When to use multiplexing and multiple data streams
For backup to tape, multiple data streams can reduce the time for large backups. The reduction is achieved by first splitting the data to be backed up into multiple streams. Then you use multiplexing, multiple drives, or a combination of the two for processing the streams concurrently. In addition, you can configure the backup so each physical device on the client is backed up by a separate data stream. Each data stream runs concurrently with streams from other devices, to reduce backup times.
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 the time to back up the device: the drive heads must move back and forth between tracks that contain the files for the respective streams.
Multiplexing is not recommended for database backups when restore speed is of paramount interest or when your tape drives are slow.
Backing up across a network, unless the network bandwidth is very broad, can nullify the ability to stream. Typically, a single client can send enough data to saturate a single 1Gb or 10Gb network connection. A gigabit network has the capacity to support network streaming for some clients. Multiple streams use more of the client's resources than a single stream. Veritas recommends testing to make sure of the following: that the client can handle the multiple data streams, and that the high rate of data transfer does not affect users.
Multiplexing and multiple data streams can be powerful tools to ensure that all tape drives are streaming. With NetBackup, both can be used at the same time. Be careful to distinguish between the two concepts, as follows.
Multiplexing writes multiple data streams to a single tape drive.
Figure: Multiplexing diagram shows multiplexing.
The multiple data streams feature writes multiple data streams, each to its own tape drive, unless multiplexing is used.
Figure: Multiple data streams diagram shows multiple data streams.
Consider the following about multiplexing:
Experiment with different multiplexing factors to find the one that is minimally sufficient for streaming.
Find a setting at which the writes are enough to fill the maximum bandwidth of your drive: that setting is the optimal multiplexing factor. If you get 5 MB per second from each of the read streams, use a multiplexing factor of two to get the maximum throughput to an LTO 7 or LTO 8.
Expect the duplication of a multiplexed tape to take longer if it is demultiplexed (unless "Preserve Multiplexing" is specified on the duplication). Without "Preserve Multiplexing," the duplication may take longer because multiple read passes of the source tape must be made. Using "Preserve Multiplexing," however, may affect the restore time (see next bullet).
When you duplicate a multiplexed backup, demultiplex it.
By demultiplexing the backups when they are duplicated, the time for recovery is significantly reduced.
Consider the following about multiple data streams:
Do not use multiple data streams on single mount points.
The multiple data streams feature takes advantage of the ability to stream data from several devices at the same time. Streaming from several devices permits backups to take advantage of Read Ahead on a spindle or set of spindles in RAID environments. The use of multiple data streams from a single mount point encourages head thrashing and may result in degraded performance. Only conduct multistreamed backups against single mount points if they are mirrored (RAID 0). However, degraded performance is a likely result.