LCD stores the following related types of data:
- Dependency Information
- Resource Status Information
- Inter-Server Equivalency Information
For each defined resource, LifeKeeper maintains a list of dependencies and a list of dependents (resources depending on a resource.) For information, see the LCDI_relationship (1M) and LCDI_instances (1M) manual pages.
Resource Status Information
LCD maintains status information in memory for each resource instance. The resource states recognized by LCD are ISP, ISU, OSF, OSU and ILLSTATE. Resources may change from one state to another when a system event occurs or when an administrator takes certain actions. When a resource changes states, the status change is reflected in the LCD on the local server as well as in the database of the backup servers for that resource.
Inter-Server Equivalency Information
Relationships may exist between resources on various servers. A shared equivalency is a relationship between two resources on different servers that represents the same physical entity. When two servers have a resource with a shared equivalency relationship, LifeKeeper attempts to ensure in its actions that only one of the two servers has the resource instance in the in-service, protected [ISP] state at any one time. Both servers can have the resource instance in an out-of-service state [OSU or OSF], but for data integrity reasons, only one server can have the resource in service at any given time.
Disks on a Small Computer System Interface (SCSI) bus are one example of equivalent resources. With the SCSI locking (or reserve) mechanism, only one server can own the lock for a disk device at any point in time. This lock ownership feature guarantees that two or more servers cannot access the same disk resource at the same time.
Furthermore, the dependency relationships within a hierarchy guarantee that all resources that depend upon the disk, such as a file system, are in service on only one server at a time.
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