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SteelEye DataKeeper for Linux Glossary of Terms

Asynchronous mirroring
A type of mirroring in which the primary system writes to the local source device and queues a copy of that write to be transmitted to the target device on the backup system. The data need not reach the target system before the write is acknowledged as complete to the operating system.
Backup system/server
This is a synonym for alternate, spare, or secondary system/server. In a DataKeeper environment, the target device resides on the backup system unless there is a failure of the primary system.
Intent log
A bitmap indicating which data blocks are out of sync between the target and source devices. The intent log can be used to avoid a full resynchronization following a failure.
md is the multiple disk driver in the Linux kernel. It is a software RAID manager that provides various RAID levels. SteelEye DataKeeper for Linux uses md raid1 in conjunction with NBD to provide network replication as a shared storage replacement.
mdadm (multiple devices admin) is a raid management package
A disk or partition on a primary server whose contents are synchronized across a network to a corresponding disk or partition on a backup server.
Mirror status
The current status of a mirror. Possible states are: Fully operational, Paused, Resyncing, or Out Of Sync. The mirror status can be viewed from the Resource Properties dialog of the DataKeeper resource.
NetRAID device
A DataKeeper device represented as an md raid1 device, which consists of a local disk or partition and a Network Block Device (NBD). 
Network Block Device (NBD)
A device driver in the Linux kernel that lets Linux use a disk on a remote server as one of its local block devices.
Primary system/server
This is the name of the server in a LifeKeeper configuration with the highest priority for a given resource hierarchy. In a DataKeeper environment, the source device resides on the primary system. All client reads and writes are made to the primary server unless there is a failure.
The mirroring of data from a source device to a target device across a network.
Resynchronization (Resync)
The process by which SteelEye DataKeeper synchronizes the mirrored data between the source and target devices. This process takes place when the mirror is created, after a pause operation, or after a break in the connection between the source and target. A resync may be full or partial. A full synchronization is performed when the DataKeeper resource is created, and after a failover (with 2.6.18 and earlier kernels – with 2.6.19 and later kernels or with RedHat Enterprise Linux 5.4 kernels 2.6.18-164 or later (or a supported derivative of RedHat 5.4 or later), a partial resync occurs after failover). Otherwise, as long as an intent log file is designated, a partial resynchronization is performed when needed.
In a DataKeeper (mirroring) environment, the source device resides on the primary server. Clients read from and write to the source device only, and writes are replicated across the network to a target device located on a backup server.
Synchronous mirroring
A type of mirroring in which writes are acknowledged only after the data has been written to both the source and target devices.  
In a DataKeeper (mirroring) environment, the target device resides on the backup server. Writes are replicated across the network from the source device to the target.

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