making files opposite from themselves (100% change)
Hi, I want to do some benchmarking and speed testing of rsync and UFS snapshots by taking existing files, doing rsyncs and snapshots of them and their filesystem, and then _changing_ those files by a certain percent difference, and rsyncing/snapshotting again. So the question is, how do I take a given file and make it 100% different from itself (but maintain its size and place on disk) ? I could just output /dev/zero to it, but that would leave unchanged all the bits that were aleady zero. So how do I flip the bits of an entire file ? Further, is there a good command line that will flip the bits of some percentage of the file ? thanks. __ Do you Yahoo!? Yahoo! Mail - 50x more storage than other providers! http://promotions.yahoo.com/new_mail ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: making files opposite from themselves (100% change)
On Mon, 5 Jul 2004 13:55:00 -0700 (PDT) Joe Schmoe [EMAIL PROTECTED] wrote: Hi, So the question is, how do I take a given file and make it 100% different from itself (but maintain its size and place on disk) ? I could just output /dev/zero to it, but that would leave unchanged all the bits that were aleady zero. So how do I flip the bits of an entire file ? Further, is there a good command line that will flip the bits of some percentage of the file ? The xor operation of a byte/word/dword with itself does that. You could setup a buffer of the desired % of bytes you want to change, read the bytes, xor them (^ in C) with itself and write back. It's trivial in C/Perl/python/whathaveyou. Or you could fetch some random data from /dev/urandom if you prefer. -- Miguel Mendez [EMAIL PROTECTED] http://www.energyhq.es.eu.org PGP Key: 0xDC8514F1 pgpNScRmyLOLI.pgp Description: PGP signature
Re: making files opposite from themselves (100% change)
On 2004-07-05 13:55, Joe Schmoe [EMAIL PROTECTED] wrote:
So the question is, how do I take a given file and make it 100%
different from itself (but maintain its size and place on disk) ?
I could just output /dev/zero to it, but that would leave unchanged
all the bits that were aleady zero.
Use an algorithm similar to the one shown below as a Perl script, to
pick a certain percentage of the bytes within a file, and at those
offsets chosen by this algorithm, use XOR with 0xFF or a random value to
alter the value of only the given percentage of bytes.
: #!/usr/bin/perl -w
:
: use strict;
: my ($filesize, $percent, $k, $nparts, $partlen);
:
: die usage: foo.pl FILESIZE PERCENT
: unless ($#ARGV == 1);
:
: $filesize = $ARGV[0];
: $percent = $ARGV[1] % 100;
:
: $nparts = int(($percent * $filesize) / 100);
: $partlen = int($filesize / $nparts);
:
: srand (time ^ $$ ^ unpack %L*, `ps axww | gzip`);
:
: print offsets:;
: for ($k = 0; $k $nparts; $k++) {
: my $tmp = int(rand($partlen));
: my $nbyte = ($tmp + $k * $partlen);
: print $nbyte
: }
: print \n;
So how do I flip the bits of an entire file ?
That's even easier:
: for each byte:
: xor(byte, 0xFF);
HTH,
Giorgos
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Re: making files opposite from themselves (100% change)
On Mon, 5 Jul 2004 22:59:55 +0200 Miguel Mendez [EMAIL PROTECTED] wrote: The xor operation of a byte/word/dword with itself does that. You could setup a buffer of the desired % of bytes you want to change, read the bytes, xor them (^ in C) with itself and write back. It's trivial in C/Perl/python/whathaveyou. This, of course, is supposed to read : the bitwise not operation. Never post before coffee. Cheers, -- Miguel Mendez [EMAIL PROTECTED] http://www.energyhq.es.eu.org PGP Key: 0xDC8514F1 pgpmBWVRdzraH.pgp Description: PGP signature
