From mboxrd@z Thu Jan 1 00:00:00 1970 From: Asdo Subject: Re: Why does one get mismatches? Date: Wed, 03 Mar 2010 12:42:48 +0100 Message-ID: <4B8E4B38.9070702@shiftmail.org> References: <20100225083936.07cd48ad@notabene.brown> <20100228080949.GA30574@maude.comedia.it> <20100302160100.621f9811@notabene.brown> <20100302073624.GA28827@maude.comedia.it> <4877c76c1003020204r477e942fo8ada66e1e9426295@mail.gmail.com> <20100302110232.GA14131@maude.comedia.it> <4B8D5581.8070509@shiftmail.org> <20100302185216.GB2243@lazy.lzy> <4B8D9EDD.2080300@shiftmail.org> <20100303091314.GA3730@lazy.lzy> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-reply-to: <20100303091314.GA3730@lazy.lzy> Sender: linux-raid-owner@vger.kernel.org To: Piergiorgio Sartor Cc: linux-raid List-Id: linux-raid.ids Piergiorgio Sartor wrote: > I've a bunch of RAID-6 volumes, made of USB disks, i.e. using > PATA<->USB bridges. > Don't your bridges ever drop out or break? What is the brand/model? Years ago I broke alot of those, just by using the disks intensively. They probably kinda overheated then failed. They couldn't last 2 days on intense disk activity... They were chinese stuff bought on ebay though. > This volumes are aggregated using LVM and, on top of that, there > is a LUKS container. > > The raw read perfomance on the RAID-6 is, in the best case, > around about 48MB/s, which is pretty good for USB, I guess it > will be difficult to get more. > 48MB/sec can be good for 1 disk, but it's bad for many disks attached separately to USB ports... > The raw read perfomance of the LVM volume is i~38MB/s. > The raw read performance of the LUKS is ~28MB/s (actually > maybe a bit less). > Might your LVM or partition within it be not aligned, or you didn't set readahead? http://www.beowulf.org/archive/2007-May/018359.html http://www.mail-archive.com/linux-raid@vger.kernel.org/msg10804.html People using LVM on arrays giving hundreds of MB/sec see slowdowns of the order of percent http://article.gmane.org/gmane.linux.raid/18302 >> LVM also does not merge requests AFAIR. (visible with mdstat -x 1) > BTW, what's that? I mean "mdstat -x 1"... I'm sorry I meant " iostat -x 1" >> But me also I have never seen mismatches and the only raid-1's I >> have are above LVM. (except /boot but that's almost never modified) >> > > Well, that's good, you confirmed my experience. > > I've also RAID-10 on LVM and never got mismatches, while the > plain RAID-10 got sometimes. > > This fact needs further investigation methinks... We could ask to the LVM people if LVM really copies the buffer. Regards A.