Linux Laptop Mode and /proc block_dump
Over at the
top-like command for disk io thread on GLLUG Kostas Georgiou mentioned
/proc file entry I’d never heard of before, and after
some digging it looks like it could be useful when debugging certain IO
problems. Assuming you have 2.6.6 or above - or a vendor patched kernel.
When you activate the option with a
echo 1 > /proc/sys/vm/block_dump as root (read the article and consider
turning syslog off first) the kernel starts to log which processes are
accessing which disk blocks and inodes. Below is a small chunk of its
output on my test VMWare system:
Mar 14 19:16:44 localhost kernel: sshd(2659): dirtied inode 388836 (sshd) on sda1 Mar 14 19:16:44 localhost kernel: sshd(2659): dirtied inode 533395 (libwrap.so.0) on sda1 Mar 14 19:17:23 localhost kernel: cat(2672): dirtied inode 888805 (.bash_history) on sda1 Mar 14 19:17:46 localhost kernel: kjournald(913): WRITE block 14016 on sda1 Mar 14 19:17:48 localhost kernel: pdflush(104): WRITE block 12487672 on sda1
The short version is, ‘dirtied’ means changed but not written to disk, pdflush
will write the rows out later, and READs are what you’d expect. A brute
force way to trace an inode to a file path is with find:
find / -inum num. The longer explanation can be found at LJs Extending Battery
Life with Laptop Mode under the “Spinup Debugging” heading.
It’s no DTrace (and no, SystemTap isn’t as good as DTrace) but it is neat and a decent addition to the debugging toolbox.