Avamar: VSR v AMS Replication

Automated Multi Streaming (AMS):

Automated multi streaming replication inflates a backup on the Data Domain (DD). It uses multiple streams to "slice" the backup up before sending it to the target DD.

AMS replication is supported in Avamar 7.0 and later.

• • It leverages multiple parallel streams to process and send backups to the replication target
  • Introduced in Avamar v7.0
  • By default, the Avamar uses six streams per concurrent process, but this value can be changed using the proper flags.
  • AMS must scan the entire backup rather than only the changes, so it often has to more data to scan than if Virtual Synthetic replication (VSR) were used. 
  • AMS typically has better performance with high change rate clients when the bandwidth between the sites is large. 
  • AMS can cause space differences between the source and target Data Domains. This is discussed further in article Data Domain: Space usage on destination DD is higher than that in the source DD when the data is the same (Log in as a registered Dell Support user may be required to view this article.)
  • No restrictions on client types for this type of replication. 

Example of Avamar choosing AMS Replication:

...
2015-11-22 10:05:55 avtar Info <6654>: Replicating backup 367, 528.7 GB
2015-11-22 10:05:56 avtar Info <40047>: id:1 VSR selected because file size is too small for NCR (container.1.cdsf)
2015-11-22 10:05:56 avtar Info <40050>: id:2 NCR selected because it could not be determined if VSR is possible (container.2.cdsf)
... 

Virtual Synthetic Replication (VSR):

Virtual Synthetic replication is a single stream replication process. It leverages the previously replicated backup so that only the changes are scanned and sent to the target Data Domain.

Once those changes have been sent, the target Data Domain can virtually synthesize a full backup. It uses the backup from the previous replication as a base file. The backup is put together with the newly replicated changes.

This is in contrast to the AMS process which requires the full backup be scanned. VSR is supported in Avamar 7.1/DDOS 5.5 and up.  

• • Single Data Domain stream consumed per concurrent Avamar process.
  • Introduced in Avamar 7.1/DDOS 5.5
  • By default, Avamar attempts to use VSR whenever possible
  • Replicates only the changed data 
  • Highly network efficient 
  • Ideal for low change rate clients
  • Only compatible with Network Data Management Protocol (NDMP), VMware Image, and File system backups

The initial replication leverages AMS to seed the base file to the replication target when attempting to replicate a backup.

Once the base files are located on the target Data Domain, VSR is used assuming all VSR requirements are met.

While VSR continues to work correctly, the base file is then updated every time the replication completes.

Example of Avamar choosing VSR replication:

...
2016-03-15 07:37:17 avtar Info <6654>: Replicating backup 471, 100.0 GB
2016-03-15 07:37:17 avtar Info <40198>: id:19 VSR selected because file size is too small for NCR (16912D3D130B98D763593BD4DCD0BC586766FE46)
2016-03-15 07:37:17 avtar Info <40198>: id:20 VSR selected because file size is too small for NCR (04C560CC90AC7A626F4E7866573CF93D7D32EFE6)
2016-03-15 07:37:17 avtar Info <40198>: id:21 VSR selected because file size is too small for NCR (9D78E4B1CD2DF728D7A45B2E374D6D28A853ED2C)
2016-03-15 07:37:17 avtar Info <40198>: id:22 VSR selected because file size is too small for NCR (41A1BDB73F3BB615E03A95FC3758CDE1A2725E6F)
2016-03-15 07:37:17 avtar Info <40198>: id:23 VSR selected because file size is too small for NCR (1D6784C9E27240132EB7FBFEE3C0D4136D81AA8D)
2016-03-15 07:37:17 avtar Info <40198>: id:24 VSR selected because file size is too small for NCR (FC295BC80B73D67BFE437291253D46FAD42650C2)
2016-03-15 07:37:17 avtar Info <40198>: id:25 VSR selected because file size is too small for NCR (93B06F7711E1F64111BCAD4FBE966E051655EB94)
2016-03-15 07:37:17 avtar Info <40198>: id:26 VSR selected because file size is too small for NCR (55A58F9CDCB82A6756853E3D84A3D2A6993F2C3D)
2016-03-15 07:37:17 avtar Info <40200>: id:27 VSR selected because all base files are available (6122D9EF93CB95529D5E59327A3FF4789B0D0911)
... 

Additional Notes on VSR: 

Sometimes where Avamar leverages partial backups VSR may not always be a viable solution. By design, Avamar expires partial backups every 7 days.

If a backup is not replicated with all its partials within that 7-day time limit, Avamar removes them.

This typically affects large backup clients like NDMP.