Hi folks We've fielded a number of mirroring questions offline as well as watched/participated in discussions here. I thought it was important to make sure some of these are answered and searchable on the lists. One major question that kept arising was as follows: q: If I have a large image file (say a VM vmdk/other format) on a mirrored volume, will one small change of a few bytes result in a resync of the entire file? a: No. To test this, we created a 20GB file on a mirror volume. root at metal:/local2/home/landman# ls -alF /mirror1gfs/big.file -rw-r--r-- 1 root root 21474836490 2011-05-02 12:44 /mirror1gfs/big.file Then using the following quick and dirty Perl, we appended about 10-20 bytes to the file. #!/usr/bin/env perl my $file=shift; my $fh; open($fh,">>".$file); print $fh "end ".$$."\n"; close($fh); root at metal:/local2/home/landman# ./app.pl /mirror1gfs/big.file then I had to write a quick and dirty tail replacement, as I've discovered that tail doesn't seek ... (yeah, it started reading every 'line' of that file ...) #!/usr/bin/env perl my $file=shift; my $fh; my $buf; open($fh,"<".$file); seek $fh,-200,2; read $fh,$buf,200; printf "buffer: \'%s\'\n",$buf; close($fh); root at metal:/local2/home/landman# ./tail.pl /mirror1gfs/big.file buffer: 'end 19362' While running the app.pl, I did not see any massive resyncs. I had dstat running in another window. You might say, that this is irrelevant, as we only appended, and that could be special cased. So I wrote a random updater, that updated at random spots throughtout the large file (sorta like a VM vmdk and other files). #!/usr/bin/env perl my $file=shift; my $fh; my $buf; my @stat; my $loc; @stat = stat($file); $loc = int(rand($stat[7])); open($fh,">>+".$file); seek $fh,$loc,0; printf $fh "I was here!!!"; printf "loc: %i\n",$loc; close($fh); root at metal:/local2/home/landman# ./randupd.pl /mirror1gfs/big.file loc: 17598205436 root at metal:/local2/home/landman# ./randupd.pl /mirror1gfs/big.file loc: 16468787891 root at metal:/local2/home/landman# ./randupd.pl /mirror1gfs/big.file loc: 9271612568 root at metal:/local2/home/landman# ./randupd.pl /mirror1gfs/big.file loc: 1356667302 root at metal:/local2/home/landman# ./randupd.pl /mirror1gfs/big.file loc: 12365324308 root at metal:/local2/home/landman# ./randupd.pl /mirror1gfs/big.file loc: 15654714313 root at metal:/local2/home/landman# ./randupd.pl /mirror1gfs/big.file loc: 10127739152 root at metal:/local2/home/landman# ./randupd.pl /mirror1gfs/big.file loc: 10259920623 and again, no massive resyncs. So I think its fairly safe to say that the concern over massive resyncs for small updates is not something we see in the field. Regards, Joe -- Joseph Landman, Ph.D Founder and CEO Scalable Informatics Inc. email: landman at scalableinformatics.com web : http://scalableinformatics.com http://scalableinformatics.com/sicluster phone: +1 734 786 8423 x121 fax : +1 866 888 3112 cell : +1 734 612 4615
