Dell NetWorker 19.9 Administration Guide

Multiplex to Data Domain VTL prerequisites and considerations

Ensure the following prerequisites and practices.

• If currently using DD OS 5.0.x, upgrade to DD OS 5.7 or later.
• The recommended settings for VTL are: max sessions=4; target sessions=4; and device block size=512 KB.
• Best max sessions and device block size values depend on the environment. For example, max sessions=2 might provide better stability and deduplication while still meeting the backup window.
• Deduplication efficiency on the VTLs is reduced by 4% to 8% per additional save stream. For example, given a sufficiently large device block size, 4 parallel streams (max sessions=4) results in deduplication ratios that are 12%-24% below the non-multiplexed rate (max sessions=1).
• Typically, deduplication ratios are initially low when you increase max sessions and device block size due to re-priming and re-analysis overhead, following which efficiency improves.
• Heavily used Data Domain systems, with 75% or more disk space that is already used, can suffer impaired performance when used with multiple sessions.
• As a best practice, do not use client-side or server-side encryption during backup to the Data Domain system.