Date: Thu, 2 Nov 2000 20:16:21 -0400 (AST) From: The Hermit Hacker <scrappy@hub.org> To: "Justin T. Gibbs" <gibbs@scsiguy.com> Cc: Tom Samplonius <tom@sdf.com>, freebsd-scsi@FreeBSD.ORG Subject: Re: iostat: tps for SCSI drives ... Message-ID: <Pine.BSF.4.21.0011022014410.494-100000@thelab.hub.org> In-Reply-To: <200011020457.eA24vea67590@aslan.scsiguy.com>
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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
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