Understanding and Designing Serviceguard Disaster Recovery Architectures

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 message 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.
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.
continental cluster A group of clusters that use routed networks or common carrier networks or both for data
replication and cluster communication to support package failover between separate clusters in
different data centers. Continental clusters are often located in different cities or different countries
and can span up to thousands of kilometers.
Continentalclusters
A Hewlett-Packard product that allows a customer to configure an Serviceguard cluster as a
disaster recovery continental cluster.
Continuous Access A facility provided by the Continuos Access software option available with the HP Storage Disk
Array XP series. This facility enables physical data replication between XP series disk arrays.
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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 contains the most recent transactions, or whether the replica database has all
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 part 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 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 recovery. 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 Primary Cluster
packages is transferred to the Recovery Cluster and made available for use on the Recovery
Cluster in the event of a recovery.
database
replication
A software-based logical data replication scheme that is offered by most database vendors.
disaster An event causing the failure of multiple components or entire data centers that render unavailable
all services at a single location; these include natural disasters such as earthquake, fire, or flood,
acts of terrorism or sabotage, large-scale power outages.
disaster recovery The process of restoring access to applications and data after a disaster. Disaster recovery can
be manual, meaning human intervention is required, or it can be automated, requiring little or
no human intervention.
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