From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: stride / stripe alignment on LVM ? Date: Fri, 02 Nov 2007 09:01:05 -0400 Message-ID: <472B1F91.8000208@tmr.com> References: <20071101101037.63b523f8@absurd> <18218.24819.569744.141171@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <18218.24819.569744.141171@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: Janek Kozicki , linux-raid@vger.kernel.org List-Id: linux-raid.ids Neil Brown wrote: > On Thursday November 1, janek_listy@wp.pl wrote: > >> Hello, >> >> I have raid5 /dev/md1, --chunk=128 --metadata=1.1. On it I have >> created LVM volume called 'raid5', and finally a logical volume >> 'backup'. >> >> Then I formatted it with command: >> >> mkfs.ext3 -b 4096 -E stride=32 -E resize=550292480 /dev/raid5/backup >> >> And because LVM is putting its own metadata on /dev/md1, the ext3 >> partition is shifted by some (unknown for me) amount of bytes from >> the beginning of /dev/md1. >> >> I was wondering, how big is the shift, and would it hurt the >> performance/safety if the `ext3 stride=32` didn't align perfectly >> with the physical stripes on HDD? >> > > It is probably better to ask this question on an ext3 list as people > there might know exactly what 'stride' does. > > I *think* it causes the inode tables to be offset in different > block-groups so that they are not all on the same drive. If that is > the case, then an offset causes by LVM isn't going to make any > difference at all. > Actually, I think that all of the performance evil Doug was mentioning will apply to LVM as well. So if things are poorly aligned, they will be poorly handled, a stripe-sized write will not go in a stripe, but will overlap chunks and cause all the data from all chunks to be read back for a new raid-5 calculation. So I would expect this to make a very large performance difference, so even if it work it would do so slowly. -- bill davidsen CTO TMR Associates, Inc Doing interesting things with small computers since 1979