From owner-freebsd-scsi Fri Oct 15 11:44:41 1999 Delivered-To: freebsd-scsi@freebsd.org Received: from front5m.grolier.fr (front5m.grolier.fr [195.36.216.55]) by hub.freebsd.org (Postfix) with ESMTP id 6B10E15044 for ; Fri, 15 Oct 1999 11:44:21 -0700 (PDT) (envelope-from groudier@club-internet.fr) Received: from localhost (ppp-165-218.villette.club-internet.fr [195.36.165.218]) by front5m.grolier.fr (8.9.3/No_Relay+No_Spam_MGC990224) with SMTP id UAA02047; Fri, 15 Oct 1999 20:44:11 +0200 (MET DST) Date: Fri, 15 Oct 1999 21:06:14 +0200 (MET DST) From: Gerard Roudier X-Sender: groudier@localhost To: "Chris D. Faulhaber" Cc: Andrew Gallatin , "Kenneth D. Merry" , scsi@FreeBSD.ORG Subject: Re: FreeBSD 3.2 / Slow SCSI Dell PowerEdge 4300 In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Sender: owner-freebsd-scsi@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org On Fri, 15 Oct 1999, Chris D. Faulhaber wrote: > da0: Fixed Direct Access SCSI-2 device > da0: 40.000MB/s transfers (20.000MHz, offset 8, 16bit) > da0: 8683MB (17783204 512 byte sectors: 255H 63S/T 1106C) >=20 > After enabling tagged queuing on this drive (by removing the quirk entry) > and found performance about 10% slower. What kind of performance are you measuring ? Tagged command queuing is intended to improve _multithreaded IOs that is a lot more realistic IO pattern than single-threaded sequential IO. I also read some decrease of performance for DCAS for single-threaded sequential IO when increasing number of tags. Unless, guys, you just want to eat the cake and to have it, I donnot see any serious problem for these drives. May-be there is some room for improvement in their firmware. They should _not_ have been quirked to 0 tags, in my opinion, if the decrease of performance observed is for sequential IOs. At most, user should be advised to use a reasonnable number of tags or the quirk should have been more soft.=20 For the DCAS, the decrease of performances has been observed for sequential write IOs that is a great stress for a disk when write behing caching is enabled with tags enabled, but still nothing has been reported for read and especially multithreaded read IOs. Castrating a disk model=20 regarding tags due to such unreaslistic results has been unserious in my opinion.=20 When tags are enabled, the drive can overlap writes by disconnecting previous to the data phase and so get rid of part of IO latency. In such a situation, write behind caching can be disabled for some good disks without significant decrease of performances, allowing drives to perform more prefetching for read. Indeed, writing meta-data synchronously does not help, but softupdate may do.=20 G=E9rard. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message