Chapter 4. Handling a data center failure

Red Hat Ceph Storage can withstand catastrophic failures to the infrastructure, such as losing one of three data centers in a stretch cluster. For the standard object store use case, configuring all three data centers can be done independently with replication set up between them. In this scenario, the cluster configuration in each of the data centers might be different, reflecting the local capabilities and dependencies.

A logical structure of the placement hierarchy should be considered. A proper CRUSH map can be used, reflecting the hierarchical structure of the failure domains within the infrastructure. Using logical hierarchical definitions improves the reliability of the storage cluster, versus using the standard hierarchical definitions. Failure domains are defined in the CRUSH map. The default CRUSH map contains all nodes in a flat hierarchy.

In three data center environment example, with a stretch cluster, the placement of nodes should be managed in a way that one data center can go down, but the storage cluster stays up and running. Consider which failure domain a node resides in when using 3-way replication for the data, in the case of an outage of one data center, it is possible that some data can be left with one copy. When this scenario happens, there are two options:

With the standard settings, and because of the randomness of data placement across the nodes, not all the data will be affected, but some data can have only one copy and the storage cluster would revert to read-only mode.

In the example below the resulting map is derived from the initial setup of the cluster with 6 OSD nodes. In this example all nodes have only one disk and hence one OSD. All of the nodes are arranged under the default root, that is the standard root of the hierarchy tree. Because there is a weight assigned to two of the OSDs, these OSDs receive fewer chunks of data than the other OSDs. These nodes were introduced later with bigger disks than the initial OSD disks. This does not affect the data placement to withstand a failure of a group of nodes.

$ sudo ceph osd tree
ID WEIGHT  TYPE NAME           UP/DOWN REWEIGHT PRIMARY-AFFINITY
-1 0.33554 root default
-2 0.04779     host ceph-node3
 0 0.04779         osd.0            up  1.00000          1.00000
-3 0.04779     host ceph-node2
 1 0.04779         osd.1            up  1.00000          1.00000
-4 0.04779     host ceph-node1
 2 0.04779         osd.2            up  1.00000          1.00000
-5 0.04779     host ceph-node4
 3 0.04779         osd.3            up  1.00000          1.00000
-6 0.07219     host ceph-node6
 4 0.07219         osd.4            up  0.79999          1.00000
-7 0.07219     host ceph-node5
 5 0.07219         osd.5            up  0.79999          1.00000

Using logical hierarchical definitions to group the nodes into same data center, can achieve data placement maturity. Possible definition types of root, datacenter, rack, row and host allow the reflection of the failure domains for the three data center stretch cluster:

Since all OSDs in a host belong to the host definition there is no change needed. All the other assignments can be adjusted during runtime of the storage cluster by:

Within this structure any new hosts can be added too, as well as new disks. By placing the OSDs at the right place in the hierarchy the CRUSH algorithm is changed to place redundant pieces into different failure domains within the structure.

The above example results in the following:

$ sudo ceph osd tree
ID  WEIGHT  TYPE NAME               UP/DOWN REWEIGHT PRIMARY-AFFINITY
 -8 6.00000 root allDC
 -9 2.00000     datacenter DC1
 -4 1.00000         host ceph-node1
  2 1.00000             osd.2            up  1.00000          1.00000
 -3 1.00000         host ceph-node2
  1 1.00000             osd.1            up  1.00000          1.00000
-10 2.00000     datacenter DC2
 -2 1.00000         host ceph-node3
  0 1.00000             osd.0            up  1.00000          1.00000
 -7 1.00000         host ceph-node5
  5 1.00000             osd.5            up  0.79999          1.00000
-11 2.00000     datacenter DC3
 -6 1.00000         host ceph-node6
  4 1.00000             osd.4            up  0.79999          1.00000
 -5 1.00000         host ceph-node4
  3 1.00000             osd.3            up  1.00000          1.00000
 -1       0 root default

The listing from above shows the resulting CRUSH map by displaying the osd tree. Easy to see is now how the hosts belong to a data center and all data centers belong to the same top level structure but clearly distinguishing between locations.