Date: Mon, 25 Oct 2004 15:45:48 -0400 From: Charles Swiger <cswiger@mac.com> To: Scott Long <scottl@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: FreeBSD 5.3b7and poor ata performance Message-ID: <77F3FD4D-26BE-11D9-9A2F-003065ABFD92@mac.com> In-Reply-To: <417D45F1.9090504@freebsd.org> 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>
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On Oct 25, 2004, at 2:29 PM, Scott Long wrote: >> Also, there is an unresolvable question. Why two 52MB/s disks >> in raid0 has a throughput of 40MB/s and for raid1 18MB/s?? > > Would you _PLEASE_ stop trying to associate RAID with performance! > RAID is about reliability and reduncdancy, not about speed. All RAID modes make tradeoffs between performance, reliability, and cost. RAID-1 mirroring and RAID-5 provide higher reliability by using partial or full redundancy. However, RAID-0 striping provides no additional reliability: the primary reason for using RAID-0 is to improve performance by accessing two or more devices in parallel. > Some cases can give you desirable performance increases as a side > effect, > but that is not the primary goal. Disagree. Why else would you use RAID-0 striping? [ If you simply want to create a logical volume bigger than the size of a physical drive, you can use concatenation instead. ] > Specifically in this case, the > GEOM raid classes are fairly new and have not had the benefit of > years of testing. I'd much rather that the focus be on stability > and reliability for them, not speed. Once the primary goals of > RAID are satisfied then we can start looking at performance. 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. -- -Chuck
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