Re: Eradic disk access during reads

[Willy Tarreau <willy@w.ods.org>]

On Sat, Sep 17, 2005 at 01:32:53PM +0300, Al Boldi wrote:
> Willy Tarreau wrote:
> > On Sat, Sep 17, 2005 at 07:26:11AM +0300, Al Boldi wrote:
> > > Monitoring disk access using gkrellm, I noticed that a command like
> > >
> > > cat /dev/hda > /dev/null
> > >
> > > shows eradic disk reads ranging from 0 to 80MB/s on an otherwise idle
> > > system.
> > >
> > > 1. Is this a hardware or software problem?
> >
> > Difficult to tell without more info. Can be a broken IDE disk or defective
> > ribbon.
> 
> Tried the same with 2.4.31 which shows steady behaviour with occasional dips 
> and pops in the msec range.
> 
> >
> > > 2. Is there a lightweight perf-mon tool (cmd-line) that would log this
> > > behaviour graphically?
> >
> > You can do " readspeed </dev/hda | tr '\r' '\n' > log " with the readspeed
> > tool from there :
> >    http://w.ods.org/tools/readspeed
> 
> Does it have msec resolution?

no, it does not. It will only show you the average transfer rate during
the past second each second. It often helps me tune network, nfs, raid, ...

Denis' tool seems clearly more suited to analyse your problem.

Regards,
Willy