Date: Fri, 19 Nov 2010 12:39:13 -0700 (MST) From: Warren Block <wblock@wonkity.com> To: Matthias Apitz <guru@unixarea.de> Cc: freebsd-usb@freebsd.org Subject: Re: copying /dev/da0 with dd(1) to file: output differs Message-ID: <alpine.BSF.2.00.1011191231200.95921@wonkity.com> In-Reply-To: <20101119183900.GA1221@tiny.Sisis.de> References: <20101119143337.GA3023@current.Sisis.de> <201011191821.09308.hselasky@c2i.net> <20101119173519.GA3933@current.Sisis.de> <201011191916.53655.hselasky@c2i.net> <20101119183900.GA1221@tiny.Sisis.de>
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On Fri, 19 Nov 2010, Matthias Apitz wrote: > El d?a Friday, November 19, 2010 a las 07:16:53PM +0100, Hans Petter Selasky escribi?: > >>> I was thinking in a tool just reading each file block by block, >>> comparing the blocks and noting the 1st diff with block offset number. >>> (some 10 lines of C code :-)) >>> >>> matthias >> >> Maybe you need to write a small C-program to do that. >> >> You can use bcmp() to compare two buffers. > > Will do that tomorrow. > > Just an idea: The USB key in question was new and I only created the > file system on it the usual way (fdisk, bsdlabel, newfs). Then I > restored the dump on it (which took 26 hours for 3.1 GByte dump file). > The USB key boots fine, btw. 26 *hours*? USB 1.1? > Could it be that unwritten/unformatted blocks are read as random data > from that USB key? Well, they could be anything. But the original filesystem has formerly-used blocks with old data from deleted files. dump doesn't copy those, but dd does. > Should I overwrite the full USB key from /dev/zero? Possibly there would still be differences. Filesystem metadata like date last mounted, for example. If you want a block-by-block duplicate, the brute-force method is to just dd the whole drive. Use bs=64k or bs=1m to help reduce overhead.
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