From owner-freebsd-stable Thu Sep 20 22:49:29 2001 Delivered-To: freebsd-stable@freebsd.org Received: from earth.backplane.com (earth-nat-cw.backplane.com [208.161.114.67]) by hub.freebsd.org (Postfix) with ESMTP id 15AAB37B420 for ; Thu, 20 Sep 2001 22:49:27 -0700 (PDT) Received: (from dillon@localhost) by earth.backplane.com (8.11.6/8.11.2) id f8L5n6r67664; Thu, 20 Sep 2001 22:49:06 -0700 (PDT) (envelope-from dillon) Date: Thu, 20 Sep 2001 22:49:06 -0700 (PDT) From: Matt Dillon Message-Id: <200109210549.f8L5n6r67664@earth.backplane.com> To: Mike Meyer Cc: "Daniel O'Connor" , "Kevin Oberman" , stable@FreeBSD.ORG Subject: Re: RE: hw.ata.wc && hw.ata.tags && softupdates short question References: <200109210329.f8L3TGR30860@ptavv.es.net> <20010920211704.C7820@gateway.bogus> <15274.48864.994229.51687@guru.mired.org> Sender: owner-freebsd-stable@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG Basically write-caching becomes irrelevant when you have tags, because the host does not have to wait for a write to complete before starting the next one. When IDE tags work, write caching no longer matters. Without IDE tags you have to turn write caching on in order to get similar write performance. Without write caching or tags for IDE, the driver must wait for a write to get completely on the disk and return an acknowledgement before it can begin queueing the next write. Unfortunately IDE drives are driven by the consumer market. They just don't work all that well if write caching is turned off (and tags are a relatively new feature for IDE), which is why we turned write-caching back on by default for IDE. While this theoretically introduces the possibility of out-of-order writes, and while out of order writes have been shown to occur under certain heavy load situations, we do not know of a single case where the IDE write caching feature has actually resulted in any corrupt data. Now with that all said it may seem that IDE + tags is the perfect solution for IDE. Unfortunately IDE is a moving target.. the chances of getting a highly reliable IDE solution are fairly low. Just look at all the problems we have with something as simple as IDE+DMA. SCSI hard drives almost universally have tags so this isn't an issue with SCSI. The simple answer is that if you care at all about reliability, pay the premium for SCSI. -Matt To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message