Designing Disaster Recovery Clusters using Metroclusters and Continentalclusters, Reprinted October 2011 (5900-1881)

of nodes, networks, application services, EMS resources, and makes failover decisions based on
where the application is able to run successfully.
cluster alarm Time at which a message is sent indicating that the cluster is probably in need of recovery. The
cmrecoverclcommand is enabled at this time.
cluster alert Time at which a message is sent indicating a problem with the cluster.
cluster event A cluster condition that occurs when the cluster goes down or enters an UNKNOWN state, or
when the monitor software returns an error. This event may cause an alert messages to be sent
out, or it may cause an alarm condition to be set, which allows the administrator on the Recovery
Cluster to issue the cmrecovercl command. The return of the cluster to the UP state results in
a cancellation of the event, which may be accompanied by a cancel event notice. In addition,
the cancellation disables the use of the cmrecovercl command.
cluster quorum A dynamically calculated majority used to determine whether any grouping of nodes is sufficient
to start or run the cluster. Cluster quorums prevent split-brain syndrome which can lead to data
corruption or inconsistency. Currently at least 50% of the nodes plus a tie-breaker are required
for a quorum. If no tie-breaker is configured, then greater than 50% of the nodes is required to
start and run a cluster.
command device A disk area in the HP StorageWorks P9000 Disk Array family or HP StorageWorks XP Disk Array
series disk array used for internal system communication. You create two command devices on
each array, each with alternate links (PV links).
complex workload Complex workloads are applications that are configured using multiple inter-related packages
that are managed collectively
consistency group A set of Symmetrix RDF devices that are configured to act in unison to maintain the integrity of
a database. Consistency groups allow you to configure R1/R2 devices on multiple Symmetrix
frames in Metrocluster with EMC SRDF.
Continentalclusters
A group of clusters that use routed networks and/or common carrier networks for data replication
and cluster communication to support package failover between separate clusters in different
data centers. Continentalclusters are often located in different cities or different countries and can
span 100s or 1000s of kilometers.
Continuous Access A facility provided by the Continuos Access software option available with the HP StorageWorks
P9000 Disk Array family, HP StorageWorks E Disk Array XP series. This facility enables physical
data replication between P9000 or XP series disk arrays.
D
data center A physically proximate collection of nodes and disks, usually all in one room.
data consistency Whether data are logically correct and immediately usable; the validity of the data after the last
write. Inconsistent data, if not recoverable to a consistent state, is corrupt.
data currency Whether the data contain the most recent transactions, and/or whether the replica database has
all of the committed transactions that the primary database contains; speed of data replication
may cause the replica to lag behind the primary copy, and compromise data currency.
data loss The inability to take action to recover data. Data loss can be the result of transactions being
copied that were lost when a failure occurred, non-committed transactions that were rolled back
as pat of a recovery process, data in the process of being replicated that never made it to the
replica because of a failure, transactions that were committed after the last tape backup when a
failure occurred that required a reload from the last tape backup. transaction processing monitors
(TPM), message queuing software, and synchronous data replication are measures that can
protect against data loss.
data mirroring See See mirroring..
data recoverability The ability to take action that results in data consistency, for example database rollback/roll
forward recovery.
data replication The scheme by which data is copied from one site to another for disaster tolerance. Data replication
can be either physical (see physical data replication) or logical (see logical data replication). In
a Continentalclusters environment, the process by which data that is used by the cluster packages
516 Glossary