaker: active/disabled
pcsd: active/enabled
We can see that WebData-clone (our DRBD device) is running as promoted (DRBD’s primary role) on pcmk-1 and unpromoted (DRBD’s secondary role) on pcmk-2.
Important
The resource agent should load the DRBD module when needed if it’s not already loaded. If that does not happen, configure your operating system to load the module at boot time. For CentOS Stream 8, you would run this on both nodes:
# echo drbd >/etc/modules-load.d/drbd.conf
Now that we have a working DRBD device, we need to mount its filesystem.
In addition to defining the filesystem, we also need to tell the cluster where it can be located (only on the DRBD Primary) and when it is allowed to start (after the Primary was promoted).
We are going to take a shortcut when creating the resource this time. Instead of explicitly saying we want the ocf:heartbeat:Filesystem script, we are only going to ask for Filesystem. We can do this because we know there is only one resource script named Filesystem available to pacemaker, and that pcs is smart enough to fill in the ocf:heartbeat: portion for us correctly in the configuration. If there were multiple Filesystem scripts from different OCF providers, we would need to specify the exact one we wanted.
Once again, we will queue our changes to a file and then push the new configuration to the cluster as the final step.
[root@pcmk-1 ~]# pcs cluster cib fs_cfg
[root@pcmk-1 ~]# pcs -f fs_cfg resource create WebFS Filesystem \
device="/dev/drbd1" directory="/var/www/html" fstype="xfs"
Assumed agent name 'ocf:heartbeat:Filesystem' (deduced from 'Filesystem')
[root@pcmk-1 ~]# pcs -f fs_cfg constraint colocation add \
WebFS with WebData-clone INFINITY with-rsc-role=Master
[root@pcmk-1 ~]# pcs -f fs_cfg constraint order \
promote WebData-clone then start WebFS
Adding WebData-clone WebFS (kind: Mandatory) (Options: first-action=promote then-action=start)
We also need to tell the cluster that Apache needs to run on the same machine as the filesystem and that it must be active before Apache can start.
[root@pcmk-1 ~]# pcs -f fs_cfg constraint colocation add WebSite with WebFS INFINITY
[root@pcmk-1 ~]# pcs -f fs_cfg constraint order WebFS then WebSite
Adding WebFS WebSite (kind: Mandatory) (Options: first-action=start then-action=start)
Review the updated configuration.
[root@pcmk-1 ~]# pcs -f fs_cfg constraint
Location Constraints:
Resource: WebSite
Enabled on:
Node: pcmk-1 (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)
Colocation Constraints:
WebSite with ClusterIP (score:INFINITY)
WebFS with WebData-clone (score:INFINITY) (with-rsc-role:Master)
WebSite with WebFS (score:INFINITY)
Ticket Constraints:
[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):
* Masters: [ pcmk-1 ]
* Slaves: [ 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=60s (WebData-monitor-interval-60s)
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)
After reviewing the new configuration, upload it and watch the cluster put it into effect.
[root@pcmk-1 ~]# pcs cluster cib-push fs_cfg --config
CIB updated
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-1 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Wed Feb 3 09:17:24 2021
* Last change: Wed Feb 3 09:17:19 2021 by root via cibadmin on pcmk-1
* 2 nodes configured
* 5 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-1
* WebSite (ocf::heartbeat:apache): Started pcmk-1
* Clone Set: WebData-clone [WebData] (promotable):
* Masters: [ pcmk-1 ]
* Slaves: [ pcmk-2 ]
* WebFS (ocf::heartbeat:Filesystem): Started pcmk-1
Daemon Status:
corosync: active/disabled
pacemaker: active/disabled
pcsd: active/enabled
Previously, we used pcs cluster stop pcmk-1 to stop all cluster services on pcmk-1, failing over the cluster resources, but there is another way to safely simulate node failure.
We can put the node into standby mode. Nodes in this state continue to run corosync and pacemaker but are not allowed to run resources. Any resources found active there will be moved elsewhere. This feature can be particularly useful when performing system administration tasks such as updating packages used by cluster resources.
Put the active node into standby mode, and observe the cluster move all the resources to the other node. The node’s status will change to indicate that it can no longer host resources, and eventually all the resources will move.
[root@pcmk-1 ~]# pcs node standby pcmk-1
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-1 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Wed Feb 3 09:18:45 2021
* Last change: Wed Feb 3 09:18:35 2021 by root via cibadmin on pcmk-1
* 2 nodes configured
* 5 resource instances configured
Node List:
* Node pcmk-1: standby
* Online: [ pcmk-2 ]
Full List of Resources:
* ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-2
* WebSite (ocf::heartbeat:apache): Started pcmk-2
* Clone Set: WebData-clone [WebData] (promotable):
* Masters: [ pcmk-2 ]
* Stopped: [ pcmk-1 ]
* WebFS (ocf::heartbeat:Filesystem): Started pcmk-2
Daemon Status:
corosync: active/disabled
pacemaker: active/disabled
pcsd: active/enabled
Once we’ve done everything we needed to on pcmk-1 (in this case nothing, we just wanted to see the resources move), we can allow the node to be a full cluster member again.
[root@pcmk-1 ~]# pcs node unstandby pcmk-1
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-1 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Wed Feb 3 09:19:47 2021
* Last change: Wed Feb 3 09:19:29 2021 by root via cibadmin on pcmk-1
* 2 nodes configured
* 5 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-1
* WebSite (ocf::heartbeat:apache): Started pcmk-1
* Clone Set: WebData-clone [WebData] (promotable):
* Masters: [ pcmk-1 ]
* Slaves: [ pcmk-2 ]
* WebFS (ocf::heartbeat:Filesystem): Started pcmk-1
Daemon Status:
corosync: active/disabled
pacemaker: active/disabled
pcsd: active/enabled
Notice that pcmk-1 is back to the Online state, and that the cluster resources stay where they are due to our resource stickiness settings configured earlier.
| [1] | See http://www.drbd.org for details. |
| [2] | Since version 2.6.33 |