Date: Fri, 12 Oct 2001 16:01:45 -0700 From: Mike Smith <msmith@freebsd.org> To: Alexey Koptsevich <alex@astro.su.se> Cc: Scott Long <scott_long@btc.adaptec.com>, scsi@freebsd.org, msmith@mass.dis.org Subject: Re: Adaptec 3210S: RAID5 performance and bus throughput Message-ID: <200110122301.f9CN1jk04719@mass.dis.org> In-Reply-To: Message from Alexey Koptsevich <alex@astro.su.se> of "Sat, 13 Oct 2001 00:37:09 %2B0200." <Pine.GSO.4.10.10110122310180.12138-100000@dioscuri.astro.su.se>
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> > Since the bus is real bottleneck, I wonder what is the purpose to have > 4-channel RAID controllers, in principle? Just to connect many discs? I > mean, 4-channel controller gives 160*4=640Mb/s, but this cannot be driven > even by 64bit/66MHz bus which provides only 528Mb/s! Also, are there x86 > motherboards which provide those 64bit/66MHz? You are making a fundamental and common mistake about the purpose of RAID controllers. You need to step back and think a little more broadly to see the answers to your questions. They are not, generally, designed to provide "improved" performance. Most of the performance-enhancing features of a RAID controller are there to compensate for the fact that the RAID process has overheads, or to take advantage of the fact that with lots of disks you can degrade more gracefully under heavy load. > I also wonder what is real performance of 3210S in RAID5 mode? Are 4-5 > disks which saturate 160Mb/s on SCSI bus (assume that load is heavy) are > able to saturate those 132Mb/s on PCI bus after performing all XOR > calculations? If not, how many discs saturate 132Mb/s? Under "heavy load" disks are largely busy with seek operations. "Heavy" sustained sequential I/O is relatively rare. > And, as a conclusion from previous question, is there real purpose to have > 2-channel controller on 32bit/33MHz PCI (which is used on my Intel > L440GX, assuming its 66MHz PCI to be weak), using it in RAID5 mode? RAID5 is relatively fragile, and two SCSI channels mostly just allows you to balance channel load, cabling load and plain, boring cabling issues around. If you are looking for reliability, one use for a dual-channel RAID controller is to mirror across channels, ie. use a RAID10 or RAID50 setup with disks split across the two channels such that losing an entire channel (eg. due to cable failure, someone unplugging, power supply failure etc.) you don't actually lose the volume. Regards, Mike To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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