Date: Wed, 07 Aug 2002 10:27:50 -0500 From: "Jack L. Stone" <jackstone@sage-one.net> To: "Siegbert Baude" <Siegbert.Baude@gmx.de>, "Rob Ellis" <rob@web.ca> Cc: <freebsd-questions@FreeBSD.ORG>, =?Windows-1252?Q?S=F8ren_Schmidt?= <sos@freebsd.dk> Subject: Re: Best "bs" for dd copies (was: Re: Questions about vinum and failure of root partition) Message-ID: <3.0.5.32.20020807102750.02d62db8@mail.sage-one.net> In-Reply-To: <001c01c23e25$b05a1180$406a3c86@whwurm.uniulm.de> 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>
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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
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