Re: Null performance during a test

From: Jens Axboe <jens.axboe_at_oracle.com>
Date: Mon, 23 Jul 2007 14:04:27 +0200

On Fri, Jul 20 2007, Jens Axboe wrote:
> On Fri, Jul 20 2007, Erwan Velu wrote:
> > Hey Jens,
> >
> > Here come the bw line from a disk_profile_zone test:
> > bw (KiB/s) : min= 0, max=66715, per=96.11%, avg=62230.60,
> > stdev=12736.12
> >
> > I was very surprised to see 0 as min. I looked into the bw file and found
> > some lines like :
> > 2193590, 0, 0
> >
> > I tried to find the number of lines and found 181 null measures.
> >
> > [root_at_max1 ~]# grep ", 0, 0" client1_bw.log | wc -l
> > 181
> > [root_at_max1 ~]#
> >
> > My job have 4569 entries in the bw.log file meaning that near 4% of my
> > requests reports a null size.
> >
> > Do you have any idea why I have such results ? Looks like an IO at 0K/sec
> > is nearly impossible for a huge set of sequential requests of 64KB blocks.
> >
> > This null results turn my average & stdev wrong :/
>
> Interesting, that definitely sounds like a bug as well. Will
> investigate.

Can't reproduce. If I were to venture a guess, I would say that it's a
combination of:

- small bw window (it's only 500 msec)
- larger block size (64kb)
- other activity on the drive?

Was the tested drive otherwise idle?

-- 
Jens Axboe
Received on Mon Jul 23 2007 - 14:04:27 CEST

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