File Handling - Its Impact on Server Backups Articles | Network Analysis Tips - Analyzers - Packet Sniffer

In one example, the backup software was reporting that the backup process was transferring at less than 30 Mbps, when both servers and all network connections between were connected at 1Gbps. Overall network utilization was very low, measuring at only 5%.

For some reason, the backup process was not taking advantage of these high-speed links.

Using Wireshark, a capture was taken during the slow backup. We first looked for retransmissions in the traffic between the two servers. This would indicate packet loss and would direct us to look at the switches for Ethernet errors such as FCS and Alignment Errors. The traces showed no retransmissions, only slow response times from the source server to file requests. Every file was requested individually and would incur a 20 millisecond delay on the source server. When this file handling was filtered out of the trace, backup throughput measured at almost 1Gbps, fully utilizing the available bandwidth.

The chart below shows the impact of file handling on data backups. This data was compiled assuming the total data is 100GB. When the backup begins, only a small amount of this data will be requested at once, which depends on the protocol in use. Typically, a SMB/CIFS based backup application will read blocks of data as shown below. The smaller the blocks requested, the more are required to move the total data amount. This will incur more read delays, which reduce overall throughput.

As shown above, file handling can have a tremendous impact on bandwidth used during file backups. In many cases, slow performance can be traced to slow file responses on the source server. Use of multi-threaded backup applications, monitoring on the source server for resource problems, or backup applications with less file handling overhead will improve backup performance in these cases. Performance can also be impacted by TCP Window issues as well as underlying network problems. Specific instances of these problems and their resolutions will be discussed in future articles.