The following are the recommendations to get the maximum data deduplication ratios with the VTL data deduplication feature
  • Disable software data compression by the backup application
  • Enable hardware compression for the tape drive to maximize space savings
  • Disable multiplexing of backup streams
  • Disable encryption of backup streams
  • Disable data deduplication by the backup application itself

Tape Block Size

The block size used to write to tape plays an important part in deduplication ratios achieved. The simple rule is that larger the block size the higher the deduplication ratios. The following lists the default block size used by different backup applications. NOTE: For certain backup applications a newer block size will only be picked up after a relabeling of the virtual tape
Symantec NetBackup
The default block size used for tape is 64K. This is not optimal for data deduplication and can be increased to 256K by specifying SIZE_DATA_BUFFERS as 256K
IBM Tivoli Storage Manager
The default block size used is 256K and is optimal for data deduplication
Computer Associates ARCServe
The default block size used for tape is 64K and is not optimal for data deduplication. This can be changed to 256K as described in https://support.ca.com/cadocs/0/CA%20ARCserve%20%20Backup%20r16-ENU/Bookshelf_Files/HTML/tapelibr/index.htm?toc.htm?ag_tl_cfg_lib_2_func_as_vtl.htm
CommVault Simpana
The default block size used for tape is 64K and is not optimal for data deduplication. This can be increased to 256K in the "Data Path Properties" of a library. For example see "Increasing Block Size" section in http://documentation.commvault.com/hds/v10/article?p=features/performance_tuning/tunable_parameters.htm
Symantec Backup Exec
The default block size used for tape is 64K and is not optimal for data deduplication. This can be changed for each drive by selecting its properties and changing the "Block Size" to 256K
HP Data Protector
The default block size used is 256K and is optimal for data deduplication
EMC NetWorker
The default block size used is 128k which is good but may be increased to 256K for optimal data deduplication
Dell/Quest Netvault
The default block size used is 32k which is not optimal for data deduplication. This can be changed for each drive by selecting its properties and changing the "Block Size" to 256K