Troubleshooting offsite synchronization speed

Topic

This article explains why offsite sync processes may not be as fast as the average bandwidth of your external connection.

Environment

  • Datto SIRIS
  • Datto ALTO
  • Datto NAS

Description

This article requires a basic knowledge of the following topics:

  • TCP window size (buffer)
  • RoundTrip Time
  • Bandwidth
  • Throughput

In general, if the upload speed of a connection is 50 Mbps, the upload rate to Datto's offsite servers should reflect this speed. However, several factors affect the overall travel time. When calculating throughput (the theoretical average rate of travel for data for a given instance), the value can never exceed the total bandwidth (the rate of transfer for a connection).

Calculation

It is important to take into consideration the TCP window size and RoundTrip Time (RTT) when dealing with throughput. Datto devices, both physical and virtual, have TCP window scaling enabled, so the system will dynamically increase as system reliability allows. The scaled max is 24 MiB (25165824 bytes), while the fall back default window size is 64 KiB (65535 bytes). The RTT can be determined by pinging a destination to test response time.

Below is an example of the determining the default window size:

ping server1492.dattobackup.com
PING server1492.dattobackup.com (8.34.181.74) 56(84) bytes of data.
64 bytes from 8.34.181.74: icmp_seq=3 ttl=51 time=9.86 ms
64 bytes from 8.34.181.74: icmp_seq=4 ttl=51 time=9.84 ms
64 bytes from 8.34.181.74: icmp_seq=5 ttl=51 time=9.76 ms
64 bytes from 8.34.181.74: icmp_seq=6 ttl=51 time=9.76 ms

While you can average out the RTT values for as many pings as you like (the average for the above ping sequence is 9.805), selecting an RTT value that is common among the others in your ping test works as well.

Once you have your RTT average or value, you can then calculate throughput. Plug in your TCP window size in bytes, and your RTT in seconds, as seen below.

65535/0.009805 = 6683834.778174401 bytes
6683834.778174401/1048576 = 6.37MBps

After converting to MB and rounding up, the throughput between sites in the above example would be 6.37 MBps.

A third-party application called iPerf can also be used to diagnose network speed issues. See the Additional Resources section of this article for external links. While Datto cannot provide support or training for iPerf, we have found the application has historically provided reliable results.

Additional Resources