Really the sysfs and /proc filesystems are worlds of magic and fantasy. Each day I discover a new trick using this filesystems. So, I decided to post a short summary of my favorites ones. Enjoy and feel free to add your tricks in comments, maybe we can a /proc and /sys knownledge database in a post

1. Scanning for LUNs in attached FC
echo "- - -" > /sys/class/scsi_host/hostX/scan

2. CPU hotplug
echp 0 > /sys/devices/system/cpu/cpuX/online

Obviously when run echo 1, put the CPU online again

3. Enable dmesg timestamp
echo Y > /sys/modules/prinkt/parameters/time

4. Restore a removed file when is still in use
cat /proc/pid/fd/descriptor number > /tmp/myfile_restored

5. Get the IO operations for a process:
cat /proc/pid/io
The syscr and the syscw are the accumulated read and write IO operations that process do where running.

6. Increase size of IO scheduler queue:
echo 10000 > /sys/block/device/queue/nr_request

7. Get the current IO scheduler enabed to a specific device:
cat /sys/block/device/queue/scheduler

8. Get the threads of pdflush process which are running:
cat /proc/sys/vm/nr_pdflush_threads

9. Set the percentage threshold for memory to start to flushd data to disk:
echo XX > /proc/sys/vm/dirty_background_ratio

10. Set the sleep time for pdflush checking (in centisecs):
echo XXX > /proc/sys/vm/dirty_writeback_centisecs

11. Set the time to live for a data in buffer, when raises, data will commit to disk (in centisecs):
echo XXX > /proc/sys/vm/dirty_expire_centisecs