Home > Storage > PowerScale (Isilon) > Industry Solutions and Verticals > Media and Entertainment > Dell Technologies PowerScale: OneFS Best Practices for DaVinci Resolve > TCP retransmits and DaVinci Resolve dropped frames
Network retransmits are a common issue that can be the result of many factors, from network congestion to bad NIC SFPs. For real-time workflows like video playback in DaVinci Resolve, high numbers of retransmits are directly correlated to dropped frames. If the storage has to keep sending the same data over again (retransmit), at a certain point, the video will not get there in time.
This guide is not meant to help troubleshoot retransmission errors. Network troubleshooting is a deep topic beyond the scope of the guide. But it is worth calling out and showing how to quickly verify that retransmissions are happening on the PowerScale storage. To be clear, if there are a high number of retransmits, it does not necessarily indicate that there is a problem with the storage. Retransmits are usually a symptom of networking issues not storage issues. But it does give the media technology administrator an avenue to explore.
Running a netstat command from the PowerScale OneFS command line will show retransmits from active storage clients. The following command outputs this information and filters out the column headings and NFS port number:
netstat -n -T -ptcp | grep -E '(OOOR|\.2049)'
Analyzing this command in some detail:
netstat show network statistics
-n this option show network addresses as ports and numbers
-T this option shows the diagnostic information
-ptcp this option filters the out to TCP traffic (assuming the connection is TCP and RDMA)
The output netstat is piped into a grep command that uses a regular expression to filter out column headings and the port for NFS (2049). Similarly SMB traffic could be analyzed by changing 2049 to the SMB port (445).
The statistic to look out for is Rexmit. During playback this number may not stay zero, but should stay relatively low, maybe in the hundreds or thousands depending on the length of playback.
Figure 13 is the output of this command from a Linux Resolve workstation. This workstation is using the nconnect NFS mount option with a value of 12, and as such it shows 12 connections to the PowerScale storage. The Rexmit value is 0, which is a result of flow control on this network. The Local Address column shows that NFS is using port 2049, which is appended to the PowerScale IP address.
It is necessary to run the command several times and observe the delta between the values on each subsequent run. There may be many Rexmit but if Rexmit values are not increasing quickly during playback, then the retransmits are not an issue.
Below is the output look at SMB clients. The command has been run several times, and while there are Rexmits present in the output, those values are not increasing over time. Also note in this output that the port is 445 (SMB) as appended to the Local Address column.
This kind of low-level analysis and troubleshooting is an advanced topic for experienced administrators. It is in this guide as background and because it has helped in real world situations. Digging into netstat and TCP statistics are not necessary for most DaVinci Resolve tuning. This type of investigation should be approached with care and an understanding of the underlying protocols.