to achieve high availability in Vertica, a segmented projection should have at least one buddy projection, and an unsegmented projection should be replicated on all the nodes. Vertica에서는, K-safety enforces these projection requirements
2. K-Safety와 Buddy Projection
K-Safety = 1인 DB에서는 auto projections (that Vertica creates) will have a buddy projection
이러한 복제본(buddy projection)은 장애가 발생한 노드에 대해 다른 노드를 대신할 수 있게 하여 DB가 계속 실행되면서 데이터 무결성 보장 가능
만약 a custom projection was creating w/o the ksafe1 keyword, the projection does not have a buddy projection, and the projection is marked unsafe until the buddy projection is explicitly created
all unsafe projections are restricted as follows:
not up to date
cannot be refreshed
cannot receive data loaded into a table
are not used for answering queries
3. K-Safety 값
Vertica에서 가능한 K 값 = 0, 1, 2
K = 1인 DB에서 노드가 하나 down되어도 DB는 계속 정상적으로 실행됨
잠재적으로, 클러스터에서 적어도 하나의 다른 노드가 down된 노드의 데이터 복사본을 가지고 있는 한, DB는 계속 실행될 수 있음
K = 0이면 node 1 or 2로 구성되며, HA구성 불가능
K=1이면 node는 3개 이상, K=2이면 node는 5개 이상 필요하며, HA구성 가능
in Vertica versions prior to 8.0.1, the get_node_dependencies API did not provide dependency bits to node mapping
if you are running an earlier version of Vertica and reading the node dependency bits, the rightmost bit represents the permanent node with the lowest node_id and the leftmost bit represents the permanent bit with the highest node_id
in Vertica 8.0.1, you can run the verbose version of get_node_dependencies to see the node names next to the bit pattern:
=> SELECT get_node_dependencies_verbose ();
you can run the following statement to get the node names in ascending order of node_id:
=> SELECT node_name FROM nodes ORDER BY node_id;
node_name
-------------------
node2
node1
node3
node4
node5
(5 rows)
a bit value of 1 means that the node representing the bit is the buddy of another node with a bit value of 1
e.g., in dependency bits 00110, a node with the second lowest node_id is the buddy node for a node with the third lowest node_id
the cnt value represents the number of projection buddy pairs in the DB
a dependency with all bits set to 1 represents an unsegmented projection and teh cnt value gives the number of unsegmented projections in the DB
if you drew a graph with the nodes as vertices and the dependencies b/w the nodes as the edges of the graph, the graph would be a ring where each vertice had two edges to maintain a K-safety of 1
the following figure shows the node dependency ring:
if all the nodes in the ring had three edges, that would represent a DB with a K-safety of 2
if only some nodes had more than two edges, that ring would indicate that rebalance activity is incomplete
you can run the following statement to recompute node dependencies
=> SELECT RECOMPUTE_NODE_DEPENDENCIES();
2. High Availability와 Data Safety
a Vertica DB with a K-safety = 1 will remain UP and fully functional if more than half the nodes in the DB are UP and no two adjacent nodes in teh dependency ring are down (this feature is known as data safety)
e.g., the DB remains UP if node1 and node4 go DOWN, b/c they are not adjacent nodes in the dependency ring
the DB will perform an unsafe SHUTDOWN if node1 and node5 go DOWN at the same time
this happens b/c the segment of data on those nodes is no longer accessible
when you have a node DOWN in your cluster, you can run the following query to get a list of critical nodes
the results table is useful when DB administrators are planning scheduled maintenance on more than one node:
=> SELECT * FROM critical_nodes;
for best resuls, Vertica engineers recomment the following:
a K-safety = 1 and do not recommend a K-safety = 2. Although you can have two adjacent nodes in the dependency ring go DOWN while the system remains UP, the chances of that occurence are slim. Additionally, a K-safety of 2 uses 50% more system resources. This usage could slow system performance
making you DB rack aware by defining fault groups for large clusters that span more than two racks
3. DB Recovery and Data Safety
when you start Vertica, you need more than half the nodes to form a quorum
no two adjacent nodes in the dependency ring can be left out
