Tracing Disk Activity

Charles Swiger cswiger at
Mon Nov 22 09:12:56 PST 2004

On Nov 21, 2004, at 2:17 PM, Hanspeter Roth wrote:
>   On Nov 21 at 10:50, Chuck Swiger spoke:
>> Hanspeter Roth wrote:
>>> I have set an idle timeout for the hard-disk. But when there is no
>>> user activity there are frequent disk accesses.
>> Yes, this is Unix.  Even when there is no user activity, a Unix system
>> normally is still running a number of daemons such as syslogd which
>> regularly write to the filesystem.  Beyond that, the syncer mechanism 
>> tries
>> to reduce the number of dirty memory buffers every thirty seconds or 
>> so.
> I guess that some daemons are causing disk access. But it must be
> not only syslogd.

That's right.  Normally, people end up running a number of daemons like 
sendmail or some other MTA, ntpd, named, etc.

> Is the syncer causing the disk to spin up even if there is nothing to 
> flush?

Probably no.  However, if you have active processes running on the 
system, it is very likely that the syncer will find data that it does 
want to write.

> [...]
>> Instead you probably will need to mount filesystems read-only and 
>> create
>> RAM disks in a fashion similar to booting off limited-write media like
> My idea is to transfer those files that are written also when the
> user is idle to a RAM disk (some from /var/log and dhclient.leases).
> But I don't want to mount the filesystems read-only.

OK.  However, you are probably not going to be able to prevent 
everything running on a normal Unix system that wants to scribble to 
disk short of heroic measures.

>> Compact Flash.  Either that, or simply shutdown the system or run zzz 
>> to
>> suspend the system via APM/APCI.
> This is less convenient and probably doesn't work on my laptop. (I
> have to check whether the upgrade to 5.3R has changed something in
> this respect.)

Hmm.  For what it is worth, it's taken about two years of effort by 
Apple to work through many of these issues in order to get MacOS X on 
their laptops to be reasonably friendly in terms of saving power, 
conserving hard drive access, and having power save/suspend to RAM 
behave properly.


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