WAN Acceleration

By combining multiple technologies such as network compression, multi-threading, dynamic TCP window size, variable block size deduplication and global caching, WAN acceleration provides sufficient capability whilst the required network bandwidth is dramatically reduced when performing Backup Copy and Replication jobs. This technology is specifically designed to accelerate Veeam job. Any other WAN acceleration technology should be disabled for Veeam traffic.

To determine whether WAN acceleration is necessary in an environment, it is important to understand what particular savings can be achieved.

Determining Required Bandwidth

When using WAN acceleration on links with very poor bandwidth, you may have to manually seed the initial copy to the target. For more information, refer to the WAN Acceleration section of the Veeam Backup & Replication User Guide.

The WAN accelerator uses its own digests based on the hashes of the blocks inside a VM disk, which means that it reads data from the backup files and re-hydrating them on the fly, or it reads directly from the source VM in case of replication. The WAN accelerator component will then process those data blocks with much more efficient data deduplication and compression algorithms. This is the reason why the WAN accelerator consumes significant amounts of CPU and RAM resources.

To determine how much data has to be transferred over the WAN link with and without WAN acceleration enabled in a backup copy job, you can compare the daily changes of the primary backup job statistics (as the same data is transported in a standard backup copy job without WAN acceleration) with the WAN accelerated backup copy job log and statistics.

Analyzing Backup Job

During both full and incremental job sessions, three metrics are displayed in the session data: Processed, Read and Transferred. To better understand the difference between direct data transfer and WAN accelerated mode, examine the Read and Transferred values:

  • Read — amount of data read from the production storage prior to applying any compression and deduplication. This is the amount of data that will be optimized by the WAN accelerator.

  • Transferred — amount of data written to the backup repository after applying compression and deduplication. This is the amount of data that will be processed by the backup copy job running in Direct Transfer mode (without WAN acceleration), assuming all VMs from the backup job are included in the backup copy job.

Analyzing Backup Copy Job

When analyzing a backup copy job you can see the same metrics in the job session Data: Processed, Read and Transferred. Comparing the backup copy job with WAN acceleration enabled and the backup job, it is possible to correlate the information in both outputs.

  • The amount of Processed blocks in the backup copy job session is equal to the amount of Read blocks in the backup job session. This is the most important metric, as it is the amount of data that has to be processed by the WAN accelerator.

  • The number of Read blocks for the backup copy job is typically higher than the amount of Processed - this is due to the backup copy job using a differing fingerprinting algorithm that works with a different block size compared to the fingerprinting algorithm and block size used by backup jobs that created the original backup file. For this reason, this metric can be ignored.

  • The amount of Transferred data is the amount of data actually transferred over the WAN link.

Comparing Direct Mode with WAN Accelerated Mode

Consider that the savings rate (18.5x) displayed in the GUI is based on Processed data ("re-hydrated" data blocks). In the example above, 283 MB would have been transferred over the WAN link in Direct Transfer mode, while only 72.8 MB were transferred after enabling WAN acceleration. The actual savings rate equals 3.9x in this relatively static demo infrastructure, whilst it would typically be significantly higher in real-life scenarios.

Note: Approximate savings ratio can be assumed as of 10x.

To calculate possible savings and needed bandwidth you may use the following calculator http://vee.am/bandwidth.

Backup Mode Effect

When planning for WAN acceleration, review the backup mode used on the primary backup job. Some backup methods produce a random I/O workload on the source repository (as opposed to sequential I/O patterns in other backup modes). The methods of reading from source is illustrated by the figure below:

For example, forward incremental and forever forward incremental methods will make backup copy jobs work much faster, as read operations will be sequential rather than random. To avoid similar fragmentation and random I/O on forward incremental modes, keep backup storage maintenance enabled when possible.

Though a workload penalty may not be significant, it can be a good idea to monitor the storage latency on the backup repository, especially if the reported bottleneck is Source. If the storage latency on the backup repository is high, it is recommended that you change the backup mode in order to increase the throughput of one pair of WAN accelerators.


Thanks to our friends at PernixData for helping with I/O analysis using PernixData Architect.

When configuring the WAN accelerator, not all configuration parameters affect both source and target WAN accelerators. In this section we will highlight what settings should be considered on each side.

Source WAN Accelerator

At the first step of the WAN accelerator configuration wizard, you can change the default setting of five TCP threads. This setting applies to the source WAN accelerator only and is automatically replicated to the target WAN accelerator at the beginning of each job. This ensures different source WAN accelerators can have different settings even when using the same target WAN accelerator. The maximum setting is 100 simultaneous threads for throughput optimization and compensation for high latency or packet loss.

If the link has low latency and high bandwidth, the default setting (5 streams) may be enough to fully saturate it. If the link is still not saturated, the number of streams may be increased accordingly.

Testing shows that with high latency links, link speed x 1.5 is a good best practice for estimating the number of streams required. Below is an example benchmark on a 10 Mbit/s WAN link with 100 milliseconds of latency.

Link (Mbit/s) Latency (ms) Packet loss (%) Streams Throughput (Mbps)
10 100 0 3 3.5
10 100 0 10 7.5
10 100 0 15 10
10 100 0 20 10

Increasing the number of streams to more than required for fully saturating the link will cause initialization of data transfers to slow down, as the data transfer will wait for all streams to initialize and stabilize before beginning transferring any data.

Tip: To test different scenarios in the lab before deploying WAN acceleration, you can use a WAN emulator (such as WANem).

When configuring the cache location for the source WAN accelerator, consider that the actual cache size on the source is irrelevant, as it is used only for digest files (where block hashes are stored). However, if a WAN accelerator will be used for bi-directional acceleration (act as both source and target), follow the guidelines provided in the "Target WAN Accelerator" section below.


When configuring the WAN accelerator on the source side, consider that all VM disk data blocks are already in the source backup repository and they can simply be re-read from the source repository when needed. This is the reason why configuring the cache size on a source WAN accelerator does not matter. It is never used for caching any data. However, there are other files residing in the source WAN accelerator folder, and the file structure will be described in the following sections.


The source WAN accelerator will consume a high amount of CPU whilst re-applying the WAN optimized compression algorithm. Recommended system configuration is 4 CPU cores and 8 GB RAM.

Source WAN accelerator IOPS

The I/O requirements for the source WAN accelerator spikes every time a new VM disk starts processing. Thus, it is recommended to deploy WAN accelerators on disk configurations with decent I/O performance.

Source WAN accelerator IO size

The typical I/O pattern is made of many small blocks, so using high latency spinning disks is not recommended.

Disk Size

Each digest file consumes up to 2% of its source VM disk size. This means, for example, that a 2 TB VM disk file can produce a digests file up to 40 GB in size.

Additionally, plan for 10 GB of working space for payloads and other temporary files.

  • Formula: (<Source data size in GB> * 2%) + 10 GB

  • Example with 2 TB source data: (2,000 GB * 2 %) + 10 GB = 50 GB

For understanding how disk space is consumed, please see the following sections.

Note: As the cache size on the source WAN accelerator will always be ignored, the digests file will be produced regardless of cache setting been configured. They may consume considerable disk space.


Only a data.veeamdrf file is located in the \VeeamWAN\GlobalCache\src folder. This file will be synchronized from the target WAN accelerator during the very first job run (or if the cache was manually cleared) to understand what data blocks are already cached in the target WAN accelerator. The size of this file is typically up to 2% of the configured target cache size; thus, it may take some time for the initial data transfer to begin.


On the source WAN accelerator there are the VM disk digests that take up disk space. For each processed VM disk, a disk digest file is created and placed in \VeeamWAN\Digests\<JobId>_<VMId>_<DiskId>_<RestorePointID>.

Note: Traffic throttling rules should be created in both directions. See Network Traffic Throttling and Multithreaded Data Transfer for more information.

Target WAN Accelerator

The following recommendations apply to configuring a target WAN accelerator:

  • The cache size setting configured on the target WAN accelerator will be applied to the pair of WAN accelerators. This should be taken into account when sizing for many-to-one scenarios, as configuring 100 GB cache size will result in 100 GB multiplied by the number of pairs1 configured for each target WAN accelerator.

  • It is recommended to configure the cache size at 10 GB for each operating system2 processed by the WAN accelerator.

  • Once the target WAN accelerator is deployed, it is recommended to use the cache population feature (see this section of the User Guide for details). When using this feature, the WAN accelerator service will scan through selected repositories for protected operating system types.

  • It is also possible to seed the initial copy of data to the target repository to further reduce the amount of data that needs to be transferred during the first run.

Populate Cache



Although a target WAN accelerator will consume less CPU resources than the source, the I/O requirements for the target side are higher.

For each processed data block, the WAN accelerator will update the cache file (if required), or it may retrieve the data block from the target repository (if possible). As described in the user guide, the cache is active on operating system data blocks, while other data blocks are being processed only with the WAN optimized data reduction algorithm (in-line compression).

Target WAN accelerator IOPS

Tests show that there are no significant performance differences in using spinning disk drives as storage for the target WAN accelerator cache rather than flash storage. However, when multiple source WAN accelerators are connected to a single target WAN accelerator (many-to-one deployment), it is recommended to use SSD or equivalent storage for the target cache, as the I/O is now the sum of all the difference sources.

Disk Size

Ensure that sufficient space has been allocated for global cache on the target WAN accelerator.

At least 10 GB per each different OS that is backed up. That is, if you plan to backup VMs running Windows 8, Windows 2008 R2, Windows 2012 and RHEL 6 (four different operating systems), you will need at least 10 GB * 4 = 40 GB

Plan for additional 20 GB of working space for cache population, payload and other temporary files.

If the cache is pre-populated, an additional temporary cache is created. The temporary cache will be converted into being the cache used for the first connected source. Subsequently connected sources will duplicate the cache of the first pair. As caches are duplicated the configured cache size is considered per pair of WAN accelerators.


  • Formula for configured cache size (insert this number in configuration wizard):
    • (Number of operating systems * 10 GB) + 20 GB
  • Formula for used disk space:
    • (Number of sources * <formula for configured cache size>)


  • Example with one source and 2 operating systems:
    • Configured cache size: (2 operating systems * 10 GB) + 20 GB = 40 GB
    • Used disk space: (1 source * 40 GB) = 40 GB
  • Example with five sources and 4 operating systems:
    • Configured cache size: (4 operating systems * 10 GB) + 20 GB = 60 GB
    • Used disk space: (5 sources * 60 GB) = 300 GB

For understanding how the disk space is consumed, please see the following sections.


For each pair there will be a subfolder in the trg directory, with a UUID describing which source WAN accelerator the cache is attached to. In each of those subfolders, the blob.bin file containing the cache will be located. That file size corresponds to the setting configured in the management console.

Note: The blob.bin file will exist for all connected source WAN accelerators.


When connecting a new source WAN accelerator, the temp folder will temporarily contain the data.veeamdrf file that is later transferred to the source containing the cache manifest.

How Many WAN Accelerators to Deploy?

As the source WAN accelerator can only process one task at a time (one VM disk in a backup copy job or replication job), you may need to deploy multiple WAN accelerator pairs to meet the performance demands.

As the target WAN accelerator can handle multiple incoming streams (as described in the Many-to-One WAN Acceleration section of the User Guide), it is recommended to maintain a 4:1 ratio between the number of source WAN accelerators per target WAN accelerator.

This guideline is very much dependent on the WAN link speed. Many source sites with low bandwidth will create little pressure on the target WAN accelerator. So, for instance, in multiple ROBO configurations a 10:1 ratio can be considered.

If there are sites with very high bandwidth (such as datacenter-to-datacenter replication), they will produce a much more significant load on both the target WAN accelerator and the target repository due to the second data block lookup (for more information, refer to the User Guide).

Note: The secondary data block lookup is used, when a data block is not available in the WAN accelerator cache. When there is a WAN cache “miss”, the secondary lookup for the same data block is performed on the target repository. If it is found here, it is read back to the WAN accelerator instead of re-transmitting over WAN.

Assuming the source and target repositories can deliver the throughput required for the optimal processing rate, use the guidelines that follow.

Note: The numbers below are processing rates. The WAN link usage is dependent on the achieved data reduction ratio.

  • Average throughput per target WAN accelerator: 500 Mbit/s (62.5 MB/s)

  • Depending on the achieved data reduction rate (typically 10x), the transfer rate over the WAN link will vary.

    • If the processing rate is 62.5 MB/s, and the data reduction rate is 10x, then it is possible to sustain 6.25 MB/s (50 Mbit/s) over the WAN link.

    • If the WAN link has high bandwidth (above 100Mbps) consider using backup copy jobs without WAN Acceleration. However, if you use WAN accelerators in that scenario, it may require deployment of multiple WAN accelerator to fully saturate the WAN link.

1. A pair of WAN accelerators means any source WAN accelerator paired with the target WAN accelerator.
2. All Linux operating systems are considered as one in terms of WAN accelerator sizing.

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