Allow multiple data streams (policy attribute)
The Allow multiple data streams attribute specifies that NetBackup can divide automatic backups for each client into multiple jobs. The directives, scripts, or templates in the backup selection list specify whether each job can back up only a part of the backup selection list. Because the jobs are in separate data streams, they can occur concurrently.
The directives, scripts, or templates in the backup selection list determine the number of streams (backup jobs) that start for each client. The list also determines how the backup selection list is divided into separate streams.
The following settings determine the number of streams that can run concurrently:
Number of available storage units
Multiplexing settings
Maximum jobs parameters
Multistreamed jobs consist of a parent job to perform stream discovery and children jobs for each stream. Each child job displays its own job ID in the Job ID column in the . The job ID of the parent job appears in the Parent Job ID column, which is not displayed by default. Parent jobs display a dash (-) in the Schedule column.
Note:
If this attribute is enabled, and a file system is in a client's exclude list, a NetBackup job appears in the for the excluded file system. However, no files in the excluded file system are backed up by the job.
The following table describes the reasons to use multiple data streams.
Table: Reasons to use multiple data streams
Reason | Description |
|---|---|
To reduce backup time | Multiple data streams can reduce the backup time for large backups by splitting the backup into multiple streams. Use multiplexing, multiple drives, or a combination of the two to process the streams concurrently. Configure the backup so each device on the client is backed up by a separate data stream that runs concurrently with streams from other devices. For best performance, use only one data stream to back up each physical device on the client. Multiple concurrent streams from a single physical device can adversely affect backup times. The heads must move back and forth between the tracks that contain files for the respective streams. Figure: Multiple stream recommendations shows why multiple concurrent streams from a single device are not recommended. |
To reduce retry time for backup failures | Because the backup streams run independently, the use of multiple data streams can shorten the retry time in the event of a backup failure. A single failure only terminates a single stream. NetBackup can restart the failed stream without restarting the others. For example, assume the backup for a 10-gigabyte partition is split into five streams, each containing 2 gigabytes. If the last stream fails after it writes 1.9 gigabytes (a total of 9.9 gigabytes is backed up), NetBackup retries only the last gigabyte stream. If the 10-gigabyte partition is backed up without multiple data streams and a failure occurs, the entire 10-gigabyte backup must be retried. The property in the Global Attributes properties, applies to each stream. For example, if the property is set to 3, NetBackup retries each stream a maximum of three times. The displays each stream as a separate job. Use the job details view to determine the files that are backed up by each of these jobs. |
To reduce administration by running more backups with fewer policies | Use multiple data streams in a configuration that contains large file servers with many file systems and volumes. Multiple data streams provide more backups with fewer policies than are otherwise required. |
The following table describes the aspects of multiple data streams that are adjustable.
Table: Adjustable aspects of multiple data streams
Item | Description |
|---|---|
The total number of streams | The backup selection list determines the total number of streams that are started. The directive lets you configure a fixed number of streams, or you can allow the client dynamically define the streams. See About the directives on the Backup Selections list. Note: For best performance, use only one data stream to back up each physical device on the client. Multiple concurrent streams from a single physical device can adversely affect backup times. Backup times are affected because the device heads must move between the tracks that contain files for the respective streams. |
The number of streams that run concurrently | The following factors determine the number of streams that can run concurrently for a policy or client:
|
The maximum jobs settings limit the maximum number of streams as follows:
Table: Job settings that limit the maximum number of streams
Item | Access method |
|---|---|
(host property) | |
Limit jobs per policy (policy attribute) | |
(host property) |
Job settings also affect the maximum number of streams. The following table describes the interdependency of these settings.
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