Re: Negative "ios_in_flight" in the 2.4 kernel

["M. Edward Borasky" <znmeb@cesmail.net>]

On Thu, 2004-12-23 at 09:08 +0100, Jens Axboe wrote:
> > We could eliminate that possibility if you ran your tests with a single 
> > non-partitioned disk, but thats just a guess.
> 
> It would be nice to know if this was a vanilla kernel or patched in some
> way. The only recent bug in this area I remember was a bad merge in the
> SUSE tree with the io_request_lock scaling patch.

I have seen this with Red Hat 2.4.18 (from RH 8.0) kernels, Gentoo
2.4.25 and 2.4.26 kernels, on both single-disk and two-disk systems. Now
that I think of it, I've seen this on both single-processor and
multi-processor systems and with both SCSI and IDE drives. I have also
seen these systems run for quite a while without ios_in_flight going
negative. And I've never seen ios_in_flight lower than -1 or higher than
0 on an idle system. So my conclusion is that an extra downcount is
fairly rare.

I saw a very similar bug listed in the LKML about a year ago. For
example, see

http://search.luky.org/linux-kernel.2004/msg00025.html

and

http://search.luky.org/linux-kernel.2004/msg03278.html

I think I'll try rebooting the two-disk box (which is easier to get one
truly idle disk on) and running bonnie++ periodically to see if I can
get steady-state ios_in_flight values other than -1 and 0 on an idle
system (between bonnie++ runs). I can set up "oprofile" on the Gentoo
boxes if that will help.

One other note: all of these systems when "idle" have a small amount of
write activity going on. The Red Hat boxes are using ext3 filesystems
and the Gentoo systems are using reiserfs. Is constant low-level writing
to be expected with journaling?