From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stan Hoeppner Subject: Re: Very long raid5 init/rebuild times Date: Mon, 03 Feb 2014 00:34:13 -0600 Message-ID: <52EF3865.4000106@hardwarefreak.com> References: <21217.9977.5869.747472@quad.stoffel.home> <20140121073540.GF10140@merlins.org> <20140121163755.GG10140@merlins.org> <52DF7976.6070808@hardwarefreak.com> <20140122174854.GF26014@merlins.org> <52E0807D.2030608@hardwarefreak.com> <20140123091319.GB2306@merlins.org> <52E10A07.3070302@hardwarefreak.com> <20140123210155.GJ10046@merlins.org> <52E1F685.3050300@hardwarefreak.com> <20140125083630.GA7555@merlins.org> <52E76054.5090900@hardwarefreak.com> <52EABA63.2060701@ubuntu.com> <52ED77A3.6020509@hardwarefreak.com> <52EE9445.2020202@ubuntu.com> Reply-To: stan@hardwarefreak.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <52EE9445.2020202@ubuntu.com> Sender: linux-raid-owner@vger.kernel.org To: Phillip Susi , Marc MERLIN Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 2/2/2014 12:53 PM, Phillip Susi wrote: > On 02/01/2014 05:39 PM, Stan Hoeppner wrote: >> It's not possible >> to accurately measure IO throughput doing buffered writes. Thus >> O_DIRECT is needed. > No, you can get there just fine with buffered IO as well, unless you > have an obscenely fast array and very slow ram, the overhead of > buffering won't really matter for IO throughput ( just the extra cpu ). Please reread my statement above. Now let me restate that as: Measuring disk throughput when writing through the buffer cache isn't a measurement of disk throughput as much as it is a measurement of cache throughput. Thus, such measurements do not demonstrate actual disk throughput. Do you disagree? -- Stan