When to Use Multiple Data Streams

Reduce Backup Time

Multiple data streams can reduce the backup time for large backups. This is achieved by splitting the backup into multiple streams and then using multiplexing, multiple drives, or a combination of the two for processing the streams concurrently.

In addition, configuring the backup so each physical device on the client is backed up by a separate data stream that runs concurrently with streams from other devices can significantly reduce backup times.

Reduce Retry Time for Backup Failures

Because the backup streams are completely independent, 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 and NetBackup can restart the failed stream without restarting the others.

For example, assume the backup for a 10 GB partition is split into 5 streams, each containing 2 GB. If the last stream fails after writing 1.9 GB (a total of 9.9 GB backed up), NetBackup retries only the last 2 GB stream. If this 10 GB partition is backed up without multiple data streams and a failure occurs, the entire 10 GB backup must be retried.

The Schedule Backup Attempts property applies to each stream. For example, if Schedule Backup Attempts is set to 3, NetBackup retries each stream a maximum of three times.

The Activity Monitor shows each stream as a separate job. Use the job details view to determine the files that are backed up by each of these jobs.

Reduce Administration---More Backups With Fewer Policies

When a configuration contains large file servers with many file systems and volumes, using multiple data streams will provide more backups with fewer policies than are otherwise required.

Tuning Multiple Data Streams

The two aspects of multiple data streams that you can tune are the total number of streams and the number of streams that can run concurrently.

Setting the Total Number of Streams

The backup selection list determines the total number of streams that are started. The NEW_STREAM directive allows you to explicitly configure a fixed number of streams, or you can have the client dynamically define the streams. See Backup Selections List Directives for Multiple Data Streams.

Setting the Number of Streams That Can Run Concurrently

The number of streams that can run concurrently for a policy or client is determined by the following:

• Storage unit and schedule multiplexing limit
• Number of drives that are available
• Maximum concurrent jobs settings for the policy and client

Each storage unit and each schedule has a maximum multiplex setting. The lower of the two settings is the limit for a specific schedule and storage unit. The maximum number of streams that can be multiplexed is limited to the sum of the multiplexing limits for all drives available in the storage unit and schedule combinations.

For example, assume there are two storage units with one drive in each. MPX on storage unit 1 is set to 3 and MPX on storage unit 2 is set to 5. If MPX is set to 5 or greater in the schedules, then 8 streams can run concurrently.

The maximum jobs settings also limit the maximum number of streams:

• Maximum Jobs Per Client (Host Properties > Master Servers > Global Attributes)
• Limit jobs per policy (policy attribute)
• Maximum Data Streams (Set the number in Host Properties > Master Servers > Client Attributes or use the bpclient command -max_jobs option as shown below)

The maximum job settings are interdependent as follows:

• If Maximum Data Streams is not set, the lowest value of Maximum Jobs Per Client and Limit Jobs Per Policy is the limiting factor.
• If Maximum Data Streams is set, then NetBackup ignores Maximum Jobs Per Client and uses the lowest value of Maximum Data Streams and Limit Jobs Per Policy as the limiting factor.

To specify a value for Maximum Data Streams with the bpclient command:

1. Determine if the client is in the client database on the master server by running the following command on one line:

   /usr/openv/netbackup/bin/admincmd/bpclient -client name -L

2. If the client is not in the client database, run the following command on the master server on one line:

   /usr/openv/netbackup/bin/admincmd/bpclient -client name -add -max_jobs number

3. If the client is in the client database, run the following command on one line:

   /usr/openv/netbackup/bin/admincmd/bpclient -client name -modify -max_jobs number