# Monitoring Reads and Writes to a File

WAR STORY: monitoring inode activity http://sourceware.org/systemtap/wiki/WSFileMonitor?highlight=((WarStories)) no script in examples

This section describes how to monitor reads from and writes to a file in real time.

inodewatch.stp

need to add references to sources/man pages that explain how "dev_nr = $file->f_dentry->d_inode->i_sb->s_dev" and "($1 << 20 | $2)".

takes the following information about the file as arguments on the command line:

- The file's major device number.

- The file's minor device number.

- The file's `inode` number.

To get this information, use `stat -c '%D %i' filename`, where `filename` is an absolute path.

For instance: if you wish to monitor `/etc/crontab`, run `stat -c '%D %i' /etc/crontab` first. This gives the following output:

    805 1078319

`805` is the base-16 (hexadecimal) device number. The lower two digits are the minor device number and the upper digits are the major number. `1078319` is the `inode` number. To start monitoring `/etc/crontab`, run `stap inodewatch.stp 0x8 0x05 1078319` (The `0x` prefixes indicate base-16 values.

The output of this command contains the name and ID of any process performing a read/write, the function it is performing (i.e. `vfs_read` or `vfs_write`), the device number (in hex format), and the `inode` number. contains the output of `stap inodewatch.stp 0x8 0x05 1078319` (when `cat /etc/crontab` is executed while the script is running) :

Sample Output

    cat(16437) vfs_read 0x800005/1078319
    cat(16437) vfs_read 0x800005/1078319

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