This section contains defintions for commonly used terms in MapR Converged Data Platform environments.




A special file in every directory, for controlling the compression and chunk size used for the directory and its subdirectories.


A special mount point in the root-level volume (or read-only mirror) that points to the writable original copy of the volume.


A special directory in the top level of each volume, containing all the snapshots for that volume.

access control expression (ACE) A Boolean expression that defines a combination of users, groups, or roles that have access to an object stored natively such as a directory, file, or MapR-DB table .

access control list (ACL)

A list of permissions attached to an object. An ACL specifies users or system processes that can perform specific actions on an object.

accounting entity (AE)

A clearly defined economics unit that is accounted for separately.

advisory quota

An advisory disk capacity limit that can be set for a volume, user, or group. When disk usage exceeds the advisory quota, an alert is sent.


A binary number in which each bit controls a single toggle.


Files in MapR-FS are split into chunks (similar to Hadoop blocks) that are normally 256 MB by default. Any multiple of 65,536 bytes is a valid chunk size, but tuning the size correctly is important. Files inherit the chunk size settings of the directory that contains them, as do subdirectories on which chunk size has not been explicitly set. Any files written by a Hadoop application, whether via the file APIs or over NFS, use chunk size specified by the settings for the directory where the file is written.


The unit of shared storage in a MapR cluster. Every container is either a name container or a data container.

container location database (CLDB)

A service, running on one or more MapR nodes, that maintains the locations of services, containers, and other cluster information.

data container

One of the two types of containers in a cluster. Data containers typically have a cascaded configuration (master replicates to replica1, replica1 replicates to replica2, and so on). Every data container is either a master container, an intermediate container, or a tail container depending on its replication role.

desired replication factor

The number of copies of a volume that should be maintained by the MapR cluster for normal operation. When the number of copies falls below the desired replication factor, but remains equal to or above the minimum replication factor, re-replication occurs after the timeout specified in the cldb.fs.mark.rereplicate.sec parameter.

disk space balancer

The disk space balancer is a tool that balances disk space usage on a cluster by moving containers between storage pools. Whenever a storage pool is over 70% full (or a threshold defined by the cldb.balancer.disk.threshold.percentage parameter), the disk space balancer distributes containers to other storage pools that have lower utilization than the average for that cluster. The disk space balancer aims to ensure that the percentage of space used on all of the disks in the node is similar.


A file on each node, containing a list of the node's disks that have been configured for use by MapR-FS.

dump file

A file containing data from a volume for distribution or restoration. There are two types of dump files: full dump files containing all data in a volume, and incremental dump files that contain changes to a volume between two points in time.


A user or group. Users and groups can represent accounting entities.

full dump file

See dump file.


A sequence number that identifies all copies that have the latest updates for a container. The larger the number, the most up-to-date the copy of the container. The CLDB uses the epoch to ensure that an out-of-date copy cannot become the master for the container.


A distributed storage system, designed to scale to a very large size, for managing massive amounts of structured data.


A signal sent by each FileServer and NFS node every second to provide information to the CLDB about the node's health and resource usage.

incremental dump file

See dump file.


The process responsible for submitting and tracking MapReduce jobs. The JobTracker sends individual tasks to TaskTrackers on nodes in the cluster.


The NFS-mountable, distributed, high-performance MapR data storage system.

minimum replication factor

The minimum number of copies of a volume that should be maintained by the MapR cluster for normal operation. When the replication factor falls below this minimum, re-replication occurs as aggressively as possible to restore the replication level. If any containers in the CLDB volume fall below the minimum replication factor, writes are disabled until aggressive re-replication restores the minimum level of replication.


A read-only physical copy of a volume.

name container

A container that holds a volume's namespace information and file chunk locations, and the first 64 KB of each file in the volume.

Network File System (NFS)

A protocol that allows a user on a client computer to access files over a network as though they were stored locally.


An individual server (physical or virtual machine) in a cluster.


A disk capacity limit that can be set for a volume, user, or group. When disk usage exceeds the quota, no more data can be written.

recovery point objective (RPO)

The maximum allowable data loss as a point in time. If the recovery point objective is 2 hours, then the maximum allowable amount of data loss that is acceptable is 2 hours of work.

recovery time objective (RTO)

The maximum allowable time to recovery after data loss. If the recovery time objective is 5 hours, then it must be possible to restore data up to the recovery point objective within 5 hours. See also recovery point objective

replication factor

The number of copies of a volume.

replication role

The replication role of a container determines how that container is replicated to other storage pools in the cluster. A name container may have one of two replication roles: master or replica. A data container may have one of three replication roles: master, intermediate, or tail.

replication role balancer

The replication role balancer is a tool that switches the replication roles of containers to ensure that every node has an equal share of of master and replica containers (for name containers) and an equal share of master, intermediate, and tail containers (for data containers).


Re-replication occurs whenever the number of available replica containers drops below the number prescribed by that volume's replication factor. Re-replication may occur for a variety of reasons including replica container corruption, node unavailability, hard disk failure, or an increase in replication factor.


A group of rules that specify recurring points in time at which certain actions are determined to occur.


A read-only logical image of a volume at a specific point in time.

storage pool

A unit of storage made up of one or more disks. By default, MapR storage pools contain two or three disks. For high-volume reads and writes, you can create larger storage pools when initially formatting storage during cluster creation.

stripe width

The number of disks in a storage pool.

super group

The group that has administrative access to the MapR cluster.

super user

The user that has administrative access to the MapR cluster.


The process that starts and tracks MapReduce tasks on a node. The TaskTracker receives task assignments from the JobTracker and reports the results of each task back to the JobTracker on completion.


A tree of files and directories grouped for the purpose of applying a policy or set of policies to all of them at once.


A MapR process that coordinates the starting and stopping of configured services on a node.