Subject: Re: wd, disk write cache, sync cache, and softdep.
To: Bill Studenmund <>
From: Manuel Bouyer <>
List: tech-kern
Date: 12/18/2004 09:07:56
On Fri, Dec 17, 2004 at 06:03:47PM -0800, Bill Studenmund wrote:
> > > send A1, drive says OK,
> > > send A2, drive says OK,
> > > send A3+FUA, drive writes A3 (due to FUA), drive says OK,
> > > drive writes A2, drive writes A1
> > 
> > If you issue A3 with an "ordered queue tag", then the FUA on A3 will force A1
> > and A2 before A3. But you can still have A2, A1, A3.
> I'm starting to feel like a broken record. "Ordered" queue tags won't 
> help, if we're trying to get around the write cache. All the ordered tag 
> does is mean we don't start writing A2 into the cache until we've said A1 
> is done, and likewise we don't start writing A3 into the cache until A2 is 
> done.

or A1 and A2 in any order, but before A3 if A1 and A2 are queued with
simple queue tag and A3 with ordered queue tags.

> With a write-back cache, we still have whatever ordering we want in 
> getting data out of the cache.

The FUA bit should for the media acess, so in this case the cache flush
for A3, and any commands queued before A3 if A3 is queued with
ordered queue tag + FUA.

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