From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Poor Software RAID-0 performance with 2.6.14.2 Date: Tue, 22 Nov 2005 16:39:07 -0500 Message-ID: <43838FFB.9060809@tmr.com> References: <4ad99e050511211231o97d5d7fw59b44527dc25dcea@mail.gmail.com> <438354B4.10604@tmr.com> <43836214.4010200@steeleye.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <43836214.4010200@steeleye.com> Sender: linux-raid-owner@vger.kernel.org To: Paul Clements Cc: Lars Roland , Linux Kernel Mailing List , Linux RAID M/L List-Id: linux-raid.ids Paul Clements wrote: > Bill Davidsen wrote: > >> One of the advantages of mirroring is that if there is heavy read load >> when one drive is busy there is another copy of the data on the other >> drive(s). But doing 1MB reads on the mirrored device did not show that >> the kernel took advantage of this in any way. In fact, it looks as if >> all the reads are going to the first device, even with multiple >> processes running. Does the md code now set "write-mostly" by default >> and only go to the redundant drives if the first fails? > > > No, it doesn't use write-mostly by default. The way raid1 read balancing > works (in recent kernels) is this: > > - sequential reads continue to go to the first disk > > - for non-sequential reads, the code tries to pick the disk whose head > is "closest" to the sector that needs to be read > > So even if the reads aren't exactly sequential, you probably still end > up reading from the first disk most of the time. I imagine with a more > random read pattern you'd see the second disk getting used. Thanks for the clarification. I think the current method is best for most cases, I have to think about how large a file you would need to have any saving in transfer time given that you have to consider the slowest seek, drives doing other things on a busy system, etc. -- -bill davidsen (davidsen@tmr.com) "The secret to procrastination is to put things off until the last possible moment - but no longer" -me