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6.3. What s Missing?

6.3. What's Missing?

All the disk I/O tools on Linux provide information about the utilization of a particular disk or partition. Unfortunately, after you determine that a particular disk is a bottleneck, there are no tools that enable you to figure out which process is causing all the I/O traffic.

Usually a system administrator has a good idea about what application uses the disk, but not always. Many times, for example, I have been using my Linux system when the disks started grinding for apparently no reason. I can usually run top and look for a process that might be causing the problem. By eliminating processes that I believe are not doing I/O, I can usually find the culprit. However, this requires knowledge of what the various applications are supposed to do. It is also error prone, because the guess about which processes are not causing the problem might be wrong. In addition, for a system with many users or many running applications, it is not always practical or easy to determine which application might be causing the problem. Other UNIXes support the inblk and oublk parameters to ps , which show you the amount of disk I/O issued on behalf of a particular process. Currently, the Linux kernel does not track the I/O of a process, so the ps tool has no way to gather this information.

You can use lsof to determine which processes are accessing files on a particular partition. After you list all PIDs accessing the files, you can then attach to each of the PIDs with strace and figure out which one is doing a significant amount of I/O. Although this method works, it is really a Band-Aid solution, because the number of processes accessing a partition could be large and it is time-consuming to attach and analyze the system calls of each process. This may also miss short-lived processes, and may unacceptably slow down processes when they are being traced.

This is an area where the Linux kernel could be improved. The ability to quickly track which processes are generating I/O would allow for much quicker diagnosis of I/O performance- related problems.

6.4. Chapter Summary

This chapter presented the Linux disk I/O performance tools used to extract information about system-wide ( vmstat ), device-specific ( vmstat , iostat , sar ), and file-specific ( lsof ) disk I/O usage. It explained the different types of I/O statistics and how to extract these statistics from Linux using the I/O performance tools. It also discussed some of the significant limitations of the current tools and areas for future growth.

The next chapter examines the tools that enable you to determine the cause of network bottlenecks.

Chapter 7. Performance Tools: Network

This chapter introduces some of the network performance tools available on Linux. We primarily focus on the tools that analyze the network traffic on a single box rather than network-wide management tools. Although network performance evaluation usually does not make sense in total isolation (that is, nodes do not normally talk to themselves ), it is valuable to investigate the behavior of a single system on the network to identify local configuration and application problems. In addition, understanding the characteristics of network traffic on a single system can help to locate other problem systems, or local hardware and applications errors that slow down network performance.

After reading this chapter, you should be able to

Determine the speed and duplex settings of the Ethernet devices in the system ( mii-tool , ethtool ).
Determine the amount of network traffic flowing over each Ethernet interface ( ifconfig , sar , gkrellm , iptraf , netstat , etherape ).
Determine the types of IP traffic flowing in to and out of the system ( gkrellm , iptraf , netstat , etherape ).
Determine the amount of each type of IP traffic flowing in to and out of the system ( gkrellm , iptraf , etherape ).
Determine which applications are generating IP traffic ( netstat ).