Re: Distinguishing network vs. local disk file I/O

[fche at redhat dot com (Frank Ch. Eigler)]

Matt Walsh <walshes@gmail.com> writes:

> [...]  I've been experimenting with read/read and vfs.read/vfs.write
> - is there a convenient way to flag whether the operation was to a
> network or local file?

One way seems to be to get at the file's mount entry's device name.

probe vfs.read { mntname = kernel_string($file->f_path->mnt->mnt_devname)
                 if (isinstr(mntname,":") { /* probable NFS */ } }

In practice, due to a bug (PR6739 and related), the mnt->mnt_devname
part can't be resolved directly.  So we can resort to embedded-C:

# cat foobar.stp
%{ #include <linux/mount.h> %}
function mntname:long (ptr:long) %{
   THIS->__retvalue = ((intptr_t)((struct vfsmount*)THIS->ptr)->mnt_devname; 
%}
probe vfs.read { mntname = kernel_string(mntname($file->f_path->mnt))
                 if (isinstr(mntname,":") { /* probable NFS */ } }
# stap -g foobar.stp
...


We could put this mntname helper widget into the vfs tapset so that a
"mntname" variable is automagically available for vfs.* probes.


- FChE