GlusterFS Server and Arbiter

When running two GlusterFS servers as a replica, there is a slight chance that a "split brain" situation will occour. To get around this a third GlusterFS server is configured as an arbiter. The arbiter does not have a copy of the data from the other GlusterFS servers, but it has all the metadata. This way the arbiter can help the other servers avoid the "split brain" scenario, without using the diskspace for the data.

In this setup we will configure 2 GlusterFS servers as a replica and one as the arbiter.

Network

The GlusterFS servers will have fixed IP-addresses. That is configured in the DHCP servers list of statically assigned IP-adresses by using the Domains MAC address.

Disk

Add hardware

To keep things separated the OS will be on one disk and the glusterfs filesystem will be on another disk. For two of the three servers we will add a disk to the domain. This disk will be used for the GlusterFS file storage.

Configure disk

Partition

Create one large partition on /dev/vdb with partition type 8e Linux LVM.

LVM

Set up a new physical volume, configure a volume group. 

pvcreate /dev/vdb1
vgcreate vg2 /dev/vdb1

Replica

Add a volume 

lvcreate --size 4G --name gluster_home vg2

Arbiter

Add a volume 

lvcreate --size 512M --name gluster_home vg2

Format

Create a filesystem on the volume. 

mkfs.btrfs /dev/vg2/gluster_home

Mountpoint

Create the mountpoint. 

mkdir /srv/home

fstab

Add the volume to /etc/fstab. 

/dev/vg2/gluster_www /srv/home               btrfs   defaults        0       0

Mount

Mount the new volume. 

mount /srv/home

If the mount command does not succeed, it it most likely because the fstab entry is incorrect.

Software

Mount Point

Glusterfs is not happy about using a directory which is also a mountpoint.

volume create: home: failed: The brick gluster01:/srv/home is a mount point. Please create a sub-directory under the mount point and use that as the brick directory. Or use 'force' at the end of the command if you want to override this behavior.

If for some reason the filesystem is not mounted, then glusterfs might misunderstand the situation and tell the other servers that the directory is now empty, and all files would be deleted on all servers. To avoid this a directory is created under the mount point. 

mkdir /srv/home/brick

Install

Install the server. 

apt-get install glusterfs-server

Introduce Servers

The GlusterFS servers need to now each other. This can be done from any one server. Here we will do it from gluster05. We need to introduce the two other servers. 

gluster peer probe gluster06
gluster peer probe gluster07

Server Status

Now check that the servers were properly probed. 

gluster peer status

Number of Peers: 3

Hostname: gluster02
Uuid: 031573c2-3b1f-4946-bd78-421563249db6
State: Peer in Cluster (Connected)

Hostname: gluster03
Uuid: ff5cec1c-6d7f-4db7-8676-08deff06b4d0
State: Peer in Cluster (Connected)

Hostname: gluster04
Uuid: 65fc398a-52e9-4292-9bbb-884becfbf5d6
State: Peer in Cluster (Connected)

Create Volume

Create the gluster volume. 

gluster volume create www replica 4 transport tcp gluster01:/srv/www/brick gluster02:/srv/www/brick gluster03:/srv/www/brick gluster04:/srv/www/brick

Start Volume

Finally we can start the volume. 

gluster volume start www

Volume Status

Check the status of the volume. 

gluster volume status

Status of volume: www
Gluster process                             TCP Port  RDMA Port  Online  Pid
------------------------------------------------------------------------------
Brick gluster01:/srv/www                    49152     0          Y       826  
Brick gluster02:/srv/www                    49152     0          Y       1355 
Brick gluster03:/srv/www                    49152     0          Y       1034 
Brick gluster04:/srv/www                    49152     0          Y       1135 
Self-heal Daemon on localhost               N/A       N/A        Y       846  
Self-heal Daemon on gluster02               N/A       N/A        Y       1377 
Self-heal Daemon on gluster03               N/A       N/A        Y       1054 
Self-heal Daemon on gluster04               N/A       N/A        Y       1155 
 
Task Status of Volume www
------------------------------------------------------------------------------
There are no active volume tasks