Media multiplexing (schedule attribute)
The Media multiplexing attribute specifies the maximum number of jobs from the schedule that NetBackup can multiplex onto any one drive. Multiplexing sends concurrent backup jobs from one or several clients to a single drive and multiplexes the backups onto the media.
Specify a number from 1 through 32, where 1 specifies no multiplexing. Any changes take effect the next time a schedule runs.
Note:
Some policy types and some schedule types do not support media multiplexing. The option cannot be selected in those instances.
Caution:
For MS-SQL-Server policies, do not enable multiplexing for a schedule that is also configured to backup with multiple stripes. Restores fail when multiplexing is enabled for a schedule that uses more than one stripe.
To configure multiplexed backups, multiplexing must be indicated in both the storage unit (Maximum Streams Per Drive setting) and the schedule (Media Multiplexing setting) configuration. Regardless of the Media multiplexing setting, the maximum jobs that NetBackup starts never exceeds the Maximum Streams Per Drive value for the storage unit.
NetBackup multiplexing sends concurrent backups from one or several clients to a single storage device. NetBackup multiplexes the backups sequentially onto the media. Multiplexed and unmultiplexed backups can reside on the same volume. Separate volume pools or media IDs are not necessary.
Figure: Multiplexed backups shows the multiplexed flow of client data to a server.
Multiplexing is generally used to reduce the amount of time that is required to complete backups. The following table describes circumstances where performance improves by using multiplexing:
Table: Circumstances where multiplexing improves performance
| Item | Description |
|---|---|
|
Slow clients |
Instances in which NetBackup uses software compression, which normally reduces client performance, are also improved. |
|
Multiple slow networks |
The parallel data streams take advantage of whatever network capacity is available. |
|
Many short backups (for example, incremental backups) |
In addition to providing parallel data streams, multiplexing reduces the time each job waits for a device to become available. Therefore, the storage device transfer rate is maximized. |
No special action is required to restore a multiplexed backup. NetBackup finds the media and restores the requested backup. Multiplexing reduces performance on restores because it uses extra time to read the images.
To reduce the effect of multiplexing on restore times, set the storage unit maximum fragment size to a value smaller than the largest allowed value. Also, on UNIX, enable fast-tape positioning (locate block), if it applies to the tape drives in use.
Consider the following configuration settings when using multiplexing.
Table: Properties and attributes that affect multiplexing
| Item | Description | Where to find it |
|---|---|---|
|
(policy attribute) |
Limits the number of jobs that NetBackup performs concurrently when a policy is run. Set this attribute high enough to support the specified level of multiplexing. |
|
|
(host property) |
Limits the number of backup jobs that can run concurrently on any NetBackup client. This property is part of Global Attributes host properties. See Global Attributes properties. Usually, the client setting does not affect multiplexing. However, consider a case where jobs from different schedules on the same client go to the same storage unit. In this case, the maximum number of jobs that are permitted on the client is reached before the multiplexing limit is reached for the storage unit. When the maximum number of jobs on the client is reached, NetBackup cannot use the storage unit's full multiplexing capabilities. Select a value that is based on the ability of the central processing unit to handle parallel jobs. Because extra buffers are required, memory is also important. If the server cannot perform other tasks or runs out of memory or processes, reduce the Maximum streams per drive setting for the storage unit. To estimate the potential load that multiplexing can place on the central processing unit, consider the following limits:
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|
(host property) |
Set the maximum number of jobs that are allowed on a specific client without affecting other clients. This property is part of Client Attributes host properties. |
|
|
(host property) |
Specifies how long the server waits for additional restore requests of files and raw partitions in a set of multiplexed images on the same tape. This property is part of General Server host properties. |
|
|
(policy schedule attribute) |
If the limit is reached for a drive, NetBackup sends jobs to other drives. When NetBackup multiplexes jobs, it continues to add jobs to a drive until the number of jobs on the drive matches the limit or the limit. |
|
|
(storage unit setting) |
NetBackup can add jobs from more than one schedule to a drive. When NetBackup multiplexes jobs, it continues to add jobs to a drive until the number of jobs on the drive matches the limit or the limit. |
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