From mboxrd@z Thu Jan 1 00:00:00 1970 From: Justin Piszcz Subject: Re: Questions about the speed when MD-RAID array is being initialized. Date: Thu, 10 May 2007 17:33:17 -0400 (EDT) Message-ID: References: <20070508170441.GB4333@scavenger.homeip.net> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Liang Yang Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Thu, 10 May 2007, Liang Yang wrote: > Hi, > > I created a MD-RAID5 array using 8 Maxtor SAS Disk Drives (chunk size is > 256k). I have measured the data transfer speed for single SAS disk drive > (physical drive, not filesystem on it), it is roughly about 80~90MB/s. > > However, I notice MD also reports the speed for the RAID5 array when it is > being initialized (cat /proc/mdstat). The speed reported by MD is not > constant which is roughly from 70MB/s to 90MB/s (average is 85MB/s which is > very close to the single disk data transfer speed). > > I just have three questions: > 1. What is the exact meaning of the array speed reported by MD? Is that > mesured for the whole array (I used 8 disks) or for just single underlying > disk? If it is for the whole array, then 70~90B/s seems too low considering 8 > disks are used for this array. > > 2. How is this speed measured and what is the I/O packet size being used when > the speed is measured? > > 3. From the beginning when MD-RAID 5 array is initialized to the end when the > intialization is done, the speed reports by MD gradually decrease from 90MB/s > down to 70MB/s. Why does the speed change? Why does the speed gradually > decrease? > > Could anyone give me some explanation? > > I'm using RHEL 4U4 with 2.6.18 kernel. MDADM version is 1.6. > > Thanks a lot, > > Liang > > > > > - > 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 > For no 3. because it starts from the fast end of the disk and works its way to the slower part (slower speeds).