Date: Mon, 25 Oct 2004 14:45:37 -0600 From: Scott Long <scottl@freebsd.org> To: Charles Swiger <cswiger@mac.com> Cc: freebsd-current@freebsd.org Subject: Re: FreeBSD 5.3b7and poor ata performance Message-ID: <417D65F1.2040809@freebsd.org> In-Reply-To: <F5F15CA0-26C5-11D9-9A2F-003065ABFD92@mac.com> References: <14479.1098695558@critter.freebsd.dk> <417D25E8.6080804@ng.fadesa.es> <200410251928.01536.victor@alf.dyndns.ws> <200410251837.58257.Thomas.Sparrevohn@btinternet.com> <417D3F12.20302@DeepCore.dk> <417D40A1.9030802@ng.fadesa.es> <417D45F1.9090504@freebsd.org> <77F3FD4D-26BE-11D9-9A2F-003065ABFD92@mac.com> <417D58B6.5030509@freebsd.org> <F5F15CA0-26C5-11D9-9A2F-003065ABFD92@mac.com>
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Charles Swiger wrote: > On Oct 25, 2004, at 3:49 PM, Scott Long wrote: > [ ... ] > >>> Your position is certainly reasonable: if a storage system is not >>> reliable, how fast it performs is something of a moot point. :-) >>> However, this being said, a RAID-0 implementation needs to improve >>> performance compared with using a bare drive if it is to be useful. >> >> >> Well, RAID-0 is a special case =-) > > > Sort of, yeah. It's hard to make generalizations about RAID performance > without considering each mode as a separate case...in which case, your > generalizations aren't very general. 8-) > >> That said, putting discrete RAID >> classes into the GEOM layer is something of a new adventure, so I'm >> not surprised to hear about performance problems, even in RAID-0. >> There might be extra data copies or path latencies that weren't planned >> for or expected. It's definitely something to look at. But it's also >> a very new subsystem, so it would be unfair to judge FreeBSD performance >> with it. > > > Oh, I'm not trying to throw stones your way, or at GEOM, or anywhere else. > > By and large, you would be right to claim that RAID generally performs > less well than direct access to bare drives. This conclusion is driven > as much by how frequently RAID-5 gets used compared with the less-common > RAID modes as anything else, however. Someone who uses RAID-0 or > RAID-1,0 modes really does expect to see a performance improvement. > RAID-0 yes, RAID-10 no, at least not for software RAID. The machine winds up having to transfer the same data twice across the PCI bus, twice through the controller, etc. If the controller is on a simple PCI-32/33 bus then it will quickly become saturated. Anyways, having spent a good part of my career with RAID, I find that I only use RAID-0 when I want to test system bandwidth, not when I want to store data. YMMV =-) Scott
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