Date: Fri, 3 Jan 1997 18:23:09 +0100 From: j@uriah.heep.sax.de (J Wunsch) To: freebsd-scsi@freebsd.org Subject: Re: Ideas on CD changers sought Message-ID: <Mutt.19970103182309.j@uriah.heep.sax.de> In-Reply-To: <199701022154.QAA23215@hda.hda.com>; from Peter Dufault on Jan 2, 1997 16:54:23 -0500 References: <199701021747.SAA08128@uriah.heep.sax.de> <199701022154.QAA23215@hda.hda.com>
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As Peter Dufault wrote: > How about round robin scheduling the platters, maybe with hysteresis > on platter switching? > > Add a "changer queue" that is a queue of CD start queues to schedule > the changer resource. A changerstart routine can then round robin > schedule across the active platters. Hmm, but that would require we start distinguish changers from regular CD drives, right? Is there any way to learn about the medium changing ability other than a hard-coded list of devices that are known to do it? > Have changerstart schedule up to N transfers in a row for a given > unit by calling a derivation of the existing cdstart. If N counts > down take the CD start queue off and enqueue it again at the tail > of the changer queue and move to the next CD start queue. You want > to select N to be a number that gives you decent utilization, maybe > enough transfers to take twice as long as the change mechanism. I think the medium change must even be artificially delayed for some amount, ``just in case'' some future request for the same platter arrives while it's swapped in. The swapin/swapout times are extremely high (10 seconds), so it would IMHO justify to wait another 2 or 3 seconds even on an empty queue. > You can still come up with loads that will thrash this. You could > add hysteresis to the platter switching with a minimum-time-at-a-platter > timeout. That's probably about the same as my thoughts above. -- cheers, J"org joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ -- NIC: JW11-RIPE Never trust an operating system you don't have sources for. ;-)
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