FS_USAGE(1) BSD General Commands Manual FS_USAGE(1)


fs_usage — report system calls and page faults related to filesystem activity in real-time


fs_usage [-e] [-w] [-f mode] [pid|cmd [pid|cmd] ...]


The fs_usage utility presents an ongoing display of system call usage information pertaining to filesystem activity. It requires root privileges due to the kernel tracing facility it uses to operate. By default the activity monitored includes all system processes except the running fs_usage process, Terminal, telnetd, sshd, rlogind, tcsh, csh and sh. These defaults can be overridden such that output is limited to include or exclude a list of processes specified by the user.

The output presented by fs_usage is formatted according to the size of your window. A narrow window will display fewer columns of data. Use a wide window for maximum data display. You may override the window formatting restrictions by forcing a wide display with the −w option. In this case, the data displayed will wrap when the window is not wide enough.

The options are as follows:


Specifying the −e option generates output that excludes sampling of the running fs_usage tool. If a list of process ids or commands is also given, then those processes are also excluded from the sampled output.


Specifying the −w option forces a wider, more detailed output, regardless of the window size.


Specifying the −f option turns on output filtering based on the mode provided. By default no filtering occurs. The supported modes are:

network Only network related output is displayed.

filesys Only file system related output is displayed.

pid | cmd

The sampled data can be limited to a list of process ids or commands. When a command name is given, all processes with that name will be sampled. Using the −e option has the opposite effect, excluding sampled data relating to the given list of process ids or commands.

If you set the DYLD_IMAGE_SUFFIX environment variable to ’’_debug’’, then an application will use the debug version of all libraries including the Carbon FileManager. See dyld(1). When fs_usage is run against a Carbon Application launched in this environment, then the high level Carbon FileManager calls will be displayed bracketing the system calls that they are built on.

The data columns displayed are as follows:


TOD when call occurred. Wide mode will have millisecond granularity.


The name of the network or filesystem related call, page-in, page-out or physical disk access.


Of the form F=x, x is a file descriptor. Depending on the type of system call, this will be either an input value or a return value.


Of the form B=x, x is the number of bytes requested by the call.


On error, the errno is displayed in brackets.


Pathname of the file accessed (up to the last 28 bytes).


Of the form A=0xnnnnnnnn, where 0xnnnnnnnn is the address being faulted.


Of the form D=0xnnnnnnnn, where 0xnnnnnnnn is the block number of the physical disk block being read or written.


Of the form O=0xnnnnnnnn, where 0xnnnnnnnn is a file offset.


Of the form S=x, x is the number of ready descriptors returned by the select() system call. If S=0, the time limit expired.


The elapsed time spent in the system call. A ’W’ after the elapsed time indicates the process was scheduled out during this file activity. In this case, the elapsed time includes the wait time.


The process that made the system call.


fs_usage -w -f filesys Mail

fs_usage will display file system related data for all instances of processes named Mail. Maximum data output will be displayed in the window.


top(1), sc_usage(1), latency(1), dyld(1)

Mac OS X November 7, 2002 Mac OS X