sysfs

Linux 2.6 introduces the /sys filesystem which you will notice in "mount" output:

/dev/sda1 on / type ext3 (rw)
none on /proc type proc (rw)
none on /sys type sysfs (rw)
..
 

Like /proc, /sys exposes kernel information as a filesystem. Why not just leave this all under /proc? Well, because /proc has become a maze of twisty passages with no real organization. Sysfs is supposed to be an organized and structured representation of the kernel device model.

/proc probably should have been named /kernel and proc should have been a directory under that. Too late now, of course, but /sys is designed to be easy for programs to parse. It's not so easy for humans, and there's a lot of symlinking (try a "find /sys -type l"). As you'll still find similar information under /proc, this is all pretty confusing. It also can be hard to find explanations of what each file represents. For example:

# ls -l  /sys/block/sda/sda1
total 0
-r--r--r--  1 root root 4096 Apr 14 13:32 dev
-r--r--r--  1 root root 4096 Apr 14 13:32 size
-r--r--r--  1 root root 4096 Apr 14 13:32 start
-r--r--r--  1 root root 4096 Apr 14 13:32 stat
 

Some of these are fairly obvious:

# for i in *; do echo  -e "$i:  \c" ; cat $i; done
dev:  8:1
size:  1060227
start:  63
stat:     90217  2203122    43751   350008
 

but just like the old-fashioned ioctl devices that /proc and /sys replace, the contents of these files are up to the device driver writer. The /sys filesystem organizes the data so that the info is in a structured place, but the meaning of the contents is up to the author of the driver. Of course, with Linux we usually would have the ability to look at the source, though even that may not tell us a lot without deep understanding of the hardware that the driver is controlling. The driver stores information by setting a Driver Attribute. Understand that (like ioctl files) nothing is really stored in these files: when you try to read or write them, some routine in the applicable driver gets called. Nor does the driver actually create the file hierarchy; that's the kernel's job.

And of course, sysfs can be abused. That's why we may need something like Debugfs ( a driver to provide a file system for device driver info, not the "debugfs" tool for ext2 filesystems). Oh, goody, yet another place to go scrounging for undocumented and unorganized info.

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