From owner-freebsd-scsi Thu Nov 2 16:18:23 2000 Delivered-To: freebsd-scsi@freebsd.org Received: from thelab.hub.org (CDR20-53.accesscable.net [24.138.20.53]) by hub.freebsd.org (Postfix) with ESMTP id 83BCB37B4D7 for ; Thu, 2 Nov 2000 16:18:20 -0800 (PST) Received: from localhost (scrappy@localhost) by thelab.hub.org (8.11.1/8.11.1) with ESMTP id eA30GLe62593; Thu, 2 Nov 2000 20:16:21 -0400 (AST) (envelope-from scrappy@hub.org) X-Authentication-Warning: thelab.hub.org: scrappy owned process doing -bs Date: Thu, 2 Nov 2000 20:16:21 -0400 (AST) From: The Hermit Hacker To: "Justin T. Gibbs" Cc: Tom Samplonius , freebsd-scsi@FreeBSD.ORG Subject: Re: iostat: tps for SCSI drives ... In-Reply-To: <200011020457.eA24vea67590@aslan.scsiguy.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-freebsd-scsi@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org On Wed, 1 Nov 2000, Justin T. Gibbs wrote: > >> Maxtags is the limit imposed by the quirk entry matching the device. > > > >Okay, under what conditions would dev_openings == maxtags? My best drive > >that I can find on my various machines is a brand new LVD drive: > > You will only see this very early in the boot process before a device > indicates that it cannot handle "maxtags" via a queue full or if you > have a device that can really achieve "maxtags". For instance, > some RAID arrays can perform the full 256 tags that can be outstanding > is SCSI-2, but most drives max out at 64 or 63 tags. Only recently have > I found some IBM drives that will support 128 tags. That said, the > system will only allocate resources to support the number of transactions > it is determined a device can use. The "maxtags" value is just an > upper bound. So, what exactly does supporting 64, or 128, tags on a drive provide? From what I've been able to gleam from reading, my guess is a FIFO queue of commands to a drive, so that as soon as the results from command 0 finishes, it doesn't have to wait for command 1 to be sent from the system ... is this correct, albeit simplistic? To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message