From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: 12x performance drop on md/linux+sw raid1 due to barriers [xfs] Date: Tue, 16 Dec 2008 09:38:57 +1100 Message-ID: <20081215223857.GF32301@disturbed> References: <1229225480.16555.152.camel@localhost> <18757.4606.966139.10342@tree.ty.sabi.co.uk> <200812141912.59649.Martin@lichtvoll.de> <18757.33373.744917.457587@tree.ty.sabi.co.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <18757.33373.744917.457587@tree.ty.sabi.co.uk> Sender: linux-raid-owner@vger.kernel.org To: Peter Grandi Cc: Linux XFS , Linux RAID List-Id: linux-raid.ids On Sun, Dec 14, 2008 at 10:02:05PM +0000, Peter Grandi wrote: > [ ... ] >=20 > > But - as far as I understood - the filesystem doesn't have to > > wait for barriers to complete, but could continue issuing IO > > requests happily. A barrier only means, any request prior to > > that have to land before and any after it after it. >=20 > > It doesn't mean that the barrier has to land immediately and > > the filesystem has to wait for this. At least that always was > > the whole point of barriers for me. If thats not the case I > > misunderstood the purpose of barriers to the maximum extent > > possible. >=20 > Unfortunately that seems the case. >=20 > The purpose of barriers is to guarantee that relevant data is > known to be on persistent storage (kind of hardware 'fsync'). >=20 > In effect write barrier means "tell me when relevant data is on > persistent storage", or less precisely "flush/sync writes now > and tell me when it is done". Properties as to ordering are just > a side effect. No, that is incorrect. Barriers provide strong ordering semantics. I/Os issued before the barrier must be completed before the barrier I/O, and I/Os issued after the barrier write must not be started before the barrier write completes. The elevators are not allowed to re-=D0=BErder I/Os around barriers. This is all documented in Documentation/block/barrier.txt. Please read it because most of what you are saying appears to be based on incorrect assumptions about what barriers do. Cheers, Dave. --=20 Dave Chinner david@fromorbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html