Shared Storage (GlusterFS)¶
While Docker Swarm is great for keeping containers running (and restarting those that fail), it does nothing for persistent storage. This means if you actually want your containers to keep any data persistent across restarts (hint: you do!), you need to provide shared storage to every docker node.
This GlusterFS recipe was my original design for shared storage, but I found it to be flawed, and I replaced it with a design which employs Ceph instead. This recipe is an alternate to the Ceph design, if you happen to prefer GlusterFS.
3 x Virtual Machines (configured earlier), each with:
- CentOS/Fedora Atomic
- At least 1GB RAM
- At least 20GB disk space (but it'll be tight)
- Connectivity to each other within the same subnet, and on a low-latency link (i.e., no WAN links)
- A second disk, or adequate space on the primary disk for a dedicated data partition
Create Gluster "bricks"¶
To build our Gluster volume, we need 2 out of the 3 VMs to provide one "brick". The bricks will be used to create the replicated volume. Assuming a replica count of 2 (i.e., 2 copies of the data are kept in gluster), our total number of bricks must be divisible by our replica count. (I.e., you can't have 3 bricks if you want 2 replicas. You can have 4 though - We have to have minimum 3 swarm manager nodes for fault-tolerance, but only 2 of those nodes need to run as gluster servers.)
On each host, run a variation following to create your bricks, adjusted for the path to your disk.
The example below assumes /dev/vdb is dedicated to the gluster volume
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( echo o # Create a new empty DOS partition table echo n # Add a new partition echo p # Primary partition echo 1 # Partition number echo # First sector (Accept default: 1) echo # Last sector (Accept default: varies) echo w # Write changes ) | sudo fdisk /dev/vdb mkfs.xfs -i size=512 /dev/vdb1 mkdir -p /var/no-direct-write-here/brick1 echo '' >> /etc/fstab >> /etc/fstab echo '# Mount /dev/vdb1 so that it can be used as a glusterfs volume' >> /etc/fstab echo '/dev/vdb1 /var/no-direct-write-here/brick1 xfs defaults 1 2' >> /etc/fstab mount -a && mount
Don't provision all your LVM space
Atomic uses LVM to store docker data, and automatically grows Docker's volumes as requried. If you commit all your free LVM space to your brick, you'll quickly find (as I did) that docker will start to fail with error messages about insufficient space. If you're going to slice off a portion of your LVM space in /dev/atomicos, make sure you leave enough space for Docker storage, where "enough" depends on how much you plan to pull images, make volumes, etc. I ate through 20GB very quickly doing development, so I ended up provisioning 50GB for atomic alone, with a separate volume for the brick.
Create glusterfs container¶
Atomic doesn't include the Gluster server components. This means we'll have to run glusterd from within a container, with privileged access to the host. Although convoluted, I've come to prefer this design since it once again makes the OS "disposable", moving all the config into containers and code.
Run the following on each host:
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docker run \ -h glusterfs-server \ -v /etc/glusterfs:/etc/glusterfs:z \ -v /var/lib/glusterd:/var/lib/glusterd:z \ -v /var/log/glusterfs:/var/log/glusterfs:z \ -v /sys/fs/cgroup:/sys/fs/cgroup:ro \ -v /var/no-direct-write-here/brick1:/var/no-direct-write-here/brick1 \ -d --privileged=true --net=host \ --restart=always \ --name="glusterfs-server" \ gluster/gluster-centos
Create trusted pool¶
On a single node (doesn't matter which), run
docker exec -it glusterfs-server bash to launch a shell inside the container.
From the node, run
gluster peer probe <other host>
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[root@glusterfs-server /]# gluster peer probe ds1 peer probe: success. [root@glusterfs-server /]#
gluster peer status on both nodes to confirm that they're properly connected to each other:
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[root@glusterfs-server /]# gluster peer status Number of Peers: 1 Hostname: ds3 Uuid: 3e115ba9-6a4f-48dd-87d7-e843170ff499 State: Peer in Cluster (Connected) [root@glusterfs-server /]#
Create gluster volume¶
Now we create a replicated volume out of our individual "bricks".
Create the gluster volume by running
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gluster volume create gv0 replica 2 \ server1:/var/no-direct-write-here/brick1 \ server2:/var/no-direct-write-here/brick1
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[root@glusterfs-server /]# gluster volume create gv0 replica 2 ds1:/var/no-direct-write-here/brick1/gv0 ds3:/var/no-direct-write-here/brick1/gv0 volume create: gv0: success: please start the volume to access data [root@glusterfs-server /]#
Start the volume by running
gluster volume start gv0
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[root@glusterfs-server /]# gluster volume start gv0 volume start: gv0: success [root@glusterfs-server /]#
The volume is only present on the host you're shelled into though. To add the other hosts to the volume, run
gluster peer probe <servername>. Don't probe host from itself.
From one other host, run
docker exec -it glusterfs-server bash to shell into the gluster-server container, and run
gluster peer probe <original server name> to update the name of the host which started the volume.
Mount gluster volume¶
On the host (i.e., outside of the container - type
exit if you're still shelled in), create a mountpoint for the data, by running
mkdir /var/data, add an entry to fstab to ensure the volume is auto-mounted on boot, and ensure the volume is actually mounted if there's a network / boot delay getting access to the gluster volume:
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mkdir /var/data MYHOST=`hostname -s` echo '' >> /etc/fstab >> /etc/fstab echo '# Mount glusterfs volume' >> /etc/fstab echo "$MYHOST:/gv0 /var/data glusterfs defaults,_netdev,context="system_u:object_r:svirt_sandbox_file_t:s0" 0 0" >> /etc/fstab mount -a
For some reason, my nodes won't auto-mount this volume on boot. I even tried the trickery below, but they stubbornly refuse to automount.
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echo -e "\n\n# Give GlusterFS 10s to start before \ mounting\nsleep 10s && mount -a" >> /etc/rc.local systemctl enable rc-local.service
For non-gluster nodes, you'll need to replace $MYHOST above with the name of one of the gluster hosts (I haven't worked out how to make this fully HA yet)
After completing the above, you should have:
[X] Persistent storage available to every node [X] Resiliency in the event of the failure of a single (gluster) node
Future enhancements to this recipe include:
- Migration of shared storage from GlusterFS to Ceph ()#2)
- Correct the fact that volumes don't automount on boot (#3)
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