Date: Mon, 28 Jul 1997 20:06:25 -0500 From: dkelly@hiwaay.net To: Chuck <crtb@capecod.net> Cc: questions@FreeBSD.ORG Subject: Re: 2940 Ultra? Message-ID: <199707290106.UAA04348@nexgen.hiwaay.net> In-Reply-To: Message from Chuck <crtb@capecod.net> of "Mon, 28 Jul 1997 09:57:34 EDT." <199707281357.JAA03512@localhost.nih.gov>
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Chuck asks: > > I can't find it in the RELNOTES, but is the Adaptec 2940 Ultra > supported in 2.2.2? Adaptec's propaganda would have one believe > that this is a different board from the 2940 (non-Ultra). In > particular, is simultaneous data transfer between multiple SCSI > devices possible with this board? Please define, "simultaneous data transfer". If you mean, "two bytes from two devices to two targets, this very instant" then no. Considering Chuck went on to discuss tape drives in his next paragraph I'd guess he means something more on the lines of, "can I backup/restore my hard drive to/from a tape drive on the same SCSI bus?" To which, the answer is "yes". While two devices can't talk at the same time on the same bus, good SCSI devices should not occupy the SCSI bus any longer than it takes to send a command. Or to reply to a command. FreeBSD should send your hard drive a request for data, then both jump off the SCSI bus. When the HD has the requested data it negotiates and claims the SCSI bus and sends the data to the SCSI card and FreeBSD. In between the tape drive gets its opportunities to do the same. But the real beauty of SCSI for real multi-user mult-tasking OS's such as FreeBSD is to consider the above and compound it with the realization multiple commands can be queued to the SCSI device(s) before the first reply arrives. And the SCSI device isn't required to reply to the requests in the same order they were sent. Haven't heard of it being used in an OS, but a host can instruct two SCSI devices to send data to each other rather than to the host. Imagine telling the HD to dump itself to the tape drive. While dumping, the host has nothing do to. Typically it copies the data in from the HD and copies it out to the tape. Considering the HD knows almost nothing about file structure (have heard some know FAT filesystems in order to better guess caching) this isn't very useful. We have at work a number of TTI (brand) Exabyte tape drives that have some kind of controller with buttons and display for error rates & such. Dual TTI tape drive boxes can duplicate a tape from one to the other without the computer host being the wiser. I'm guessing they use a serial interface Exabyte provides to issue commands to both tape drives, and then the transfer goes thru the SCSI bus. -- David Kelly N4HHE, dkelly@hiwaay.net ===================================================================== The human mind ordinarily operates at only ten percent of its capacity -- the rest is overhead for the operating system.
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