Date: Thu, 5 Mar 1998 00:10:14 -0700 (MST) From: "Justin T. Gibbs" <gibbs@narnia.plutotech.com> To: Karl Denninger <karl@mcs.net> Cc: hackers@FreeBSD.ORG Subject: Re: SCSI Bus redundancy... Message-ID: <199803050710.AAA17706@narnia.plutotech.com> In-Reply-To: <19980303200652.07366@mcs.net> <XFMail.980303212310.shimon@simon-shapiro.org> <19980303232444.59397@mcs.net>
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> The best I've seen off our RAID systems right now is about 11MB/sec (that's > megaBYTES, not bits). That's on an Ultra bus, with 2 ultra busses going to > the RAID disks. > > Neither the disk buses nor the RAID controller CPU are saturated. I > believe this is pretty much the wall on one SCSI channel, at least with > 16 SCBs. I'm going to try it with SCBPAGING turned on and see if that > helps, but for sequential reads it probably won't matter much. The problem is that the largest I/O you can send to your RAID box is 64k which it will stripe over N disks. As soon as you drop to 16 or 8k a disk per transaction, you will never be able to saturate the system. This is one of the reasons CCD performs so much better than a RAID 0 external box. CCD can perform up to a 64k transaction per disk. We really need to get buffer chaining into the kernel so we can get past this silly 64k I/O barrier. > I suspect the bottleneck is in the AIC code at this point, or the bus > itself, or the interrupt latency on the DMA completion is killing me. > There is no appreciable difference between running at 40MB/sec (ultra > full-bore) and 20MB/sec, indicating that perhaps the hold-up is in the > Adaptec microcode, driver, and/or the Adaptec/PCI bus interface. You need to up the number of transactions handled in parallel. The ahc driver has no problem saturating the SCSI bus, if you feed it enough to do. > -- > Karl Denninger (karl@MCS.Net)| MCSNet - Serving Chicagoland and Wisconsin > http://www.mcs.net/ | T1's from $600 monthly to FULL DS-3 Service > | NEW! K56Flex support on ALL modems > Voice: [+1 312 803-MCS1 x219]| EXCLUSIVE NEW FEATURE ON ALL PERSONAL ACCOUNTS > Fax: [+1 312 803-4929] | *SPAMBLOCK* Technology now included at no cost -- Justin To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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