Date: Thu, 5 Mar 1998 08:56:54 -0600 From: Karl Denninger <karl@mcs.net> To: "Justin T. Gibbs" <gibbs@narnia.plutotech.com> Cc: hackers@FreeBSD.ORG Subject: Re: SCSI Bus redundancy... Message-ID: <19980305085654.20326@mcs.net> In-Reply-To: <199803050710.AAA17706@narnia.plutotech.com>; from Justin T. Gibbs on Thu, Mar 05, 1998 at 12:10:14AM -0700 References: <19980303200652.07366@mcs.net> <199803050710.AAA17706@narnia.plutotech.com>
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On Thu, Mar 05, 1998 at 12:10:14AM -0700, Justin T. Gibbs wrote: > > 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. Will going to paged SCBs do this? Some disks have had trouble with this in the past, which is why I run it disabled right now. But with the CMD controllers in the loop now, that's no longer a factor. -- -- 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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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