Date: Wed, 7 Aug 2002 11:38:33 -0400 From: Rob Ellis <rob@web.ca> To: freebsd-questions@FreeBSD.ORG Subject: Re: Best "bs" for dd copies (was: Re: Questions about vinum and failure of root partition) Message-ID: <20020807153832.GA53552@web.ca> In-Reply-To: <3.0.5.32.20020807102750.02d62db8@mail.sage-one.net> References: <005e01c23dcb$061acbb0$6602a8c0@swbell.net> <200208070101.g7711iU06306@clunix.cl.msu.edu> <005e01c23dcb$061acbb0$6602a8c0@swbell.net> <3.0.5.32.20020807085441.02d62db8@mail.sage-one.net> <3.0.5.32.20020807102750.02d62db8@mail.sage-one.net>
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i chose 102400 also by trial and error -- a biggish number that might be a multiple of disk blocks. :-) smaller numbers made the dd slower, bigger numbers didn't seem to be making it significantly faster... but i wasn't being scientific. - rob On Wed, Aug 07, 2002 at 10:27:50AM -0500, Jack L. Stone wrote: > At 05:18 PM 8.7.2002 +0200, Siegbert Baude wrote: > >Hi Jack, > > > >first an apology to Soeren for cc'ing him, but he probably is the best > >man to answer this question (and a second one for not being able to > >produce the correct letter for the spelling of his name *g*). > > > >Rob wrote: > >> >then create a backup of the fbsd partitions on > >> >the first disk, copying everything from ad0s1 to ad2s1: > >> > > >> > dd if=/dev/ad0s1 of=/dev/ad2s1 bs=102400 > > > >Jack wrote: > >> My side question is about the "dd" command. Why did you choose the > >> parameter "bs=102400" rather than any other?? I've been using 8192, > >but > >> have seen this switch all over the map, including 1024 to 1M. I know > >it can > >> make a difference in the time to do an image because of the sizing. > >With > >> 8192, I do an entire 40GB HD in 39 mins (1.4GHz CPU) but takes 49 mins > >for > >> a 1GHz CPU. > > > >I just experimented a bit last weekend, when I dd'ed a 80GB IBM > >IC35L080AVVA07-0 to another disk of exactly the same type (needed a > >bit-copy as backup, because the partition table was corrupted). > > > >I tried bs from standard 512 up to 8MB by always doubling the value from > >try to try and found that 128k worked the best for me. That is quite > >near to the proposed 100k of Rob. The transfer rate varied from 6 MB/s > >to 20MB/s, if my memory works right. At least it was far away from the > >33MB/s the UDMA-33 mode should give (the disks could do UDMA 100 and the > >highpoint controller even UDMA 133, but I only had normal cables). IBM > >claims its disk should transfer a sustained rate from 48MB/s to 23MB/s > >depending on the zone. The disk cache is 2MB, btw. The board was an > >Abit-BX133 with 256MB RAM and a PIII-850. > > > >So, if anybody knows how to calculate the best value out of the > >technical parameters or can explain, why ~100k seems the best value (and > >not e.g. something in the area of disk cache size) I also would be very > >interested to hear. > >What is the maximum at all, one can expect? Is it possible to reach the > >maximum rate IBM claims for its disk with dd? > > > >Ciao > >Siegbert > > > > Thanks for the follow-up on the "dd bs" side question. Clearly this > parameter makes a big difference as does the CPU speed, UDMA, etc. I too > would like to know of any way to calculate, but probably what you did by > trial is a good approach for each particular specific setup/environment. > > Best regards, > Jack L. Stone, > Administrator > > SageOne Net > http://www.sage-one.net > jackstone@sage-one.net > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-questions" in the body of the message To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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