You can get data in whatever chunk size you want using read(). Here is an example that takes 24 seconds to process a 100MB file on my PIII with slow disks. That gives a throughput of 4MB per second which will process your 81GB file in under 6 hours. The optimal chunk size empirically is around 1MB with modest benefits increasing it to 2,4 and 8 MB. With smaller chunks you can here the heads flipping from one file area to the other - bigger chunks allow the heads to chill. At 64kB the run time was 57 seconds and the disks screamed. At 4MB the runtime was 23 seconds.

If possible I would suggest reading from one disk and writing to a completely separate one (I did the testing on a single partition of a single disk). You could also roughly double the speed by forking a kid to do the disk write while the parent reads and processes more info. This will only help if you are reading from one disk and writing to another.

#!/usr/bin/perl -w
use strict;
my $chunk = 2**20;  # try 1MB to start but it may be faster to go bigg
+er/smaller
my $infile = 'c:/test.txt';
my $outfile = 'c:/out.txt';
open IN, $infile or die "Can't open $infile $!\n";
open OUT, ">$outfile" or die "Can't open $outfile $!\n";

my $buffer;
my $partial_line = '';
my $start = time;

while (read(IN, $buffer, $chunk)) {
    # we should only process full lines so we trim off the partial lin
+e 
    # that we inevitably get at the end of our read and save it into $
+2
    $buffer =~ s/^(.*\n)([^\n]+)\z/$1/s;
    # add last partial line to front of buffer
    $buffer = $partial_line.$buffer;
    # save the current partial line for next loop so we can add it bac
+k on
    $partial_line = $2 || '';
    # make changes
    $buffer =~ s/this/that/g;
    print OUT $buffer;
}

print "Took ", time - $start, " seconds\n";

close IN;
close OUT;
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cheers

tachyon

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