Date: Mon, 01 Apr 2002 14:29:04 -0700 From: "Justin T. Gibbs" <gibbs@scsiguy.com> To: Michael Smith <msmith@FreeBSD.ORG> Cc: "Kenneth D. Merry" <ken@kdm.org>, Lukas Ertl <l.ertl@univie.ac.at>, freebsd-scsi@FreeBSD.ORG Subject: Re: ciss driver and tagged queuing Message-ID: <200204012129.g31LT4939657@aslan.scsiguy.com> In-Reply-To: Your message of "Fri, 29 Mar 2002 14:16:32 PST." <200203292216.g2TMGWN01304@mass.dis.org>
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>The arrays don't support tagged queueing, but they do support multiple >outstanding commands. So then the boards aren't really emulating SCSI disks. 8-) Tagged queing is the only way to support multiple outstanding commands in SCSI. >I don't want to have to handle tags; I just want CAM to keep giving me >commands until I ask it to stop. 8) Don't you know in advance the maximum number of commands you can handle concurrently per-array and per-controller? If you do, you can just tell CAM the numbers and it will do what you request. If the limits are squishy, you have to freeze your device and/or sim queues when you hit hit a limit or perhaps simulate a queue full response if you are going to emulate tagged queuing. >One other complication is that the limit on outstanding commands is >per-adapter not per-array, so the automatic tag-backoff that CAM does >won't work correctly under load. I'm confused by this comment. CAM has both per-SIM and per-device limits and will round-robin resources amongst devices that share those resources when there is contention. The tag backoff is *per-device* not per controller. -- Justin To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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