linux disk access when idle

[Matt Garman <matthew.garman@gmail.com>]

The scope of this question is probably broader than Linux RAID, but
I thought I'd try here before going to the LKML...

I have a fileserver/NAS box that has four Western Digital GreenPower
drives in software RAID-5 (i.e. Linux md).  There was some noise
about these drives not too long ago: when used under Linux, they
tend to have really high (and rapidly increasing) SMART
Load_Cycle_Count values[1].

Basically, in order to save power, these drives park the read/write
heads after so many seconds of inactivity.  According to
SilentPCReview[2], the 1 TB model uses 5.7W when idle and only 3.7W
when the heads are unloaded.  Apparantly, each head parking event
causes teh Load_Cycle_Count value to increase.

Most people seek to disable this head parking behavior to stop that
Load_Cycle_Count value from growing alarmingly high.  I actually
want to use it to it's full potential.

I've got a Kill-A-Watt electricity meter connected to my NAS box.  I
see a difference of 10 watts power usage when the heads park (I can
hear them unload, so I know when it happens).  Allowing for some
power supply inefficiency, I'd say my observation is consistent with
SPCR's numbers.

The problem is, the drives don't stay in this parked state very
long.  I haven't actually timed the state changes, but average power
consumption over a long time (e.g. a week or more) is at the higher
(i.e. +10) level.

So I figure, something is accessing these drives shortly after the
heads park, causing them to un-park (and increasing power
consumption).  But this machine is idle 95% of the time.  And even
then, the overwhelming majority of the accesses are reads, with very
few writes (literally, a handful of writes per week).

So what I'm trying to figure out is, what is causing the disk
access?  It could be any one of:

    - Kernel
    - RAID subsystem (i.e. md)
    - XFS filesystem
    - NFS
    - Samba
    - ???

I'm hoping someone has enough knowledge of these systems to point me
in the right direction for tuning things.  The goal is that when the
machine is idle, it is "truly" idle, meaning, no disk accesses take
place and the heads can stay parked (thus saving energy).

Thank you,
Matt

LINKS:
[1] http://groups.google.com/group/linux.kernel/browse_thread/thread/505ccf760023d132/7e4f4e996f911efd

[2] http://www.silentpcreview.com/article804-page2.html