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Date:      Thu, 23 Nov 2006 14:20:11 -0500
From:      Kris Kennaway <kris@obsecurity.org>
To:        Dieter <freebsd@sopwith.solgatos.com>
Cc:        freebsd-questions@freebsd.org
Subject:   Re: processes not getting fair share of available disk I/O (was: Re: TCP parameters and interpreting tcpdump output )
Message-ID:  <20061123192011.GA81772@xor.obsecurity.org>
In-Reply-To: <200611231735.RAA19097@sopwith.solgatos.com>
References:  <20061123075214.GA51991@xor.obsecurity.org> <200611231735.RAA19097@sopwith.solgatos.com>

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On Thu, Nov 23, 2006 at 09:35:08AM +0000, Dieter wrote:
> > > > > 	hw.ata.wc=3D3D3D0
> > > >         ^^^^^^^^^^^
> > > > "Make my hard drive go reeeeally slow please (just in case I crash)=
" :)
> > >=3D20
> > > Slower, yes, but not *that* slow.
> > >=3D20
> > > Normal ls : 0.032 second.  Two processes using same disk, multiply by=
 two,
> > > so 0.064 second.  Maybe the multiplier is more than 2, call it 10x, so
> > > 0.32 second.  But I'm seeing a factor of over 9100x.
> >=20
> > Humour me and turn it back on, then see what happens.
>=20
> Where is the knob to turn the write cache on/off on a per-drive basis
> in FreeBSD?  I can do this in NetBSD, but the only knob I can find in
> FreeBSD affects all drives, and requires a reboot.

Yes, I think you need to do it globally at boot time.

> Humour me and read the Subject line.  The ls does not get its fair share
> of disk I/O.
>=20
> Both times are with the disk's write cache in write-through mode.
> I'm not comparing times with the write cache in different modes.
> I'm comparing ls by itself against ls competing with cp.

Your cp is going to be running synchronously, i.e. spend a lot of time
waiting on the disk to perform the writes.  This may well be the cause
of your problem.  Once we have established whether or not it is the
cause, we can proceed to whether this behaviour can be improved.

Kris
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