Avamar: Choosing Between VSR and AMS Replication

When choosing between these two forms of replication, it is important to have a good understanding of the difference between the two. The differences between them are discussed in detail in Avamar: VSR vs AMS Replication, but this article covers the basics. 

Virtual Synthetic Replication

Virtual Synthetic Replication (VSR) was introduced in Avamar 7.1/DDOS 5.5 and uses a single stream to replicate only the changed data to the DR site. This method is highly efficient and uses minimal resources on the Data Domain. Typically speaking this process works best with larger clients that have a relatively low change rate due to its low stream count. Another benefit of VSR is that it uses relatively low amounts of bandwidth when compared to the alternative method, so should perform better when link speed is a concern. 

Characteristics of an ideal VSR candidate: 

• VSR is supported by this client Avamar: VSR vs AMS Replication
• Relatively low change rate day over day 
• Resource restricted environment, either from an I/O or network standpoint

Automated Multi-Streaming

Automated Multi-streaming (AMS) also known as Ninja Chopping Replication (NCR) is the default method of replication when Avamar and Data Domain are configured together. AMS works by moving a copy of the full backup from the cur directory within the Avamar Mtree on the source Data Domain to the STAGING directory. Once there the whole backup is then scanned using multiple Data Domain streams before sending the unique data to the replication target.

AMS replication is not as network efficient as the VSR method, but can often be considerably faster depending on the replication conditions. This is because each AMS replication context uses multiple TCP connections based on how many streams are allocated. In general, additional streams mean additional network traffic. 

When considering AMS replication, it is important to understand the difference between concurrent processes on the Avamar and Data Domain replication streams. A concurrent process set by the drop-down menu in the replication dataset refers to how many avtar sessions are running at a given time, or how many clients can run in parallel. By contrast the Data Domain stream count refers to how many streams are active per concurrent process. This is important when attempting to balance the number of streams used, since any changes to the Data Domain stream count would then be multiplied by the number of concurrent processes. 

For example: If 4 concurrent processes are selected, and each process given 12 streams, then the total number of streams in use would be:

To allocate additional Data Domain streams per Avamar concurrent process:

a. Edit the appropriate replication group: Replication>Edit Replication Group>Overview within the Avamar UI

b. Select More Options>More

c. Apply the following flag: 

(By default this value is set to 6 streams per process). 

AMS replication is typically most effective on clients that have a higher change rate. The real decision when choosing between the two replication methods is can a single stream replicate the changed data faster than multiple streams can scan the whole backup and replicate the changed data.

Characteristics of an ideal AMS client:

• High change rate day over day. 
• High link speed between Data Domains
• VSR is unsupported for this client type per Avamar: VSR vs AMS Replication

For more detailed information about replication tuning when leveraging AMS replication, see Employees/Partner only view article (Internal Only - *Advanced*) Modifying the Automated MultiStream Replication (AMS) for DD Client Performance on Avamar v7.

Factors to consider when choosing between the Replication Methods:

• Client Change Rate:

• Network link speed between sites:

•  Available stream count on the DD backend:

• Data Domain I/O limitations based on the DD model:

By default Avamar will automatically choose between AMS or VSR based on a few key factors. If manual intervention is required, do the following:

To force a particular type of replication:

a. Edit the appropriate replication group by going to Replication>Edit Replication Group>Overview within the Avamar UI.

b. Select More Options>More

c. The replication method can be controlled using the following flag:

[avtar]ddr-repl-method-control    =     N

• • • Setting this value to 0 forces AMS/NCR
    • Setting this value to 10 forces VSR 

(By default this value is set to 5).

Reference: Avamar: VSR vs AMS Replication