From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: Performance of a software raid 5 Date: Tue, 21 Apr 2009 00:46:54 +0100 Message-ID: <49ED096E.1000002@anonymous.org.uk> References: 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: Johannes Segitz Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 20/04/2009 18:12, Johannes Segitz wrote: > i'm currently trying to create a raid 5 out of three 1 TB hdd. For now > there is one hdd missing so i get 3 TB of usable space. [...] > Stride and stripe-width will be correct when i add another two hdd of > which one will carry data. Can someone please give me a hint why i > could get such bad performance especially while reading? I would have thought it's because you're running in degraded mode and one in 3 sectors is having to be regenerated from the parity. It still seems a bit slow, though. Here I have a 3-disc RAID-5 of similar drives: # hdparm -i /dev/sda /dev/sda: Model=SAMSUNG HD103UJ , FwRev=1AA01112, SerialNo=S1PVJ1CQ602164 Config={ Fixed } RawCHS=16383/16/63, TrkSize=34902, SectSize=554, ECCbytes=4 BuffType=DualPortCache, BuffSize=32767kB, MaxMultSect=16, MultSect=?0? CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=268435455 IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120} PIO modes: pio0 pio1 pio2 pio3 pio4 DMA modes: mdma0 mdma1 mdma2 UDMA modes: udma0 udma1 udma2 AdvancedPM=yes: disabled (255) WriteCache=enabled Drive conforms to: unknown: ATA/ATAPI-3 ATA/ATAPI-4 ATA/ATAPI-5 ATA/ATAPI-6 ATA/ATAPI-7 # mdadm --detail /dev/md1 /dev/md1: Version : 00.90.03 Creation Time : Mon Jul 28 15:49:09 2008 Raid Level : raid5 Array Size : 1953310720 (1862.82 GiB 2000.19 GB) Used Dev Size : 976655360 (931.41 GiB 1000.10 GB) Raid Devices : 3 Total Devices : 3 Preferred Minor : 1 Persistence : Superblock is persistent Intent Bitmap : Internal Update Time : Tue Apr 21 00:35:26 2009 State : active Active Devices : 3 Working Devices : 3 Failed Devices : 0 Spare Devices : 0 Layout : left-symmetric Chunk Size : 256K UUID : d8c57a89:166ee722:23adec48:1574b5fc Events : 0.6134 Number Major Minor RaidDevice State 0 8 2 0 active sync /dev/sda2 1 8 18 1 active sync /dev/sdb2 2 8 34 2 active sync /dev/sdc2 It has LVM and an ext3 filesystem on it. Here are my timings: # time dd if=/dev/zero of=big_file bs=4096 count=2560000 2560000+0 records in 2560000+0 records out 10485760000 bytes (10 GB) copied, 264.448 seconds, 39.7 MB/s real 4m25.740s user 0m2.272s sys 0m34.470s # time dd if=big_file of=/dev/null bs=4096 count=2560000 2560000+0 records in 2560000+0 records out 10485760000 bytes (10 GB) copied, 53.9577 seconds, 194 MB/s real 0m54.026s user 0m0.556s sys 0m4.944s I'm not quite sure whether I should be disappointed at my writes being so slow. Certainly there's a lot of rattling during writing, which probably indicates lots of seeks to write ext3's journal. But reads are roughly what I expected, at about three times the single-disc throughput. Cheers, John.