From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: RAID5 Date: Sun, 02 May 2010 18:45:34 -0400 Message-ID: <4BDE008E.8050105@tmr.com> References: <4BCEFE66.6010607@tmr.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Michael Evans Cc: Kaushal Shriyan , linux-raid@vger.kernel.org List-Id: linux-raid.ids Michael Evans wrote: > On Wed, Apr 21, 2010 at 6:32 AM, Bill Davidsen wrote: > >> Michael Evans wrote: >> >>> On Sun, Apr 18, 2010 at 8:46 PM, Kaushal Shriyan >>> wrote: >>> >>> >>>> Hi >>>> >>>> I am a newbie to RAID. is strip size and block size same. How is it >>>> calculated. is it 64Kb by default. what should be the strip size ? >>>> >>>> I have referred to >>>> http://en.wikipedia.org/wiki/Raid5#RAID_5_parity_handling. How is >>>> parity handled in case of RAID 5. >>>> >>>> Please explain me with an example. >>>> >>>> Thanks and Regards, >>>> >>>> Kaushal >>>> -- >>>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in >>>> the body of a message to majordomo@vger.kernel.org >>>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>>> >>>> >>>> >>> You already have one good resource. >>> >>> I wrote this a while ago, and the preface may answer some questions >>> you have about the terminology used. >>> >>> http://wiki.tldp.org/LVM-on-RAID >>> >>> However the question you're asking is more or less borderline >>> off-topic for this mailing list. If the linked information is >>> insufficient I suggest using the Wikipedia article's links to learn >>> more. >>> >>> >> I have some recent experience with this gained the hard way, by looking for >> a problem rather than curiousity. My experience with LVM on RAID is that, at >> least for RAID-5, write performance sucks. I created two partitions on each >> of three drives, and two raid-5 arrays using those partitions. Same block >> size, same tuning for stripe-cache, etc. I dropped an ext4 on on array, and >> LVM on the other, put ext4 on the LVM drive, and copied 500GB to each. LVM >> had a 50% performance penalty, took twice as long. Repeated with four drives >> (all I could spare) and found that the speed right on an array was roughly >> 3x slower with LVM. >> >> I did not look into it further, I know why the performance is bad, I don't >> have the hardware to change things right now, so I live with it. When I get >> back from a trip I will change that. >> >> >> > > This issues sounds very likely to be write barrier related. Were you > using an external journal on a write-barrier honoring device? > Not at all, just taking 60G of free space of the drives, creating two partitions (on 64 sector boundaries) and using them for raid-5. Tried various chunk sizes, better for some things, not so much for others. -- Bill Davidsen "We can't solve today's problems by using the same thinking we used in creating them." - Einstein