Date: Sun, 2 Oct 2005 08:59:26 -0700 (PDT) From: Arne "Wörner" <arne_woerner@yahoo.com> To: Steven Hartland <killing@multiplay.co.uk>, freebsd-performance@freebsd.org, Patrick Proniewski <patpro@patpro.net> Subject: Re: dd(1) performance when copiing a disk to another Message-ID: <20051002155926.19631.qmail@web30313.mail.mud.yahoo.com> In-Reply-To: <003e01c5c764$3391d720$b3db87d4@multiplay.co.uk>
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--- Steven Hartland <killing@multiplay.co.uk> wrote: > From: "Patrick Proniewski" <patpro@patpro.net> >> # dd if=/dev/ad4 of=/dev/ad6 bs=1m >> >> It yields to poor performances: >> > That's actually pretty good for a sustained read / write on a > single disk. > Does somebody know, why this is "pretty good"? I mean: Where is the bottleneck? As far as I know, SATA is quite fast... And memory to memory copies are quite fast... disc<->memory should be quite fast, too. >> Is it normal that data rate won't go upper than 35/38 MB/s ? >> Hmm... Can u find out, if DMA transfers are enabled for those discs? What does dmesg say? What does "sysctl hw.ata.ata_dma" say? Maybe atacontrol(8) says something useful about SATA discs, too (e. g. atacontrol mode 0)? Can u try the following commands, when the system (especially the discs) is idle? #dd if=/dev/ad4 of=/dev/null bs=1m count=1000 #dd if=/dev/zero of=/dev/null bs=1m count=1000 (Maybe you could find a way to copy /dev/zero to /dev/ad6 without destroying the previous work... :-)) E. g.: # dd if=/dev/ad6 of=/tmp/arne bs=1m count=1000 # dd if=/dev/zero of=/dev/ad6 bs=1m count=1000 # dd if=/tmp/arne of=/dev/ad6 bs=1m count=1000 ) > one more question: is dd(1) a good way to duplicate a boot > drive to make a bootable spare disk ? > I say, is the file system on /dev/ad4 read only during the "dd"? If /dev/ad4 changes before "dd" completes, ad6 might need a fsck or ad6 might be useless... Btw.: I use gmirror(8)... But then an unintentional, fatal change von ad4 would be fatal for ad6, too... :-)) So I have to hope, that I do not type things, I shall not type (luckily I have some boot CDs for that unlikely case ;-)) )... -Arne __________________________________ Yahoo! Mail - PC Magazine Editors' Choice 2005 http://mail.yahoo.com
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