Data Domain: Multiplexing in Backup Software

Zhrnutie: Multiplexed write streams often cause lower compression and is unnecessary with a disk-based backup system.

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Symptómy

Compression is lower than expected due to multiplexing being enabled in the backup software being used.
  • It is difficult to tell that multiplexing is happening solely from statistics on a Data Domain. Multiplexing should be suspected if other reasons for low compression do not apply.
  • Some backup applications are more likely to multiplex by default than others. For example, Oracle/RMAN multiplexes eight streams by default.

Príčina

Multiplexing is not supported on any Data Domain model.
 
When backing up to a physical tape or virtual tape library (VTL), the backup application is usually configured to multiplex multiple input streams into a single output stream per tape drive.
 
There are two reasons for this multiplexing:
  • First, multiple incoming streams cannot be written to a single tape drive at once.
  • Second, multiple input streams may be needed to feed the tape drive with a data rate that is high enough to avoid frequent starts and stops, also known as shoe shining.
However, multiplexing is not necessary when backing up to disk or virtual tape, because modern disk-based systems inherently support multiple streams at variable speeds. In fact, multiplexing is undesirable and makes restores slower. It can also degrade compression, because different streams might be multiplexed differently from backup to backup.
 
The smaller the granularity of multiplexing, the more likely the potential degradation.

Riešenie

Multiplexing causes existing data to come into the Data Domain in a different order every time a backup is performed, making it impossible for the system to recognize repeated segments, so this option is not supported.
 
Disable multiplexing. It is not supported on any Data Domain.
 
Since disabling multiplexing can cause performance degradation for backups, system throughput can be improved by increasing the number of write streams. Do not exceed the supported stream limits for the model and DDOS version in question.
  • CIFS, NFS, and OST users should increase the number of streams that are allowed to backup to file-type devices. This is a configuration setting under the options for the backup application.
  • VTL users should create additional drives on the Data Domain. Each drive is an additional stream.

Ďalšie informácie

REFERENCE:

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Data Domain

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Data Domain
Vlastnosti článku
Číslo článku: 000028900
Typ článku: Solution
Dátum poslednej úpravy: 13 máj 2026
Verzia:  7
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