Date: Tue, 16 Feb 1999 19:18:05 -0500 (EST) From: Harvey Fishman <fishman@panix.com> To: "Robert G. Brown" <rgb@phy.duke.edu> Cc: aic7xxx Mailing List <AIC7xxx@FreeBSD.ORG>, Doug Benjamin <dbenjamin@fnal.gov> Subject: Re: Two controllers or a dual... Message-ID: <Pine.GSU.4.05.9902161911410.985-100000@panix3.panix.com> In-Reply-To: <Pine.LNX.3.96.990216150406.14746A-100000@ganesh.phy.duke.edu>
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On Tue, 16 Feb 1999, Robert G. Brown wrote: > Dear List Humans: > > We are trying to design systems intended to hold as much as 100-200 GB > of disk (to hold events collected at Fermilab and simulations thereof, > where the actual total dataset sizes can run into the terabyte range > quite easily). > > A lot of the processing is likely to be disk I/O bound, and the question > has arisen -- is it better to get a single dual scsi controller (e.g. > the 7895) or two separate scsi controllers (e.g. 2940's)? Presumably > the dual controller will share an interrupt for both channels, and two > controllers would be on different interrupts. Does the answer depend on > whether or not the controllers are U, UW, U2W? I realize that 2 U2W > controllers will saturate the PCI bus anyway, but they should still give > some gain over a single U2W controller. > > The system(s) in question will probably run linux, but the question > itself is open to anyone with either measurements (ideal) or theoretical > statements to make for either operating system. > > Thank you, > > rgb > > P.S. -- if anyone wishes to comment on building the required disk out of > e.g. 3 or 4 Cheetahs per U2W controller vs buying a commercial disk > array (speed, cost comparisons, support in linux) that would be welcome > as well. It seems to me that you should be considering just where your bottlenecks are going to be. The classical one is the head-disk bandwidth on the drives. If you have enough discrete requests occurring in parallel to different drives, only then does the SCSI channel bandwidth become of consequence. And remember that you also have DMA bandwidth in the processor box to worry about. I suspect that two U2W channels running at the same time are going to more than fill that. The processor will be accessing that same memory over the same bus so there is more contention to worry about. If your data is in L2 cache, then you don't need the disks to start with. So I think that you need a LOT more understanding and detail about the nits and grits of the application BEFORE you start thinking about what hardware will best serve your needs. Harvey ---------------------------------------------------------------------------- Harvey Fishman | fishman@panix.com | A little heresy is good for the soul. 718-258-7276 | To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe aic7xxx" in the body of the message
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