A helper for DRBD which makes/reverts links into DRBD volumes.
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README.markdown

DRBDLINKS README

Sean Reifschneider jafo00@gmail.com
Homepage: http://www.tummy.com/Community/software/drbdlinks/
Code/bugfixes: https://github.com/linsomniac/drbdlinks
drbdlinks is under the following license: GPLv2

Status

drbdlinks is stable and in maintenance mode. It is included in many distributions (Debian, Ubuntu, Fedora, CentOS and variants).

Overview

drbdlinks is a program that will manage creation and removal of symbolic links. It is primarily used with clusters of machines using either shared storage or the "DRBD" replicated block device. It has the ability to fix SELinux contexts and restart cron and syslog as part of the linking process.

While the name of the program is "drbdlinks", it can be used in any shared-storage sort of environment where the shared storage is only mounted on the active node. In cases like NFS where the shared storage is always mounted on all nodes, drbdlinks is not necessary.

The advantage over creating static symbolic links is that package updates often require that directories point at real files, so updates can often fail if you do not have the shared storage mounted.

drbdlinks also supports multiple instances of links, in the case of active/active clusters. For example, if you have MySQL running in one resource group, and Apache running in another, you can use the "-c" switch to specify a configuration file for each resource group.

A simple configuration file, "/etc/drbdlinks.conf", specifies the links. This can be used to manage links for /etc/httpd, /var/lib/pgsql, and other system directories that need to appear as if they are local to the system when running applications after a drbd shared partition has been mounted.

Optionally, configuration directives can also be written to files in "/etc/drbdlinks.d" with the suffix ".conf", which are loaded after the "/etc/drbdlinks.conf" file, in sorted order.

When run with "start" as the mode, drbdlinks will rename the existing files/directories, and then make symbolic links into the DRBD partition. "stop" does the reverse. By default, the rename appends ".drbdlinks" to the name, but this can be overridden.

An init script is included which runs "stop" before heartbeat starts, and after heartbeat stops. This is done to try to ensure that when the shared partition isn't mounted, the links are in their normal state.

Getting Started

The first thing you need to do is create a "/etc/drbdlinks.conf" file. The package ships with an example, primarily you will need to uncomment and modify the "mountpoint" and "link" directives. The "mountpoint" directive tells drbdlinks the root of where your shared storage for this resource group is mounted.

Next, you will need to set up the shared storage.

Automatic population: drbdlinks now includes an "initialize_shared_storage" mode that will look at the links in the config file, and if they don't exist in the shared storage it will populate them from the root file-system. Parent paths that do not exist will be set to the same ownership and mode as the same directory from the source file-system, if they share the same name. So for example, if you have a "link('/etc/apache2', '/shared/etc/apache2')", it will create "/shared/etc" and "/shared/etc/apache2" with the same permissions/ownership.

Manual population: Create the directories specified by the "link" directives in the configuration file and copy the appropriate files into them.

Now run "drbdlinks checklinks". This will test the configuration file and make sure all the specified links exist in the shared storage.

Now you just need to list "drbdlinks" in your resources, it needs to be after the shared storage is mounted, but before any references to it are used. I usually bring it up immediately after the resource that mounts the shared storage.

About Apache

The easy route is to just set up all of "/etc/apache2" or "/etc/httpd" as the directory that is linked. However, this contains many sub-directories or links that may best be left on the root file-system, so that updates don't get the shared storage and the root file-system out of sync.

In particular, the links to Apache modules may become out of sync.

However, it also requires some discipline to ensure that all of the configuration changes you make are not in system directories or files. For example, ideally you would customize the configuration only in the "conf.d", "sites-available", "sites-enabled", and "mods-enabled" directories.

I haven't yet tested these, but I'd propose you'd need to use the following.

For RHEL/CentOS/Fedora:

link('/etc/httpd/conf.d')
link('/etc/httpd/conf/httpd.conf')
link('/var/log/httpd')

For Debian/Ubuntu:

link('/etc/apache2/mods-enabled')
link('/etc/apache2/sites-enabled')
link('/etc/apache2/sites-available')
link('/etc/apache2/conf.d')
link('/etc/apache2/ports.conf')
link('/etc/apache2/envvars')
link('/etc/apache2/apache2.conf')
link('/var/log/apache2')

Please let me know if these values work or do not work for you.

OCF Resource

drbdlinks can also be used as an OCF resource. Following example could contain RHEL/CentOS/Fedora specific paths and names, but should give an impression how drbdlinks could be used in a Pacemaker cluster setup. It of course requires Corosync and Pacemaker configuration before, see the ClusterLabs documentation for details.

Create a Pacemaker resource for DRBD resource "data", assumes existing DRBD configuration:

pcs cluster cib data_drbd_cfg
pcs -f data_drbd_cfg resource create data_drbd ocf:linbit:drbd drbd_resource=data
pcs -f data_drbd_cfg resource op add data_drbd start interval=0 timeout=120s
pcs -f data_drbd_cfg resource op add data_drbd stop interval=0 timeout=60s
pcs -f data_drbd_cfg resource op add data_drbd monitor role=Master interval=59s timeout=30s
pcs -f data_drbd_cfg resource op add data_drbd monitor role=Slave interval=60s timeout=30s
pcs -f data_drbd_cfg resource master data_clone data_drbd master-max=1 master-node-max=1 clone-max=2 clone-node-max=1 notify=true
pcs cluster cib-push data_drbd_cfg

Filesystem resource to mount ext4 filesystem on /dev/drbd0 to /data (after DRBD device got primary), assumes existing ext4 filesystem on DRBD device:

pcs cluster cib data_fs_cfg
pcs -f data_fs_cfg resource create data_fs ocf:heartbeat:Filesystem device="/dev/drbd0" directory="/data" fstype="ext4" op monitor interval=60s
pcs -f data_fs_cfg constraint colocation add data_fs data_clone INFINITY with-rsc-role=Master
pcs -f data_fs_cfg constraint order promote data_clone then start data_fs
pcs cluster cib-push data_fs_cfg

Resource for drbdlinks to create symbolic links as per /etc/drbdlinks.conf; on same node but after the filesystem was mounted, assumes proper drbdlinks configuration before:

pcs cluster cib drbdlinks_cfg
pcs -f drbdlinks_cfg resource create drbdlinks ocf:tummy:drbdlinks op monitor interval=60s
pcs -f drbdlinks_cfg constraint colocation add drbdlinks data_fs INFINITY
pcs -f drbdlinks_cfg constraint order data_fs then drbdlinks
pcs cluster cib-push drbdlinks_cfg

Apache resource that shall handle its configuration and log files on DRBD partition (after drbdlinks was started):

pcs cluster cib httpd_cfg
pcs -f httpd_cfg resource create httpd systemd:httpd
pcs -f httpd_cfg resource op add httpd monitor interval=60s timeout=30s
pcs -f httpd_cfg resource op add httpd start interval=0 timeout=120s
pcs -f httpd_cfg resource op add httpd stop interval=0 timeout=120s
pcs -f httpd_cfg constraint colocation add httpd drbdlinks INFINITY
pcs -f httpd_cfg constraint order drbdlinks then httpd
pcs cluster cib-push httpd_cfg