From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tim Moore Subject: Re: paralellism of device use in md Date: Tue, 17 Jan 2006 16:23:15 -0800 Message-ID: <43CD8A73.90703@nsr500.net> References: <20060117120927.GU7017@strugglers.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20060117120927.GU7017@strugglers.net> Sender: linux-raid-owner@vger.kernel.org To: linux-raid List-Id: linux-raid.ids Andy Smith wrote: > ... > For example, are *writes* to a 2 device RAID-0 approaching twice as > fast as to a single device? If not, are they any faster at all? > Are reads from a 2 device RAID-1 twice as fast as from a single > device? If there are benefits, how quickly do they degrade to > nothing as disks are added? > Server development where I work uses a 3-way mirror for system bits but this would be a costly solution for any sort of real storage and/or write performance. Here's the last chunk of a pair of 120GB WD SATA-I drives, SiI3112 chipset, sata_sil driver, 2.4.32 kernel. A three or four way stripe should get proportionally more provided separate controller channels and of course, risk scales with performance. [16:05] abit:~ > cat /proc/mdstat | head -7 Personalities : [raid0] [raid1] [raid5] read_ahead 1024 sectors md14 : active raid0 sdb13[1] sda13[0] 114575616 blocks 32k chunks md13 : active raid1 sdb12[1] sda12[0] 20113216 blocks [2/2] [UU] [16:06] abit:~ > hdparm -tT /dev/{md14,sd{a,b}13,md13,sd{a,b}12} /dev/md14: Timing buffer-cache reads: 1908 MB in 2.00 seconds = 954.00 MB/sec Timing buffered disk reads: 272 MB in 3.01 seconds = 90.37 MB/sec /dev/sda13: Timing buffer-cache reads: 1904 MB in 2.00 seconds = 952.00 MB/sec Timing buffered disk reads: 156 MB in 3.01 seconds = 51.83 MB/sec /dev/sdb13: Timing buffer-cache reads: 1912 MB in 2.00 seconds = 956.00 MB/sec Timing buffered disk reads: 136 MB in 3.00 seconds = 45.33 MB/sec /dev/md13: Timing buffer-cache reads: 1876 MB in 2.00 seconds = 938.00 MB/sec Timing buffered disk reads: 164 MB in 3.00 seconds = 54.67 MB/sec /dev/sda12: Timing buffer-cache reads: 1904 MB in 2.00 seconds = 952.00 MB/sec Timing buffered disk reads: 166 MB in 3.02 seconds = 54.97 MB/sec /dev/sdb12: Timing buffer-cache reads: 1892 MB in 2.00 seconds = 946.00 MB/sec Timing buffered disk reads: 146 MB in 3.00 seconds = 48.67 MB/sec [16:08] abit:~ > -- | for direct mail add "private_" in front of user name