Subject: Re: wd, disk write cache, sync cache, and softdep.
To: Bill Studenmund <>
From: Manuel Bouyer <>
List: tech-kern
Date: 12/17/2004 10:22:09
On Thu, Dec 16, 2004 at 04:10:39PM -0800, Bill Studenmund wrote:
> [...]
> > The thing I liked about the completion queue idea is that it stays
> > etirely within the disk driver layer, and merely restores the disk
> > semantics that are assumed by all the upper layers: biodone buffers
> > are safely on stable storage. 
> But those semantics are not always the right ones for the upper levels.
> Yes, those are the ones everything grew up with. But write-back caches    
> were added to disks (and enabled by default) for a reason - they let the
> disk perform very well. Other OSs cope well with this. If we want to
> perform well, we will need to as well.
> I agree it's wrong to assume we don't have write caches when we do, but I 
> think you tackle the problem backwards. Rather than hide the write caches, 
> I think we need to start changing the upper code to deal with them.

What he said.

Manuel Bouyer <>
     NetBSD: 26 ans d'experience feront toujours la difference